Transactions of the Geological Society, 1st series, vol. 3/On the Geology of Glen Tilt
VIII. A Geological Description of Glen Tilt.
Read Dec. 3, 1813, and the Supplementary Additions Dec. 16, 1815.
By john Mac Culloch, M.D. F.L.S. President of the Geological Society, Chemist to the Ordnance, and Lecturer on Chemistry at the Royal Military Academy at Woolwich.
It is well known that the appearance of the granite in various parts of the valley of the Tilt afforded to Dr. Hutton one of the original arguments on which his theory was founded. Hence this spot has acquired among geologists a sort of classical celebrity, sufficient to render it an object of interest, independently of that which is excited by the intricate and remarkable disposition of its rocks. Observations, now more numerous than in his day, have brought to light so many facts similar to those which appeared to him so important, that there is no difficulty in pointing out abundant instances of an occurrence once supposed rare. But having had occasion while examining these rocks to observe other interesting appearances, which have been either misapprehended or overlooked, I consider that a notice of them will not be unacceptable to the Society. The notice is accompanied by such specimens and drawings as are requisite for the illustration of the subject.
In describing Glen Tilt I have been under the necessity of differing on certain points from those who have preceded me. An examination of many of these points of difference would lead to discussions as endless as they would be fruitless, and give this paper the aspect of a controversial essay, rather than of that to which it alone pretends, a descriptive one; for which reason I have chosen to describe it precisely as it appeared to me. Where my description differs from that of others, the differences will be found to consist at times in a difference of opinion respecting the denominations of rocks, while on other occasions they are much too great to admit of their arising from a different use of the same terms, or a different mode of contemplating the same phenomena; they are differences respecting facts, and are therefore the less fit subjects of discussion.
As the local circumstances which require to be examined before
an adequate notion can be formed of the true structure of this interesting
place occupy a considerable range of country, and as
inevitable confusion would follow any attempt to describe the mineral
beds and their geological connections in their natural order,
on account of their perpetual interference with the geographical
disposition of the ground, I have chosen to adopt a geographical
method. In so doing I shall the more readily be understood as far
as the simple specification of facts goes, and those who shall
incline to follow me in the investigation of this spot will also be
furnished with a clue by which they may trace the description, and
the more easily confirm or refute, as it may happen, that which I
shall relate. In the present state of geological knowledge, this
method of proceeding seems absolutely necessary. Like the detail
of chemical experiments it enables the reader to follow step by
step the appearances from which the general results have been deduced,
to examine the inferences as they are drawn from the phenomena,
and finally to determine on the legitimacy of the conclusion.
Such inferences or such general conclusions, as appear
to me to result from a comparison of these facts duly approximated,
will be stated afterwards.
In examining such a valley as that which is the subject of the present paper, the course of the river, as well as the ranges of hills which bound it, must necessarily enter into the history of its geological formation. Where these are of similar structure it is easy to comprehend the whole in one line of description, taking the course of the river for a guide. But in Glen Tilt a dissimilarity so complete is observed between the opposite sides of the valley, and the numerous and interesting phenomena which occur in the bed of the stream itself are so dissimilar to both, that it is impossible to convey an adequate idea of the whole structure without a distinct examination of the three several lines. A perpetual transference of the imaginary spectator from one point to another, would produce as much confusion in the narration, as such a proceeding in examining the ground would obscure the judgment and puzzle the investigation of the real observer. I have therefore chosen to detail the examination of the three lines separately, and have commenced with the river as the most natural if not the most useful method, describing in succession the ranges of hills, of which the history is required to render the phenomena which are to be seen in the river intelligible.
The Tilt has its rise in a small loch not far from that long valley which is the common division of the waters running east and west to join the Dee on one hand and the Tumel on the other. As it enters that valley it falls immediately into a line tending westward, from which it deviates towards the south-west before its termination in the Garry, the narrow bottom of the glen affording but little room for lateral deviations, and its direction being subject to no material variation. For two or three miles its course lies so much through alluvial matter that it offers nothing to the spectator; nor does any material feature occur before its junction with the Tarff. Occasionally it may be seen cutting its way through quartz rock, through schist, and, not long before its arrival at the Tarff; through beds of dark bluish limestone. One solitary portion of pure white limestone or marble is also visible in this part of its course. Where it joins the Tarff it also receives another stream of considerable magnitude, flowing from Glen More and collecting the waters from a set of vallies that lie between the foot of Carn-ree and the head of Glen Fernat, as well as from those corys of Ben Gloe which tend in this direction.
It will be unnecessary to particularize the several streams which it receives during its course to the Garry, unless where their sections offer material information respecting the structure of the neighbouring hills.
Having passed the Tarff it begins to run in a more rocky channel, and though often, and for considerable spaces, its true bed is concealed by the alluvial matter which it has accumulated, enough remains exposed to excite the curiosity, and employ no small portion of the time of a geologist. Although in enumerating and describing these several points where the natural bed is visible, I have used all my endeavours to specify the particular places where they occur, yet in this respect the description will still be deficient. Even if an accurate survey of the ground were made, the want of points of reference in a valley which possesses so few permanent marks for distances, would prevent me from referring to them as accurately as would be desired by those who might wish to follow the same track. The difficult nature of the banks, and the many inaccessible points which the river possesses, combined with the dangerous violence of its waters, frequently also deny access, and may consequently have led to omissions. Such omissions will also be found to arise from another cause, to which may equally be referred some discrepancies and errors in the measurements of the remarkable rocks which are seen in its bed. This cause is the variable state of the water, which at times exposing more or less of these projections in its bottom, alters their apparent magnitude as well as their number. I have been rendered sensible of these differences by examining its course at, different periods, but since some confusion would have resulted from any attempt to correct the first examination which I made, I have preferred giving it as it was first recorded, with the caution against implicit reliance in the measurements which I have now laid down. It is only necessary to say that the observations were made in a kind of medium height of the water, and that as the geological consequences are in no wise affected by this sort of inaccuracy, I held it unworthy of correction. The statement of the circumstance was however necessary, lest the discovery of inaccuracies of this nature might lead so doubts with regard to more important observations. The measurement given in yards is merely a rude computation from pacing, but it is fully adequate to the purposes in view; since neither the facts nor reasonings can in any way be altered by a greater or less nicety of admeasurement.
Descending therefore the course of the river from Poll Tarff (See
Map plate 13) the junction of that stream with the Tilt, a large body
of granite will be seen crossing it above the Shepherd's hut, and a
small granite vein may also be observed above it at a little distance.
The bed of the river from Poll Tarff to this point, wherever it is visible
consists of dark blue limestone alternating with quartz rock.
At the distance of four or five hundred yards from the burn of
Aldianachie, a large rock consisting of a complicated mixture of
granite and schist crosses the Tilt again. I shall not here dwell
upon the circumstances which attend this mixture, as better opportunities
of describing them will occur hereafter.
It is not far below this place, but at a distance which I have neglected to note, that a great portion of a limestone bed will be found crossing the river. It is much contorted, and is also reticulated with a few granite veins. Here the alternation of the limestone with the quartz rock is also visible, and they cross the river together. The limestone is in this place uncommonly hard, and in its composition very siliceous. Its external aspect where it is worn by the action of the water, is not much unlike that of a granite or porphyry, and it has in fact been sometimes mistaken for one or other of these rocks. Continuing to descend nothing remarkable occurs till we arrive within about two miles of Forest lodge, where a rock like those last mentioned, traverses the bed of the river. This rock consists of a great mass of red granite so mixed with quartz rock and hornblende schist, that neither pen nor pencil can describe their confusion. Limestone may be observed both at its upper and lower edge, and this is traversed and reticulated by small granite veins. The whole mass occupies a space of about 150 yards. It is proper to remark here, that the blanks which occur in this account of the bed of the river are such as in general arise from its course being over a bed of alluvial matter which covers and conceals the fundamental rock, while in other cases they arise from those portions of the natural bed which are visible being trivial or unimportant, or from the state of the water which prevented its bottom from being seen.
The next, and of all perhaps the most remarkable rock to be observed in the course of the Tilt, occurs at a bridge a short distance above Forest lodge. A large mass of red granite is first seen occupying the bed of the river for a considerable space both above and below that bridge. Associated with this mass of granite are various rocks, so disposed and intermingled that neither description, nor drawing, nor specimens, can give an adequate idea of their disposition and confusion. The drawings which accompany this paper may serve to illustrate a description which can only be general, since no drawing less in size than the rock itself could give an accurate representation of the place. In contact with this great mass of granite, which for a certain space offers no very particular feature, are seen rocks of a schistose nature. These are succeeded by blue limestone, and subsequently by schist and granite, but in a state of disorder so inexplicable, that I do not attempt to describe their relative positions. No appearance of parallelism is to be seen in the schistose rock, but the limestone, although much bent and twisted, has the aspect either of a complicated vein, or of the edges of a bed in a vertical position, the portions of which have been split asunder and filled with other materials. The lamellar form of this limestone and its connections with the surrounding rocks, leave however no doubt of its being the exposed edge of a bed, and it must at the same time be remarked that besides being placed in a vertical position, the line of its course is at right angles or nearly so to the general bearing of the strata, of which it seems once to have formed a regular constituent part. Viewing the whole of this compound mass of schist and limestone as a single rock, and the granite as another, it is easily seen on the most superficial glance, from the strong contrast between the dark grey of the former and the red of the latter, that the dark rock is intersected and disturbed by innumerable veins of granite. These traverse it in every possible direction, and are of various sizes, the smallest not exceeding that of a thread, and reticulating the dark rock in a most intricate and amusing manner. Such is the ordinary view which has been taken by one class of observers of this rock, as well as of all the other rocks seen in Glen Tilt. They have been looked upon as mere masses of schist intersected by veins of granite, the larger bodies of granite being considered as veins, the origin and direction of which were unknown, and the smaller ones as ramifications proceeding from them. A different class of observers, seeing in this imagined schist nothing but its approximate position with granite, have considered it as offering an example of the alternation of these two rocks. In whatever way the different theories may be affected by these distinct views of this case, I have no inclination at present to enquire. If one of them be incorrect, the other is imperfect, and the object of this paper is rather to give a more full account of the appearances to be observed in this place, than to renew a controversy on the same grounds on which it has already been agitated.
Independently of such considerations, the disposition of the rocks at this place, as well as in other parts of the Tilt, are so curious, and as yet so solitary and new, as to render them highly attractive and well worthy of record. It will however be seen hereafter, that the history of these several appearances, even if it should not be supposed to add any strength to the general theory respecting the posteriority of granite, serves at least to give us an accurate idea of the real structure of the valley itself.
The remarkable novelty here visible, and in which the junction of the granite with the superincumbent rocks differs from all similar junctions formerly described, is that of the interference of granite and limestone. The granite veins which traverse the schist, pass equally into the limestone which accompanies it. These veins are occasionally of large size. In this case they can sometimes be traced into the larger masses of granite with the same ease as when they traverse the schist: in others, however, they appear both to originate and end in the limestone, and present rather the aspect of detached lumps and irregular processes than of veins. So confused is this interference, that a fragment of granite is often found entangled in the limestone, and a lump of the latter will sometimes be found intruding into the former. The minuter veins of granite are generally if not always connected with the larger pieces, and they intersect and reticulated the whole limestone as they do the schist, diminishing at length to the thickness of a leaf of paper or a thread. But there is a still more remarkable arrangement of the limestone and granite. Parallel to the limestone bed or beds, and following every flexure and contortion which it undergoes with the most perfect regularity, are to be seen narrow lines rising above the general surface, and accompanying the course of the bed through its whole extent. On examining them they are found to consist of a harder substance which has resisted the action of the water, while the softer and intermediate parts, being of limestone, have been dissolved and washed away. The fracture of the rock shows that these are the edges of laminæ formed of a reddish siliceous substance precisely similar to that which constitutes the reticulations, and which from the contrast of its colour to that of the blue limestone with which it alternates, is always easily distinguished.
Having blown up a considerable portion of this rock, I am enabled to say that it is of a laminated texture throughout, being a bed of which the alternate layers are limestone and that siliceous red rock which I consider as a modification of granite. A perfect notion of such a limestone may be formed by recollecting the appearances of those laminated limestones which contain alternate layers of micaceous schistus or clay slate.
There is yet one other disposition of these two classes of rock; this consists in minute points or fragments, if they may be so called, of the same siliceous matter, inhering in the limestone, and which from their smallness are scarcely to be detected, unless where from having been exposed to the action of water they are found to give a rough and echinated surface to the calcareous rock. This appearance is very widely diffused through the whole extent of the body of limestone hereafter to be described, as forming a great portion of this district, wherever it is found in the vicinity of the great mass of granite.
The singularity of these appearances renders it proper to dwell a little on them, and to enquire into their connections and probable origin. Saussure indeed has mentioned a transition from granite to limestone, and, as there are some situations in Glen Tilt where the limestone in contact with granite becomes so siliceous and indurated as by degrees entirely to lose its mineralogical, and pretty nearly its chemical character, so it is possible that he may have met with some similar fact, although no very accurate notion can be derived from his account of it and consequently no assistance obtained from his observation.
As I shall have occasion to enter fully hereafter into the mineralogical description of this and of the other rocks which occur in Glen Tilt, and shall then vindicate the term granite, which as a general term I have applied to it, I think it only necessary to say at present that the mass of rock above described as traversing the stratified rocks, is a portion of a more continuous one which may be traced to the hills constituting the right hand or northern boundary of Glen Tilt. It is not an independent vein. So far therefore the geological connection, as well as the identity in mineralogical character of the mass of granite which is approximate to the limestone and schist with the granite mountains of the northern ridge, is indisputable. From this mass, processes or veins of different sizes are seen to issue, until they gradually terminate, as I before said, in a thread. As long as these veins continue of a few inches in breadth, their mineral character remains unaltered. As they diminish, however, the hornblende gradually disappears, although, in the cases in which the vein traverses hornblende schist, this mineral is increased in quantity, and the vein assumes rather a more decided character of that rock which, as I conceive, is inconveniently distinguished by the term syenite. But the vein of mixed quartz and felspar is the most common, and this, as it continues to diminish in size ultimately becomes mere felspar, or else a compound of felspar and quartz so intimate, that the magnifying glass discriminates the particles no longer, and the whole is only distinguishable from common felspar by its peculiar fracture and superior hardness. This circumstance, the varying composition of granite veins, is not unusual, and it may be observed, among many other places, in the Corpach bason of the Caledonian canal. Now, however inaccurate it may be in a mineralogical point of view to designate the substance which constitutes the minuter veins by the term granite, yet for the purpose of geological reasoning it is unavoidable, nay, proper, since the substances have an absolute continuity or geological identity. It would be unjust either by a misapplication of terms, or an undue nicety in their use, to reject any geological argument which might be founded on such a fact as this. The same is true of the siliceous lamina found in the limestone, as well as of the detached pieces and small points, since they are absolutely identical in composition with the smaller reticulations.
With respect to the limestone, it is subject to a variation of aspect where it approximates to the granite which it is interesting and important to notice. The colour of the great limestone mass is lead blue of various intensity, and its texture is almost universally large grained and highly crystalline. It is true that in some few instances it preserves this aspect in the situations described, but, in the greater number of cases, particularly in those where it is penetrated by the smaller veins and where the beds have undergone an elongation or a contortion, the crystalline texture disappears. It then assumes an appearance exactly resembling that of hornstone or compact felspar, having a smooth texture, with a thin-edged fracture intermediate between the splintery and flat conchoidal. I may as well add here that the schist is often of an argillaceous, but highly indurated, character, and also that it often passes into hornblende slate, and this (it is worthy of notice) the more frequently as it approximates nearer to the granite.
So much has been said with respect to the origin of granite veins, and on the mode by which their intrusion into the schistose rocks which generally accompany them has been effected, that it is superfluous to repeat it, since nothing new can be offered on the subject. By whatever mode it has been produced, it is obvious that the same explanation will apply to the case of the limestone which is interstratified with the schist, as far at least as the veins are concerned, although the fact itself be a new one. But how has the alternation of laminæ of granite with those of limestone been effected?
It is necessary here to anticipate that which will be fully described, and, I trust, proved hereafter in a more proper place, the original regular position and the posterior disturbance of these stratified rocks. The limestone which is here visible, is a portion of an enormous series of beds, which are to be found in a regular position occupying the left ridge which bounds Glen Tilt, interstratified with quartz rock and with different varieties of schist. This regularity of position is destroyed wherever these beds come in contact with the mass of granite which occupies the right ridge before mentioned. Consequently, the limestone involved in the schist and granite now under review, is a portion of a bed the original position of which is perverted and lost. Are we now to suppose that this bed of limestone was originally deposited in its present form with alternating layers of granite? Admitting that the reticulating veins have been the consequence of a posterior intrusion, this cannot possibly be true of the lamina, as independently of the difficulties, or impossibility as it may more properly be called, of such a process, it is plain that the simultaneous flexure and contortion of the laminated mass is the result of the disturbance produced by the granite veins, and consequently that it was deposited before the intrusion of those veins. Must we then allow that there are cases where granite, or a matter resembling it, has been deposited like schist and bedded limestones from solution or suspension in a fluid? In the mean time, however, the igneous hypothesis respecting granite may perhaps allow of another mode of explaining this appearance. It is conceivable that a mass consisting of alternate layers of micaceous or argillaceous schist and limestone, a compound of which abundant examples are seen in the immediate vicinity of the rock under review, might be so acted on by heat as to admit of the conversion of its siliceous layers into the substance now found interstratified in the limestone, while this latter resisting as it is well known to do, any change from the application of heat under such circumstances, might undergo no other affection than I that softening which has led to its flexure and contortion.
The remaining appearances to be seen at this rock are more curious as adding to the unexampled confusion of the whole mass, than as offering any thing very new to the geologist in addition to that which has been already detailed.
Among the remaining substances entangled in this confusion, a large vein of felspar is the most remarkable. Its course is not long, nor does it appear to have any connection with the granite. It varies in colour through this limited extent, being sometimes white, at other times of a pink or dead lilac hue, and in some few places greenish. This latter colour seems to proceed from a mixture of epidote, which mineral is found in other parts of the rock in thin veins accompanying the granite and colouring the schist. Some massive garnet is also to be seen involved among the other substances, and it seems principally connected with the limestone. In a few cavities there occur crystals of flesh coloured calcareous spar, but so incomplete from the narrowness of the spaces in which they have crystallized, that I could not assign their figure. This I believe completes the catalogue of the minerals found in this singular place.
The granite is again seen crossing the river between this bridge and Forest Lodge, and near it lies a body of quartz rock which is evidently a continuation of some beds which may be observed in the hill above.
About 300 yards below the Lodge it crosses the stream again, and here the limestone is again mixed with it. A set of beds of a yellowish and greyish colour, resembling some varieties of foreign marble, and exceedingly indurated, may be seen occupying a space of about twenty feet. If a careless view of these beds be taken, they will be found to offer what appears for the space of about ten feet, to be an alternation of granite in a very regular disposition with limestone. It has been mistaken for a real alternation. If this rock be more carefully examined it will be evident that it consists of a mere fragment, and that it is abruptly cut of where it meets the hill on the right, by abutting against the mass of granite. As the lower end dips under the water, a clear view of that part was not to be obtained when I examined it, but it appeared there also to be cut of by a granite vein. Those parts therefore of the granite which seem to alternate with the limestone can only be considered as portions of veins, the disposition of which, like that of trap veins in similar cases, has accidentally coincided with the direction of the limestone beds. If granite does really occur in beds, I know no reason to prevent it from alternating with limestone, but the appearance here is much too limited, and too doubtful in its origin to be admitted as an instance of such alternation. The granite which accompanies this junction may be seen in the bed of the river for seventy or eighty yards, after which it disappears. Three or four hundred yards of a blank alluvial space occur immediately after this, followed by a rocky space of about 200 yards. This rocky portion exhibits the following alternation, granite, schist, granite, limestone, succeeded by a general confusion of all these substances, and the granite which is in general limited to the right bank, now crosses to the left of the river. When I say that there is such an alternation, I do not mean that the several substances are bedded in this order, the case to which the term alternation is more properly applied. It is particularly necessary to attend to this remark, and to ascertain the real nature of this kind of alternation, since it has been quoted at different times as proving a regular alternation between granite and schist, the limestone, as it happens, having been here overlooked. It is no further an alternation than because these substances, which in numerous other instances are confounded and irregularly mixed together, happen in this particular one and for a very short space, to have assumed a disposition accidentally more regular. The question of alteration must rest on other facts than these. A blank of about eighty yards follows this rock, and is succeeded by one of the most remarkable junctions of the different rocks already described, which occurs in the course of the river. The main body of this compound mass consists of white limestone or marble, without any tendency to that regularly bedded form which is the general characteristic of the limestone in Glen Tilt. It is accompanied by a small portion of schist as well as by a mass-of granite, with both of which it is variously intermixed. The marble itself is in most places of a pure white, a fine grain, and dry aspect, and is extremely hard. In some places it is of an ochry colour, and is interspersed by thin veins of the same substance, so as to resemble some of the palest and worst specimens of that marble known by the name of Giallo Antico. The schist which accompanies it is argillaceous and of a bluish colour, and the granite, both in the larger masses and in the ramifications, bears so near a resemblance to that formerly described that it is superfluous to describe it again. The veins which traverse the marble, like those at the bridge above mentioned, are of various sizes and are placed in every possible direction, but the ramifications are neither so numerous nor so minute as in that instance, nor is there any appearance of the laminated structure which occurs in the former. Where the marble is at the greatest distance from the granite it differs little or nothing in hardness or composition from ordinary specimens of this substance. But wherever it approaches or comes into contact with the granite it becomes highly indurated, effervesces slowly with acids, and gives on analysis a large portion of siliceous matter. In other respects there is no obvious change at the planes of contact, and the union between the granite and limestone is so slight that the action of the water separates them. The union of the limestone with the schist is much more intimate. Wherever the granite traverses both these substances together, which is generally the case where the schist is present, such a confusion takes place, that the boundary of neither can be defined, a troubled mixture and then a complete gradation between these two rocks being the result.
On the right bank of the river above this rock a bed may be seen, consisting of a brownish hornstone, or rather a compact felspar, with a character somewhat porphyritic. It is one of that great variety of rocks associated under the general name of porphyry of which frequent examples occur in the course of the Tilt on both sides of the valley; more than I have thought it necessary to mention. These are sometimes considered as veins, and sometimes as beds; I believe that the former disposition is by far the most common, but their real nature is often so difficult to ascertain that I have been obliged to abandon the point in despair. The present one appears to be a bed.
I have purposely avoided noticing the rocks on both sides of the river which lie beyond its immediate bed, as they offer, on the right side at least, a succession so mixed and so confused, that any view of them which can be obtained in following the stream is nearly unintelligible. I may only say that schist, granite, and quartz rock of different aspects, with occasional masses of porphyry, succeed each other so frequently that the whole might be considered by those who are prepossessed respecting systems of alternation, as a demonstrable instance of this disposition. I hope to give a more correct view of their connections hereafter. On the left bank I may also generally remark that a succession of dark blue limestone, rarely interrupted by schist, quartz rock, and granite, is seen on the left bank from Poll Tarff down to this place, and even beyond it as far as Gow's bridge.
An alluvial blank of about 800 yards follows this marble rock, which is succeeded by 100 yards or thereabouts of granite; and after 150 yards more of an alluvial space, a large mass of rock for the space of 200 yards and upwards is found traversing and obstructing the stream, so as to form a cascade. This very irregular mass of rock consists of granite and quartz rock united. The quartz rock is of a peculiar aspect, being of a bluish colour, very uniform and compact and approaching near to common quartz in character, but still shewing evident marks of foliation. It is disturbed and intersected by the granite in a way which I need, not now repeat. A thin stripe of limestone and schist may also be seen interwoven in the granite. Below the cascade there is once more a blank alluvial space of 100 yards, which is succeeded by quartz rock for about the same space; immediately after which occur about fifteen yards of a yellowish and greyish limestone or marble, very hard, and disturbed and traversed by granite in the way already described. This is followed by a series of limestone beds for about 200 yards, the first of which is blue and the last five or six of a yellowish aspect: these are almost the first limestone beds observed in descending the river which have the same general dip and position with the beds of the mountain on the the left hand. But that position can only be called general, as in every instance all particular or minute regularity disappears wherever the limestone beds are found in the immediate vicinity of the granite. Nothing very remarkable is to be seen between this place and the entrance of the Criny. Near the point of confluence beds of quartz rock are distinctly seen crossing the river, and dipping under the limestone to which they are parallel.
Although various sections of the hills on each side are given by the streams which fall into the Tilt, few of them are sufficiently deep or explicit when singly taken to afford much information about the structure of the hills. The Criny however presents a section of considerable importance, which, as it illustrates the views that I shall hereafter give of the whole structure of this glen, may as well be described here.
The quartz rock which I have noticed as crossing the river, may be traced through the chasm formed by this torrent, and it is found reposing on limestone, which is immediately succeeded by argillaceous schist. I am uncertain whether there is not a thin bed of schist interposed between the quartz rock and the limestone, and I have represented such a one in the engraved section; but it is not important, and the place is so difficult of access, while the confusion produced by the waterfall is so great, that it is difficult to make the investigation minutely. This alternate arrangement is found immediately reposing on the granite, and the schist being the bed in actual contact with the granite, is penetrated by veins precisely similar to those which have already been described. The limestone is here of a peculiar character, being mixed with quartz, sometimes in distinct concretions, sometimes in laminæ irregularly alternating. These veins originate in the mass of granite which forms the mountain, and this observation is important, as I shall hereafter shew that the granite seen in the Criny is a portion of the great central granite mass which forms the whole range of hills on the right bank of the Tilt.
From Glen Criny a succession of schist and quartz rock is to be observed, extending till within a few hundred yards of Gow's bridge; but the bed of the river being here inaccessible, we can only obtain a general view of it. The last bed of quartz rock in this direction is immediately followed by a bed of limestone, and this again is succeeded by an alternation of beds of schist, limestone, and quartz rock for some hundred yards below that bridge, together with masses of porphyry both of a reddish and a greyish colour, but whether disposed in beds or veins I have been unable to ascertain, though I imagine that they are beds, since they seem to conform to the stratified rocks. As this collective mass presents some interesting circumstances it will require a more minute detail.
The position of the whole of these beds is regular, with two very slight exceptions, and they occupy a space extending from 320 yards or thereabouts above Gow's bridge to 200 below it.
We have seen that all the irregularities of the beds take place wherever the granite comes into contact with them. Here there is no granite present, but, on the contrary, the mass of limestone of which these beds form a part, and which has hitherto been found terminating in the bed of the river, is continued to the right hand ridge, and may he traced high up in the hill, where the contact with the granite takes place. The only exception with regard to the granite, is that of one solitary and small vein, of a grey colour, which intersects the schist immediately above the bridge; the disturbance however produced by it is trifling. Another disturbance, although small in extent, is very remarkable, though not unique, since I have found a similar occurrence in Iona. Below the bridge on the left bank a bed of white marble is seen surmounted by a bed of hornblende schist. This is followed by a second bed of the marble about two feet in thickness, of which the greater part is suddenly cut off at right angles to the bed by a second mass of hornblende schist, while the lower part continues in the same direction, but bent, and of the breadth of only two or three inches. The lower part of the mass of hornblende schist, which thus intersects the marble, is also protracted in a thin plane continuous with the thin part of the marble and lying above it, while the upper side of the same becomes continuous with a regular and thick bed of the schist. Between these two portions of the schist there is interposed a thick bed of the marble, which is therefore included on three sides within the hornblende schist, its outer extremity being cut through at right angles by the schist. As the beds dip into the hill, the further progress of this extraordinary arrangement cannot be traced. A drawing accompanies this description, which words can scarcely render intelligible. The appearance now described is interesting, since it is connected with phenomena of no very uncommon occurrence, which have given rise to much controversy. It is evident that both the prolongation of the limestone and that of the schist bear a great resemblance to the veins, which, in the case of granite, may be traced from a mass of that substance into the neighbouring rocks: yet there is no doubt that both the limestone and the schist are stratified rocks. I have observed the same appearances in clay slate, and they are frequent in the islands of Scarba and Jura, where this substance alternates with quartz rock, and where great contortions of these rocks have occurred. A more remarkable example occurs in a rock which constitutes one of the numerous beds of which Schihallien is composed. This rock is a micaceous schist, containing imbedded fragments of granite and of quartz rock, often of considerable magnitude. The larger fragments of quartz rock are sometimes partially split at right angles to its laminar structure, and these fissures are filled with the substance of the mica slate, putting on the same pseudomorphous appearance of a vein. I have found similar veins of red sandstone in the limestone of Arran, and they have also been seen in trap. But in a paper on Kinnoul I have described and delineated a specimen from a very extensive set of appearances of this nature, where a schist of the graywacké character becomes prolonged into ramifying veins in the interstices of a trap rock by which it is broken and disturbed. It is unnecessary therefore to enter on the subject in this place, but it will be sufficient to say that such pseudomorphous veins must be considered as portions of the strata, which, during their soft or softened state have been forcibly compressed and elongated into that form. The appearance occurring at Schihallien might obviously be produced by the mica while in a loose state in water, falling into and occupying the open spaces of the fragments deposited with it; and, in a similar manner, rifts in limestone might have been filled by the loose sand which formed the surrounding strata. The explanation of the other circumstances which occur at the junction of the limestone and hornblende schist which has given rise to these remarks, will be as obvious to those who shall inspect the drawing as it is to me, and it is probably not very obvious to any one.
It is not far from this place, and on the left bank of the river, that a greenstone vein may be seen traversing the strata at nearly right angles. It is the only specimen of genuine greenstone which I observed in Glen Tilt, and it appeared to me to resemble those veins which are found in Cruachan and in the neighbouring mountains, where they also frequently assume a porphyritic aspect. Still however they differ considerably in their general appearance from those greenstone veins which are most usual in Scotland, and which are found to predominate in the vicinity of the larger formations of trap in the Western islands, and in many well known parts of the continent of Scotland.
The great mass of limestone which we shall hereafter find forming the whole of the left boundary of Glen Tilt, is of a dark blue colour, with one or two exceptions which I have already described in the progress down the river. But the beds at this place are of various colours, and offer some of the most beautiful ornamental marbles which Scotland has yet produced.
The basis of nearly the whole is a white, rather larger grained, and crystalline marble. Beds of this variety occur in a pure state, and of considerable dimensions. But as all these marbles contain more or less of mica, with which substance they are interstratified, the white colour is seldom pure, being mottled with the slight grey tint which mica in similar cases always produces. It cannot therefore be considered as a statuary marble, since modern artists, acquainted with the beautiful stone of Carrara, have confined their labours to this more perfect variety. It is however perfectly applicable to various architectural as well as economical objects. A marble perfectly similar to it has lately been imported from America for the same purposes, to many of which its greyish hue and low tone of colour are more applicable than the dazzling white of Carrara. It is of a larger grain and a more compact texture than the Pentelic, with which the beautiful and interesting remains imported by Lord Elgin have lately made us acquainted. But the Pentelic marble, like that of Glen Tilt, contains mica, and from this contamination arises its fissile nature, to which we unhappily owe so much of the injury which these wonderful works have suffered. When polished, the two can scarcely be distinguished from each other; the difference in the size of their grains disappearing, and the grey and watery stains, with the brown stripes of the micaceous laminæ, equally characterizing both.
The discovery of statuary marble in the British dominions has been long a desideratum, but having already in the present volume discussed this question, I shall only briefly remark that how much soever we may admire those wonderful sculptures by Phidias which have been executed in a marble scarcely differing in colour or quality from this of Glen Tilt, we are very well assured on examining the progress of art in Greece, that the marble of Pentelicus was only used in the deficiency of a purer and more uniform stone, and that it was abandoned when later discoveries had made the sculptors of that country acquainted with a better class of marbles. It would be a fruitless attempt to introduce the marble of Glen Tilt, or even those whiter varieties which Scotland produces, in competition with the exquisitely beautiful and easily wrought stone of Carrara, as long as these quarries remain accessible to us. For that higher class of sculpture on which the powers of genius are exerted, the proportion between the price of the wrought and unwrought article, (to use commercial phraseology) is so unequal, that no difference in the value of the raw material can compensate for even the most trivial defects in its quality. But there remains even in the class of the fine arts a great number of uses to which the marble of Glen Tilt might with advantage be applied. Such are all those works in architectural decoration, in which absolute whiteness and uniformity of colour, are not only unnecessary, but from their dazzling effect even injurious. The subdued tone and slight air of antiquity given to this marble by its stained and unequal colour would in these works render it of the greatest use. Its durability for the purposes of interior architecture must also be equal to that of Carrara, although there is little doubt that when exposed to the action of the weather it would like the Pentelic be liable to corrosion in those parts which abound in mica.
Besides mica, steatite and noble serpentine are found mixed with the white marble. The colours of these substances offer various gradations from bright yellow down to the darkest sap green. It is by these admixtures that the green and white marbles which form by far the largest portion of these beds are produced. The colours are so variously mixed, blended, and dispersed throughout the stone that numerous varieties are the result; and these are further increased by the occasional presence of dark lead blue. This mixture of serpentine with marble is by no means uncommon in Scotland. On the contrary it may be said that all the white marbles found in this country contain it in a greater or less It is common in the white marble of Sky and in that of Balahulish. In that of Assynt it is more rare, but it occurs also abundantly in a white marble which I found in Glen Fernat, not far from the junction of the Brerachan with the Airdle. The well known ancient green marbles owe that colour to the same admixture, which indeed in the Egyptian green and the Verde antico, is such that the calcareous matter is overpowered by the serpentine. The aspect of the green marble of Glen Tilt is however perfectly different from that of any marbles ancient or modern which have yet been wrought, and it offers therefore a valuable addition to the arts as well as a new commodity to the list of our mineral productions. I may add that, with a similarity of composition, many specimens bear a considerable resemblance both in character and colour to some varieties of the Cipolino of the Italians. Two other distinct varieties of marble occur where the calcareous beds terminate. The one is of an uniform ochre yellow, but of a much paler tint than the giallo antico; the other is of a flesh colour graduating into dark blue, but neither of these beds is of great magnitude.
The extent of the limestone which I have now been describing is considerable. The mass is of great thickness, and from the obliquity of its section a very large horizontal surface is exposed. As the beds run towards the north they may be traced to a considerable distance up the hill on the right of the river, but dipping to the south they plunge into the opposite hill and disappear. Independently of the steatitical matter which is mixed with this rock, some interesting minerals are found imbedded in it. The beds where they come in contact with micaceous schistus contain so much mica that this latter becomes at length the predominant substance, and there is then a perfect transition from micaceous schist to marble. In these cases it is raised in thin flags of great extent; between these flags there are often seen beds of steatitical clay, of a pure white or greenish hue, and often of considerable thickness. In the same situation may be seen thin beds of a compact pale green and transparent steatite, and irregular lumps of the same substance are frequently found dispersed about the beds.
It is rare to find distinct pieces of noble serpentine of any magnitude, but they are also met with, and of a transparency and greenness equal to that of the well known specimens from Iona.
Talc of various colours, white, lead-blue and green is likewise met with among the marble beds, but in small quantity: and asbestus, although it is seen here and there in considerable pieces, may also be considered rare. But it produces abundance of tremolite, and offers some interesting and uncommon varieties of that beautiful mineral.
Of these, the first is fibrous and very compact, having a slight purplish tinge, particularly when viewed according to the axis of its crystallization. It is imbedded in the rock, with which it is perfectly compacted, presenting on fracture a small stellated or a longer radiated appearance, and giving a peculiar character to the marble.
The next variety consists of a congeries of regular prismatic crystals, varying from a tenth to a sixth of an inch in diameter. These are seldom radiated, but most commonly form a dense mass of which the crystals are irregularly interwoven together, and sometimes curved. Thin beds of this are found between the marble beds, and in some cases the crystals have shot in cavities so is to present terminations, which, however formerly complete, have been spoiled by the action of the water on them.
It is well known to mineralogists that many substances when found in the earth are in a soft state, and only acquire induration when removed from their native beds. If we are to believe Patrin the same is true even of the beryl. It is so with the tremolite which I have now described, which is exceedingly soft and flexible when first up, but hardens after some days of desiccation. The asbestus taken which I found here is likewise so soft that it may be wrought into a paste with the lingers, but it acquires in a few days a hardness equal to that of fir wood. I have observed the same fact in the disthene which I collected at Boharm in Bamiffshire. It is worthy of remark that the reapplication of water does not restore these minerals to their flexible state. We have yet to learn the chemical explanation of this circumstance.
There is no novelty in the fact I have here brought forward, but sufficient attention has not perhaps been paid in geological writings, to this differing condition of many strata as they exist in the earth, and after they have undergone that change to more perfect induration which they acquire after being removed from their native places and exposed to the air. With regard to many rocks used for economical purposes it is notorious even to workmen. It is easy to see how this circumstance affects many of the reasonings which have been brought forward on the consolidation of strata; a condition of which we see perpetual examples before us, without any ground for supposing that the agency of heat was conducive to that end, and in circumstances indeed where no agency of heat can he imagined capable of producing the complicated effects which have resulted. We are too little acquainted with the chemical laws which regulate the affinity of earths in a state of extreme division, to decide on results which may or may not be produced from either solution or suspension in water. One solitary fact well known in the potteries showing the strong affinity which exists among earths in such a state of extreme division and suspended in water, is sufficient to suggest to its the possibility of affinities still more intense existing among earths in similar circumstances, when their proportions perhaps are different, or where, in addition to a more minute division, many of them may perhaps also be in a state of solution. On a small scale indeed examples of stony matters so formed are not wanting in nature. Such are the hard deposits of calcareous and other matters in water pipes, and in vessels at no greater temperature than that of boiling water. It is easy to see in the case of the semi-indurated strata, and of the soft minerals above described, how the further abstraction of the water in which these affinities were first brought into action, may by increasing them in consequence of the nearer approximation of the earthy particles, induce that stony hardness which these substances acquire in drying.
A third and more beautiful variety of tremolite is also found here. This like the last forms thin beds interposed between those of the marble, and occasionally also entering as a constituent part into the stone. It is that well known variety which so much resembles spun glass, that this comparison forms the best description that can be given of it. It is generally of a pure white colour, and is radiated in stellated forms varying from an inch to two feet in diameter, and exhibiting specimens of uncommon magnificence. I could not perceive that any of the specimens of this variety were so decidedly flexible as the latter, although they were taken from the bottom of the river, but there was still a very sensible difference in their power of cutting the hands and entering into the skin when first taken up, and after they had been kept for a few days. More rarely the same fibrous and silky variety is found passing through all the shades of colour, from a pale to a very dark sea-green, and these latter specimens are particularly marked by the I crowded and beautiful groups of their small stellated radiations.
At no great distance from these beds and in a similar position, a fourth and hitherto undescribed variety is also found, constituting a bed of two or three feet in thickness. In its general aspect it so nearly resembles a granular quartz as to be at first sight difficultly distinguished from it. As it produces no effervescence with acids, it probably contains no great mixture of carbonate of lime, and may therefore be considered as a granular tremolite. It may be distinguished from quartz by its superior lustre, translucency, and specific gravity, as well as by its inferior hardness. The sp. gr. of the specimens which I tried was 2,840.
A fifth variety is found in the same place, but without so very marked a character as that of the one preceding. It is foliated, with a slight tendency to a fibrous structure, and forms large concretions in the marble. Its aspect is vitreous, and it is of a greenish grey colour.
Another modification of this beautiful mineral which occurs here can scarcely be considered of sufficient importance to constitute a mineralogical variety. It splits into thin laminæ, of which the flat surface has the splendour and almost the uniform smoothness of white satin, but on examination it is seen to consist of very minute fibres of tremolite interfoliated with very white and compact carbonate of lime. Some specimens are again found which seem to consist of a mixture of tremolite and schiefer spar, a substance which is also found although but rarely, among the calcareous beds.
The various forms indeed under which the several mixtures of tremolite with the surrounding rocks occur are such as to elude description, and not a little to puzzle a mineralogist. I shall only enumerate two more of the most remarkable. One of these is a compact schistose rock in which the union of the tremolite with the limestone is so intimate that it can only he distinguished in the first instance by its hardness and specific gravity, as the eye cannot detect the mixture. In the other the tremolite is mixed with a kind of steatitic slaty marle, very friable, of a snow white colour and great specific gravity.
Associated with this limestone and in a similar position, I also discovered beds of sahlite, a mineral of considerable rarity in this country. These vary in thickness from two inches to a foot and more, and are sometimes intimately mixed with thin laminæ of the fibrous variety of tremolite. This mineral, from its superior hardness, resists the action of the water of the river better than the limestone does with which it is interstratified, and may therefore be distinctly traced projecting from those beds of which the calcareous portions have been more or less worn away.
The beds of sahlite may be seen extending for a space of from
fifty to a hundred yards along the bank of the river at the same
place. Considerable variety of aspect occurs throughout their
course, from a greater or less intermixture of calcareous matter with
the pure sahlite. Thus, like the more common specimens of this
mineral it sometimes effervesces with acids, while in other cases
it appears to contain no uncombined calcareous matter. When
it is unmixed it resembles at first sight the whiter varieties of
felspar, exhibiting a large platy fracture, and a similar lustre.
In the angle of its fragment as well as in its specific gravity,
hardness, and habitudes with the blow-pipe, it perfectly resembles
the better known varieties of this mineral. As it becomes more
and more mixed with the calcareous matter its platy fracture first
becomes less regular and gradually disappears, the mixed mineral
however still retaining a specific gravity greater than that of the
neighbouring limestone. Its sp. gr. varies from 3,153 to 3,246, the
most purely lamellar being the lightest. It is invariably white with
a slight tinge of grey.
I should not however conclude this account of the minerals which occur in this part of Glen Tilt, without mentioning that fibrous limestone is also found in some of the rifts, showing a little of that pearly and changeable lustre, for which the satin spar is so remarkable.
I have thought it unnecessary in describing either these or the other rocks which I have examined, to particularize the several anomalous mixtures of substances which are found about them, since they are extremely partial and generally limited to the vicinity of the junctions. They all appear to be of a mechanical nature, or at least to have resulted from the same causes which produced the several disturbances already described, and they offer no particular instruction, while at the same time they can scarcely be defined by words.
Quitting these marble beds and descending still the course of the Tilt, schist, quartz rock, and limestone, are seen alternating for about 500 yards. In one place a bed occurs in the quartz rock, forming a regular part of the series, but composed of the constituents of granite, these being sometimes disposed in a granitic manner, and sometimes possessing the foliated texture of gneiss. This is followed by a schist, which seems intermediate between gneiss and micaceous schist, having the aspect of the latter with its shining surface and even foliated structure, but shewing in the cross fracture the grains of felspar which belong to the former.
Proceeding further on, a great series of these thin beds may be observed, of which some on being broken present the aspect of true granite, inasmuch as the mica is irregularly placed and the laminæ of the rock, however foliated in position, are by no means foliated in structure. A fragment so broken as to be divested of its external flat form, would be considered as a fragment of granite since it must be esteemed an essential character of gneiss that the mica should possess an arrangement, at least so far parallel, as to give the appearance of a foliated structure.
As whatever view we can take of this occurrence is followed by important consequences, it is necessary to consider it somewhat more particularly, and I shall first examine that case in which the gneiss is found alternating with the schist. We have already seen in numerous instances, that great anomalies take place in the aspect of the schistose rocks, as well as of the limestone, wherever they approximate to the granite and are traversed by granite veins, I must therefore premise that no such veins are seen in this place. The mass of granite is here indeed removed to a considerable distance, and the schistose beds are perfectly straight and undisturbed.
The general series consists of mica slate, alternating in a very irregular manner with hard argillaceous schist and quartz rock; and with a few beds of limestone. With these beds is found that micaceous schist which contains felspar, and which will probably be considered by most observers as a modification of gneiss. If then it be called gneiss, that rock alternates with quartz rock, clay slate, mica slate, and limestone, an anomaly at variance with the almost universally received doctrines respecting precedence in rocks. If we now turn our attention to those specimens which have the granitic aspect, I may remark that, however decided this character may be in some particular points about this place, I shall here lay no stress on it, as better examples can be adduced from the hills at no great distance. Although the granite, in the instances which I shall enumerate, occupies but a small space among the laminæ, its texture is perfectly granitic, and bears not the slightest resemblance to that of gneiss, since the mica is placed in all directions, and the portion in question, if detached from the neighbouring parts, could not be distinguished in the generality of hand specimens from genuine granite: at the same time I must add, that it bears but little resemblance to veins. The neighbouring rocks furnish abundant examples of ordinary granite veins, which are readily distinguished from the present appearances by the decision of their junctions, while in this case a regular gradation exists between the granite and the schist, and no boundary to the former can be assigned. The appearance which I have now described is not limited, as I have just said, to this place: other examples of it may be found. Besides the loose specimens of it which are to be observed among the transported blocks on the hills which bound the left side of the valley, it is also of frequent occurrence through a large tract which extends from Blair to Dalwhinnie, and thence to the head of the Spey. The specimens seen in this direction frequently exhibit various and repeated alternations of granite and schist, from the thickness of three or four inches down to that of a quarter of an inch. The tract in question is wholly, or in a great measure, composed of hard argillaceous schist, graduating into schistose quartz rock and more rarely into micaceous schist, with granite veins dispersed here and there through it. Between the lamina of these schists the granite is found graduating into the schist, many varieties of coarser or finer texture appearing in rapid succession in the space of a few inches. It is only by the comparison of a number of specimens that the true nature of the rock is discovered, and it affords a curious example of the facility with which false conclusions may be formed on this intricate subject. However parallel the laminæ of granite may be to those of the schist for a certain space, they invariably quit that direction when the specimen is of sufficient magnitude, and may be seen holding an oblique course through these laminæ, often indeed becoming true granite veins in their course, and even crossing the layers at considerable angles. A rock observable at Garviemore illustrates this accident well. Here the transit of the granite vein through the schist is at right angles, and is attended with a flexure of the layers on each side of the vein, in a curve directed from the thickest to the thinnest part of it. At each side of the vein the granite passes from it between the laminæ of the schist, producing a partial effect similar to the more extensive one above described, and which has doubtless owed its origin to a similar cause.
The other case which I reserved for consideration, is that where similar beds of granite are found alternating with, and graduating into quartz rock. This alternation is visible in the bed of the river not far from the entrance of the Mark. But a much better instance occurs in the hill of Grianan on the right of Glen Tilt, where the transition from the quartz rock to the granite is so perfect that the boundary cannot be defined. I have met with similar appearances in the vicinity of Dalnacardoch.
In a paper which I presented to the Society last year, I described among several varieties of quartz rock, that, perhaps the most common of all, in which grains of felspar are found mixed with grains of quartz, neither of them being crystallized; and I then pointed out the distinction between this rock and granite, attempting to shew that it was a recomposed rock, and that it had been formed from the wearing of more ancient granites. Among the several transitions of quartz rock into mica slate, clay slate, &c. I did not notice this transition into real granite, as I had not then met with this appearance: it will add one to the number of those transitions. It will at the same time be obvious, that it can in no respect affect the account which I then gave of the nature and connections of quartz rock. On the contrary, as I have shewn that this rock belongs to the same order of rocks as mica slate, and clay slate, it will only serve to point out their concurrence in another common circumstance. It is perhaps almost superfluous to say that the transition, which I am now describing, of quartz rock into granite, is by no means a dubious one, and that it does not bear the least resemblance to the common variety above referred to, which has so often been mistaken for granite. It is perfectly crystallized throughout, the felspar, the quartz, and the mica, having that aspect and relation to each other which cannot for a moment be mistaken by any one acquainted with the characters of rocks. The aspect of the mica alone is so remarkable, and so different from its appearance when found in quartz rock, that it is sufficient of itself to distinguish this modification from every other. Before quitting this subject it is necessary to remark, that one of the transitions between the granite and quartz rock takes place at the junction of the latter with the great body of granite.
I have been thus particular in describing this singular rock, because it has often been confounded with gneiss, from which it is strongly distinguished in its mineral character, and because it is important that a substance of so much consequence in the system of rocks as this is should have its relations accurately ascertained, and should not from laxity of definition be introduced into a series with which it has no geological connection. Many of the Western islands offer examples of it in its most genuine form.
From the point last described, following still the course of the stream, nothing is seen for the space of a mile and upwards, but a continued succession of the same varieties of schist. These rocks are also found occupying the hills on both sides of the glen, as far as they are within sight of the river.
If there are any other remarkable varieties, or any peculiar circumstances connected with the schist which forms the bed of the river throughout this space, they were concealed from me. The channel becomes so deep and the sides so clothed with wood, that it is scarcely possible to examine the banks; and the bed of the river itself may safely be called inaccessible during nearly the whole of its remaining course to Blair. Within about two miles of Blair, however, at a point which is marked by a cascade and a small bridge, a large mass of yellow marble is seen crossing the river. It is, like that before described, of an uniform ochre colour, and like it, applicable to architectural uses. Lower down, near the junction of the Fender, where there is again access for a short space, the great body of blue limestone is found. This may be traced across the grounds of Blair, and in the bed of the Banavie, where it exhibits those minute reticulations of siliceous matter which I have already described. From hence it is again to be traced to the opposite side of the Garry, where it forms a great part of the green hill of Tulloch. On this hill, at Invervach, many of the beds are white, and resemble those already described at Gow's bridge, being interlaminated in a similar manner with steatitical clay, and accompanied by mica. But I have passed my proposed boundary.
Having anticipated no more of the description of the two ridges which bound Glen Tilt, than, was necessary to understand the nature of the rocks which are seen in the bed of the river, I must now return to them, taking up the right hand ridge, or north side of the glen, at the place where I have now stopped.
That portion of the ridge which lies from Blair to Gow's bridge is so encumbered with wood in some places, and offers so little interest in others, that I am content to pass it by in a superficial manner, particularly as its more minute examination would throw no light on those interesting appearances in the bed of the Tilt from which it is so remote, and which are the principal objects of this paper. It is sufficient to say in general, that it consists of the several varieties of schist already noticed, which will doubtless be found associated both with quartz rock and limestone, and reposing somewhere on granite, as the other parts which I have really examined are found to do.
On the declivity of the hill above Gow's Bridge the schist and quartz rock may be seen irregularly alternating with each other and with the limestone. On the hill Stron na 'Chro the green marble beds are found at a considerable elevation, associated, as they are below, with these rocks, being in fact the continuation of that series of beds, the dip of which however various in the quantity of the angle, is always in a southern direction. A little higher in the hill the granite occurs: it is of a red colour, and precisely similar in character to that which is observed throughout the bed of the Tilt, and which I have already described.
The burn Alderiny having worn a deep channel in the rocks, it is easy to see the granite for a considerable extent in situ; and its connections which are visible at the lower part of the channel of the Criny having been described before, I need not here repeat them.
The granite continues along the ridge to Grianan, and is accompanied by quartz rock, of which the connection with the granite is unintelligible in the hill, however it may hereafter be explained by comparison with other observations. This rock itself is of a very compact and somewhat transparent quartz, containing irregular grains of felspar: it breaks in a flaky manner, and appears to consist of beds extended in a north-easterly direction, and dipping to the westward of north. It is impossible, however, either here or in any other instance which I have met with, to give a correct description of the dips of the schistose rocks which occur in the highland mountains, without running into endless details, since they are perpetually varying. It is perhaps fully as useless as it would be laborious to the observer and tiresome to the reader. These beds, at least for a short space, lie on the northern bank of the granite mass, and stretch away in a south-east line to Cairn na 'Chlavhan and Connalach more, which elevations consist entirely of quartz rock. The granite continues still visible to the eastward in the direction of the hills, but ceases to be red. It is now grey, and shews a slight tendency to a foliated structure.
This peculiarity of structure, imperfectly as it is marked, is an object of curiosity. I have already described a quartz rock as passing by a regular transition to granite, and here the granite in the vicinity of the quartz rock is found to have a foliated tendency, a tendency which not improbably unites it at some point with the quartz rock, although I did not discover that point of perfect transition. On the same summits I also picked up a specimen of genuine gneiss, but did not discover whence it had come.
The hill of Connalach beg shews a summit of granite, but it is a small space surrounded by quartz rock. The granite here passes into a variety which, with the same general aspect, contains crystals of hornblende. The two varieties are perfectly continuous, as continuous as the red and the grey, which I have just described.
Here then we have, as in numerous other instances, a confirmation of the geological identity of common granite, and of that which is considered as a separate substance by the name of Syenite. However distinct these two rocks may be considered as cabinet specimens, there is no distinction in their habits and connections, nor have we any reason to believe that the æras of their formation are different. It, is also no less inconvenient than unnecessary and improper, that the designation of a granite vein, when on entering a schistose rock it acquires hornblende, should be changed from granite to syenite. This mineral ought to be regarded as accidental, and whether present or absent it makes no alteration in the rock considered as a mass, nor can it alter or affect the validity of any geological reasonings which may be deduced from the relations of the granite in which it is found, to other rocks.
It has with equal want of proof been said that granite of different aspects belonged to different epochas or periods of formation, and we thus read of newer and older granites, as if mineralogists had established criteria by which these several varieties could be referred to a prior or posterior æra. If the connection of trap, sometimes with primary rocks, sometimes with those which contain animal remains, gives a support to these speculations where the rocks of this family are concerned, there are no such connections between granite and the surrounding rocks known as to justify them in this case.
In this place as in many others different varieties of granite are found together, not forming veins nor distinct masses, but graduating into each other by an indistinct transition. The several colours succeed each other at the same time with as little order as do the aspects and proportions of the constituent parts.
The quartz rock appears again to the eastward of Connalach beg, similar in its aspect to that one last named, and after continuing for a few hundred yards, it is succeeded by another variety of granite of a small grain and flesh colour, which forms a large mass of the mountain.
A superficial view of this perpetual repetition of granite and quartz rock, would lead to the false conclusion that these two rocks alternated, and that quartz rock was, as it has too often been considered, a modification of granite. This supposition I have elsewhere shewn to be unfounded, and that this rock is a member of those schistose formations which are found reposing on granite. I shall prove hereafter, that in this instance also the apparent alternation of the quartz rock is a deception, and that it is, like the limestone and the schist with which it really does alternate, superimposed on the granite.
I have in former papers presented to the Society, shown that the polarity of granite, or its power of affecting the magnet, is by no means so rare an occurrence as it was supposed. I may add to these observations, that almost all the granite found here possesses the same properties. But as this is a question involving some important considerations which would interrupt the chain of the present investigation, I shall consider it in a separate paragraph at the end of this paper.
As I reserved the mineralogical description of the granite which is found in the bed of the river in contact with the schistose rocks, till I had described the larger masses with which it is connected, I shall now describe it, as well as such other varieties as appear worthy of particular notice. I may here also say, that if a term be required for the purpose of distinguishing it from that granite which does not contain hornblende, it may be called syenitic granite; in treating however of its geological relations, I do not think it necessary to adopt so long a term, which it will be sufficient to use whenever it is requisite to distinguish its peculiar character as a compound rock. One advantage to be gained by the use of such a term will be, that the term granite will thus become a sort of generic term, including species, or varieties, which differ from the compound of mica, quartz, and felspar, and of which there are instances to which no distinct denomination bas been applied, as this term of syenite has been to those which contain hornblende. And since the granite of mica, felspar, and quartz, is sometimes found to contain here and there a solitary crystal of hornblende, with which in a series of gradation those varieties in which hornblende at length prevails to the exclusion of the mica are connected, it is impossible to define the point at which granite is to end and syenite is to begin: there is no difficulty in modifying the application of the adjective term. But another, and the principal advantage arises from the limitation of the term syenite to a rock of which the mineral character is much more constant, at the same time that it is in general very strongly distinguished from granite in its appearance. In its geological connections also it differs so widely that it is absolutely necessary to remove all chance of their being confounded together, since this rock is connected with the trap family, and is found in a position in which neither granite nor syenitic granite has ever yet been seen, namely, incumbent on the rocks of secondary formation.
To proceed to the description of the rock at the junctions formerly
described. It consists chiefly of high red felspar and white
quartz, of which the latter is generally in the smallest proportion,
the compound mass containing obscure crystals of dark green hornblende.
These are seldom in large quantity, more commonly
they are thinly dispersed through the rock, and not seldom even in
the largest masses of rock they are altogether absent. I have not
observed mica in any of the granite at the junctions described,
but in several parts of the hills the same rock, or one not to be
distinguished from it except by this circumstance, is found to contain
mica together with hornblende, which latter not infrequently
becomes excluded altogether and the simplest form of granite remains.
The various granites described as occupying the principal parts of the ridge, are also mixtures of quartz, mica, and felspar, differing however as much in colour and texture as they do in structure; to describe such varieties is not possible, were it useful.
The chief varieties of the syenitic granites are found on the slopes which descend to the Tilt, and they are almost invariably of a grey and black colour, from the predominance of the hornblende: their texture is as various as their shades of colour, the crystals of hornblende being sometimes very large and decided, and imbedded as it were in a paste of quartz and felspar, while in other extreme cases the minuteness and mixture of the several ingredients is such, that the constituents can scarcely be discerned. In these cases the specimens have sometimes the fallacious appearance of the greenstones of the trap family. They may, however, be distinguished from these by attending to the white ingredient, which frequently consists of quartz with little or no felspar, while in all cases it contains quartz, a substance which is but rarely found in greenstone. They may generally also be recognized for what they really are, by tracing them for some space, when their characters will be found to become more conspicuous, either by the enlargement of the constituent parts, or by the admixture of mica, a substance which, except incidentally, is not found as an ingredient of greenstone properly so called. These rocks, like common granite and gneiss, frequently contain epidote as a constituent incidental part, and in this also they are distinguished as well from that syenite to which I propose to limit the term, as from the greenstone of the trap family, since epidote is but very rarely found among these latter rocks. They are equally distinguished from them by the frequent presence of another mineral which, as well as the former, is peculiar to the elder rocks, Sphene. This is found in considerable abundance, minute, but well crystallized and dispersed throughout the stone. It is by no means uncommon in similar rocks in Scotland, and occurs, among other places, in Criffel as well as in Braemar; and I may here add, that I have found it in specimens of Egyptian granite which contained no hornblende. I think it unnecessary to enter into any further details of this nature.
It is not far from the granite which I have now described, that the quartz rock with the transition into granite which I have already described, occurs, and it is not necessary to make any further observations on it in this place. At Craig Crocha the granite again makes its appearance; and here, as in other places, it passes by a gentle gradation into syenitic granite, that is, it contains crystals of hornblende.
It is quite superfluous to continue the details of the structure of this ridge with the same minute accuracy, as they will be found little else but a repetition of those which have preceded. It is not however the less important to give a general view of the whole, and I shall therefore state it to as wide a range as is necessary for understanding the structure of Glen Tilt, and the true nature of the interesting appearances there visible.
Ben Dearg may properly be considered as the most conspicuous portion of the granite which forms the basis of this range of hills. It is connected with the right ridge of Glen Tilt which I have now been describing, by a succession of granite and quartz rock, exactly similar to that already noticed. Pursuing the direction of the country across the Tarff to Cairnmuir and the head of Dee, the same succession gradually unites the granite of this tract to that which forms Cairngorm and Ben Avon, the great central granite of the Grampian hills, and the most extensive tract of this rock in Scotland.
There is no reason then to doubt that the granite which forms the right ridge of Glen Tilt is part of this great mass, and it in fact constitutes its termination in a southern direction. To ascertain its connection with the quartz rock, it is necessary to compare numerous observations made on the slopes of the hills, and in the channels of the torrents where such connections are visible. I have already mentioned the very conspicuous junction visible at Glen Criny, and it would be tedious to repeat the rest of the observations from which the general conclusion is drawn. That general conclusion is, that the quartz rock is superimposed on the granite, and that every instance of apparent alternation in these rocks may be resolved into this. The confusion which arises in the disposition of these rocks depends on two causes, the discontinuous arrangement of the quartz rock, and the irregular protuberances, into which the surface of the granite is formed. These latter are the cause of the semblance of alternation before noticed, both on the sides of the right ridge, and in the bed of the Tilt. In no instance have I discovered them proceeding to such a distance from the body of the granite as to deserve the name of veins, and assuredly they are not veins traversing the main body of granite, but on the contrary portions of its solid mass. The junctions therefore of the limestone and schist with the granite, which are visible in the bed of the Tilt, are not to be viewed as consisting in the passage of granite veins through those rocks, a phenomenon sufficiently common in other places, but as the points of junction, as I shall soon proceed to show, between a great bed of stratified rocks and a central mass of granite.
It is necessary for that purpose to enquire into the structure of the left ridge or southern side of Glen Tilt.
Beginning from Gow's bridge and ascending the hill at right angles to the stream, a succession of limestone continuous with that at Gow's bridge before described is found to a considerable height in the hill. It will be useful to point out the general bearing of this great range of limestone. It crosses the hills at Lude, tending towards the south, whence it passes through the Glen of Fincastle, and across the valley of the Tumel. Limestone is again seen at Mount Alexander, and at the base of Schihallien, from whence it appears to proceed through Glen Lyon to the side of Loch Tay; but I have not been able to trace any connection between these detached places. Eastward it extends by the side of the Scarsough to Mar, beyond which point I am unacquainted with the country.
To examine it more particularly. From Gow's bridge up the course of the Tilt to Forest Lodge it is seen skirting the banks of the river, and alternating with schist and quartz rock, while in the places formerly described it joins the granite, and undergoes those disturbances which I need not repeat. From the river it extends upwards to a height of many hundred yards in the hill, although not so exposed that we can truly ascertain what alternations it may undergo through this space. Every where its course is marked by the most lively verdure, forming a strong contrast with the brown and barren aspect of those hills where the subsoil is granite or quartz rock. Authyllis vulneraria, Cistus helianthemum, Satyrium hircinum, and other plants which affect calcareous soils, are found on the faces of these green hills.
This limestone is of a blueish colour, and almost always of a large grained fracture. Where it lies in contact with quartz rock, it is commonly of a harder texture than in more distant portions, and where it is about to alternate with schist it is often interfoliated with thin lamina of micaceous or argillaceous slate. Although dark blue or grey is the predominant colour, it occasionally varies through different shades to pale grey and greenish grey, and its texture is equally liable to variations. It is a pure carbonat of lime with the exception of the colouring ingredient iron, containing no notable proportion of other earths, except where it is intermixed with the siliceous or argillaceous laminæ. The same is true of the white varieties, except that where they are much mixed with steatite and serpentine they yield magnesia upon being analyzed.
The dip of the beds is invariably to the south, but the quantity of that angle is not sufficiently constant to render its measurement an object of interest. It seems to vary from five to fifty and even to sixty degrees. In one place and one only I observed a considerable contortion of the beds, and in many others there are fractures and dislocations to be seen. Yet with such partial irregularities we may still safely consider the general parallelism and stratification as regular, and the dip as a medium constant quantity of perhaps twenty degrees. A few porphyry veins are found to traverse these beds, an appearance too common to call for any particular notice. Nor is it possible for want of marks to refer to the places where they occur. Contortions similar to those here described are not infrequent in Scotland, and they have been often supposed to depend on the vicinity of trap or granite. They are however to be seen in many places where neither of these rocks can be found, and I have observed in the island of Sky a series of stratified rocks of which the evenness and parallel horizontality is perfect, although they are traversed by trap veins of enormous magnitude and great frequency. No general conclusions therefore respecting a cause can be deduced from these appearances. The beds of limestone which I have thus in a general manner described on account of their perfect resemblance through their whole course, continue with scarcely any alteration as far as the burn of Glenmore, extending upwards to a height in the hill similar to that which they have at the western end of the ridge. The alternation of the beds with schist and quartz rock is however more visible on the sides of the Tilt in this part of its course; and at the river Aldianachie in particular, there is so be seen an alternation in this order; limestone, quartz rock, limestone, schist, limestone. The beds of limestone do not cease at the burn of Glenmore, but after crossing it they are less continuously visible, while at the same time they lose the almost rectilinear course which their elevated edges have hitherto preserved. A bed of white marble is found among them at Fealair, and some rolled stones of pink marble in the channel of the river point out also the probable existence of a bed of this colour. Alternations of limestone with quartz rock and schist continue to be seen in this southern ridge of Glen Tilt towards Scarsough, but the pursuit being unnecessary for the purposes of this paper I made no accurate record of it. It is to be regretted on this account as well as many others, that no geographical Survey of Scotland exists on which reliance can be placed. The errors are in fact beyond enumeration. It would otherwise be a desirable object to trace the real line of direction taken up by this elevated edge of limestone through the long course I have pointed out: but it must be reserved for future examination.
Resuming now the left hand ridge of Glen Tilt at the point where the limestone terminates, the remainder of the task of description is easy. The whole mountain of Ben Gloe is one mass of quartz rock, with scarcely any notable quantity of schist interstratified with it. It is the immediate boundary therefore of the limestone bed, and from the relative position of both, it must consequently be considered as lying above the limestone. The extent of this rock towards the cast is considerable, since it stretches into Mar, and occupies a great part of Glen Shee. To the west it soon disappears, or at least its continuity is broken, since on the west side of the Garry it is only found in alternation with schist. To the south of Ben Gloe this rock is once more seen alternating with limestone, which is again succeeded by the mica slate, and this without further changes, except a gradation into clay slate, terminates at the well known boundary of the secondary strata near Dunkeld and Blairgowrie.
The contortions and the conglomerate aspect of some of the beds are the most remarkable circumstances respecting the quartz rock of Ben Gloe; the former proving that quartz rock, like the schists with which it is associated, has been in a flexible state, and the latter proving beyond doubt the partially mechanical nature of this deposit. Besides these it produces three coloured varieties of quartz, pink, amber brown, and blueish grey; the former equalling in colour the most beautiful varieties of that well known substance.
I have reserved to this place what remains to be said concerning the rock which I have hitherto spoken of by the general name of schist. To have described the several varieties on all occasions where the geological history only was required, would have created much confusion, as they rarely maintain the same character for any considerable space. The most abundant is a clay slate, rarely fissile, and generally of a very compact texture and dark blackish blue colour. It sometimes possesses a glossy and unctuous surface, and passes into a sort of talcaceous schist. It is often also much penetrated with quartz, and on some occasions finely interlaminated with the same substance, very beautifully banded specimens of this variety occur near Gow's bridge. On some occasions it becomes intermixed with hornblende, which by degrees predominating, the compound passes into a hornblende schist similar to that which is so common in gneiss. This occurrence is frequent at the junctions, and here also the schist, even where it is merely argillaceous, generally displays unusual hardness. Hornblende schist is also found in beds unconnected with the clay slate.
Micaceous schist of aspects infinitely varied is also found in this ridge of hills. In general however it is characterized by the predominance of the quartzy ingredient, and in this way it passes into quartz rock by a series of gradations which cannot be defined. I have already had occasion to speak of those places where it assumes the character of gneiss, and need not therefore repeat the description here. I have only to add that green fibrous actinolite is occasionally to be found in this micaceous schist, a substance which, however radically identical with hornblende, is sufficiently distinguished in its appearance by characters which need not be detailed here. In some of the rolled porphyries which are found in the hills which bound this Glen I have observed pinite. This mineral has not hitherto been found in Scotland, and as indeed it seems to have been as yet seen in only one or two situations besides the place whence its name is derived, it will not be superfluous to describe the specimens and the substance in which they are imbedded. The base of the porphyry is a brownish hornstone or compact felspar, containing numerous crystals of reddish-yellow felspar and a few grains of quartz, some of which appear to have the primitive form of this substance. Together with these, numerous crystals of pinite are imbedded in the stone, varying in diameter from one tenth to one sixth of an inch, sometimes of similar length, and at others mere scales. Brongniart describes it as having been found in porphyries, and I may add that the specimens which I procured differ in no respect from the foreign specimens with which I have compared them.
A similar porphyry, I may here say, is to be observed in Glen Shee. Oxide of titanium in a pulverulent or scaly and investing form, is also to be seen in the rifts of the quartz rock in Ben Gloe. The same disposition of this mineral occurs in Ben-na-caillich, near Killin, and it must not be confounded with oxide of iron, the more common metallic substance in rifted quartz. Among the loose stones on the skirts of Ben Gloe I also found a singular substance. It is a laminated schist of which the basis is clay slate, occasionally mixed with hornblende and with quartz, and containing small lamellar and conchoidal scales of a crystalline white carbonat of lime. I know not how to describe this rock so well as by saying that the calcareous bodies are precisely like fragments of shells, and that the whole on a first view would pass for a shale containing shells; if found in a secondary country it would be thrown aside as a bad specimen of such a rock. I would not be understood to mean that the substances in question are really the fragments of shells, whatever general resemblance they may possess; but if considered merely as crystallizations they are sufficiently singular to be worthy of notice, since nothing analogous to them occurs in the different schistose rocks which I have examined in Scotland.
One other rock is yet deserving of notice which, although not precisely situated in Ben Gloe itself, is found among the beds which appertain to this great mass of quartz rock. It is visible in Glen Fernat.
This is a mass of porphyry intermixed with a mass of quartz in such a manner that it is impossible to ascertain precisely the relation which either of them bears to the surrounding rocks. There is however little reason to doubt that the quartz forms a vein, but I suspect that the porphyry also is disposed in a similar manner, and that the appearance in question is the result of the interference of two veins. In either case the porphyry as a mineral specimen offers an aspect I believe as new as it is difficult to explain. Its basis is the usual indurated claystone, or compact felspar, or hornstone if that term be preferred, which is the most common basis of the porphyries that occur among these strata. It contains distinct crystals of felspar, but together with these, fragments of quartz are also found in it. These are most obviously fragments not crystals. They are irregular, of different sizes from that of a pea to that of an egg, and their angles are sharp. In addition to that, where the porphyry and quartz masses are in contact larger fragments are to be observed mutually connected both with the porphyry and with the quartz. Porphyry has generally been considered as a crystallized rock, yet here it offers the mixed structure of a crystallized and conglomerated one. The island of Arran indeed produces some porphyry which appears to possess a structure analogous to this, inasmuch as grains of quartz apparently rounded, not crystallized, are found imbedded in a common base with glassy and earthy felspar. But that structure might admit of dispute.
Such are the most remarkable of the independent facts which I have noticed in this tract of country. Let us now, rejecting all the minuter features which accompany this succession of rocks, extend our views over a wider range, and attempt to investigate their general bearing.
The most continuous bed which has occurred in this view is the limestone. We have seen that it is regularly bedded, without any material disturbance, with an elevation somewhat varying and consequently in an undulating plane, and that it is continuous over a very large tract of country. We have also seen that its elevated edge is prolonged in a line which is straight for a great distance, and which perhaps may prove to be so even through a greater space, when an accurate survey of Scotland shall have been produced. It is therefore the most regular rock of the series now under examination, and may consequently be taken as the point of comparison for the others and as that from which the true relations of the rocks on each side of it must be investigated.
It is indifferent to the object which I have at this moment in view, whether the beds of limestone are considered as having been originally deposited in their present position, or whether by subsequent changes they have been diverted from one more horizontal. It will not be denied that they are deposited rocks.
If we now trace upwards from this continuous bed of limestone awe shall find that it is followed by a large and continuous bed of quartz rock, and we have also seen that the calcareous beds themselves alternate with beds equally continuous of quartz rock and of schist. We have seen that the great mass of quartz rock is followed by a small bed of limestone, and that this again is succeeded by micaceous and clay slate, terminating the series of the bedded rocks in this direction. Here then is an order of rocks different from that which we are taught to believe universal, since the micaceous schist is far removed from the granite, and only follows a succession of limestone and quartz rock. It is the limestone which immediately follows the granite, and that granite is probably a portion of the great mass which forms the central granite of Scotland.
This contact of the limestone and granite is too well marked to admit of dispute, however, like many of the other remarkable circumstances attending on Glen Tilt it has been overlooked.
But there are other important phenomena which accompany the junction of the limestone with the granite. Recurring to the alternations between the limestone and the beds of schist and quartz rock with which it is continuous, we find that these alternations are regular, even, and defined. But if we now trace downwards to the granite, we do not find any one of these beds continuously in contact with the granite; on the contrary it is sometimes the schist, sometimes the quartz rock, sometimes the limestone, or there is a want of conformity between the granite and the rocks which lie above it. This is not the appearance which we ought to expect had the superincumbent strata been deposited on the previous basis of granite. If the granite had been the lowest of a series of deposited rocks and the basis on which the incumbent ones were precipitated or crystallized, all the strata which lie on it should have followed upwards from it in a regular order of succession. That rock which was contiguous to the granite in one place should have been contiguous to it every where, since this law is invariably found to hold good among rocks of which the stratification is demonstrated. This observation is of importance, and I have illustrated it by two sections, the one showing the fact as it actually exists, and the other the order which should take place had the strata been regularly deposited on the granite. In addition to the irregularity of this line of contact, symptoms of confusion and disturbance are apparent, in every instance where the granite is actually visible in contact with the neighbouring rocks. These consist in a general mixture of all the stratified rocks with the granite, and a total discomposure of their regularity, being at the same time accompanied by the passage of minute veins from the mass of granite into the stratified rocks. Where on the contrary the beds lie out of the immediate vicinity of the granite, they retain their parallelism and regularity, its influence appearing to extend to a very short distance beyond the point of actual contact.
The real structure of Glen Tilt, and the true cause of the appearances which have excited so much notice, must now be seen so clearly to follow from the history of the rocks which bound it, that it is almost superfluous to give a summary view of the whole. I will however conclude this account with such a sketch, illustrating it by a map and such sections as may render the whole more obvious to those who may be inclined to follow me over the same tract. The map does not pretend to minute accuracy with respect to every rock over this large space. It would have been in fact impossible to have laid down such details on a survey of this mature, the only one which has yet been executed of this district. But however defective and however misplaced in point of distance many of the most remarkable boundaries can scarcely fail of being, it is sufficiently accurate in the general features for all geological purposes. The sections are necessarily general, and are given not as actual sections of any particular points, but as illustrative of the relative superpositions of the rocks.
The course of the Tilt may therefore be considered as bounded on one side by the outer edge of the granite mass of the Grampians, and on the other by the primary rocks which follow and are superimposed on it. These rocks consist of an alternation of limestone, schist, and quartz rock.
The bed of the river is cut upon the line of contact of these two separate classes of rock, lying upon the surface of the granite, and against the elevated edges of the stratified rocks.
Its action has in various instances exposed the junction of the granite with the stratified rocks, and these exposed parts are the confused mixtures in the bed of the river which have already been described
Although the river follows this line of junction in a general view, it does not follow it so accurately as always to keep the granite on its right bank, and the stratified rocks on its left. For this reason the stratified rocks are sometimes seen crossing to the right bank, and even ascending high up the right side of the hill. The granite also crosses to the left in a few cases, but as it dips under the stratified rocks it is not found in the hill.
The Sections which are given illustrate this variation and explain its cause.
The apparent alternation of the granite with the stratified rocks is also explained by attending to this arrangement.
The granite masses which extend beyond the general surface of the granite, and are found in the bed of the river, do not run to any extent through the mass of strata. It is therefore probable that they are not veins, but simply irregularities, of the granite.
The junctions of granite in the Tilt are not therefore the transit of independent granite veins like those of Portsoy or Rona, but the interrupted portions of a continued line of junction between a great surface of stratified rocks, and an equally extensive but irregular surface of granite.
Wherever this junction is found, a complicated disturbance of the whole rocks at the point of junction is seen, and small veins of granite are observed penetrating the stratified rocks.
In these places of junction the granite becomes so intimately mixed with the limestone as to alter its character, and wherever the granite is in contact both with schist and limestone, a similar mixture and transition between those two substances takes place.
In all other cases the schistose and calcareous rocks preserve both
their regular disposition and their ordinary chemical characters.
I have reserved for a separate description some interesting circumstances relating to Glen Tilt, because they are extraneous to the general appearances, which required to be considered in one collective point of view, and because they would have interrupted the continuity of the narrative. The first of these is a deposition of calcareous matter, which was pointed out to me by the Duke of Atholl, to whose unwearied kindness and friendship I am no less indebted for the facility which I experienced in examining this district, than the science of geology is for this very interesting fact. On the brow of the hill above Gow's bridge a large bed of shell marle is to be seen. It is found on the left side of the river commencing under a face of limestone and extending for about 250 feet or more down the hill. It is between 80 and 90 feet wide, and its depth seems to vary from one foot to six or more. This appearance is unexpected but it is not solitary, as another similar instance occurs at the foot of the limestone ridge at the south of Ben Gloe, as well as on the hill of Tulloch formerly mentioned, but in neither of these cases so well marked or so extensive as the present.
The formation of shell marle in the peat mosses of Scotland is I believe well known to the Society, as it has long been to the agriculturists of Scotland. These deposits are by no means uncommon, whether in the larger tracts of peat, or in the smaller ones which are found occupying insulated patches on those hollows in the mountains which are favourable to the growth of this substance. They are found forming beds, generally of no great thickness, but varying from an inch or two to as many feet, and lying under the covering of peat. The calcareous matter is commonly small in proportion to the clay, sand, and other foreign substances which they contain, in so much that they are seldom of any value to the agriculturist. A few which I examined were found to contain from ten to fifty per cent. of carbonat of lime; and independently of the clay and sand which enters into their composition, they are usually blackened by a mixture of half decomposed and carbonized vegetable matter. It is easy to perceive from the flatness of their surfaces and their tolerably uniform thickness, that they have been formed at the bottom of water in lakes of different dimensions, which have been gradually obliterated, partly by the influx of earth, and partly by the growth of those well known vegetables which have covered them with their present stratum of peat. In the instances which I have had an opportunity of examining, the shells from which this calcareous matter has originated have been either so thoroughly decomposed, or from their tender structure so mutilated and broken, that I have never been able to collect a specimen capable of being ascertained. Other mineralogists however have examined the shells found in these beds, which were long since known to the late Dr. Walker, and of which an account has been given in some of his works.
The formation in question is of a nature entirely different, and has never yet been noticed by mineralogists: its novelty at least renders it a matter of some interest.
Where the great limestone bed, which I have described as occupying the southern side of Glen Tilt, is about to meet the quartz rock, it forms a range of small abrupt faces or scarps extending in an interrupted manner for perhaps a mile. These may vary from ten to thirty feet in height. In wet weather small streams fall in cascades over them in two or three distinct places; and generally the surface-water from above trickles down their faces. In these parts, thus subjected to the action of water, the limestone is dissolved and deposits here and there a white crust of carbonat of lime resembling the usual deposits of calcareous waters, so as to whiten in many places the surface of the rock. One cascade of considerable height has thus left the marks of its intricate course in the dark and broken rocks so perfectly, that even in dry weather it appears to be always flowing, and only a narrow inspection discovers it to be an imitation; a painted cascade formed by the hand of Nature. I have been the more particular in describing this fact and its extent, for the purpose of showing that the marle bed is not produced by a deposition of calcareous earth from the water thus impregnated. Were this the case it should be found under the whole extent of this range of rocks, whereas it is limited to one spot and that one a point where the water flowing from above is diverted from it on each hand by sinuosities in the scarped face. I do not assert that there is absolutely no other deposition of a similar nature in any other place throughout the great extent of the limestone range of Glen Tilt, but after a considerable search I did not find any in those neighbouring parts where the surfaces of the rock were whitened, and the greater flow of water would have led me to expect it, had it been a mere deposit from water. There is little doubt that a portion of the lime contained in the bed has been produced in this way, but that portion must necessarily be small, or the same earth would be easily discovered in the soil of the immediate vicinity which lies in similar situations, which has nevertheless only the ordinary aspect of the whole mass of soil incumbent on the limestone. On examining the nature of this deposit it is found to consist of a substance much more spungy and loose than chalk, although on a first glance strongly resembling its more friable varieties. It is soluble without residuum in muriatic acid. On a narrow inspection it is found to contain minute fragments of shells, particularly towards the surface, while deeper in the bed all marks of organization disappear. It is also every where penetrated by the fibrous roots of grasses and other plants, or by the minute channels which their decay has left. It does not acquire any induration on drying, as the specimens before the Society will show. On the immediate surface, or at small depths where the decomposing causes have had less time to act, entire shells are found, and numbers of these may be collected in a very perfect state. Among those which I did collect I only found the following in a state sufficiently entire for examination.
|No. 1.||Helix arbustorum.|
|2.||Appears to be Helix rufescens, or bispida.|
|3.||Two broken specimens, of which I cannot speak with the least decision.|
|4.||I conjecture it to be Helix fusca of Montagu; and perhaps it agrees with Draparnaud's genus Vitrine.|
|5.||A small Planorbis; but of what species I am unable to determine.|
|6.||Appears to be a minute Lymneus; perhaps Lymneus minutus of Braard.|
All appear to be terrestrial shells.
This then is to be considered as a land formation of calcareous matter, if we may use such a term, and the result of the successive renewal and death of these minute animals for a length of time unknown. Whether this process is still going on I was unable to discover. After much search I did not find any living animal, but among the helices were some with their colours so perfect, that their death could not in all probability have long taken place. As the animals themselves are known to be existing species, it is indeed probable that this bed is still in a state of increase, however tedious and imperceptible the process may be.
Here then we have a fact which presents us with another modification of those strata which owe their origin to the action of living animals, or to the accumulation of their remains. It has always been known that marine animals acted an essential part in the production of the secondary, and even the latest of the primary, (or transition,) strata. It has lately been shown that considerable deposits and large beds of rock have also in former times been produced by testacea inhabiting fresh waters. But it has never been suspected that similar deposits could be formed on dry land. That such is the case here, is sufficiently evident from the angle of acclivity on which this bed is formed, on a surface which has assuredly undergone no alteration of its position since its formation, or at least since the formation of the granite. The fact itself is much too insulated and too narrow to admit of any generalization, or to justify us in supposing that similar formations might have taken place in a more ancient state of the globe. Else speculative minds might conceive that of the numerous elevated and apparently displaced strata which are now found containing organic remains, of which, not only the species, but the very genera as well as habits are unknown, some at least might have been derived from land animals, whose remains were converted into rocks in the very places where they now exist. Should causes with which we seem but very imperfectly acquainted at present, hereafter convert the marle bed now described into a rock, and all traces of its recent formation disappear, a circumstance at least within the limit of possibility, assuredly future geologists would not be a little puzzled to reconcile its appearance with that of the surrounding rocks, of which the intricacies already described in this paper, are sufficient in themselves without the further addition of such a deception as this.
It were to be wished that this specimen could be preserved for the future examination of others, but the wants of agriculture must be satisfied, and the pick-axe and spade have already commenced their depredations.
The next appearances in Glen Tilt which I consider worthy of notice, and which are extraneous to the general structure already described, are the marks which it bears of the action of water. If we consider the general characters of the rocks which form the opposite sides of the valley, and attend to the section which accompanies the description, it will be seen that the channel of the river during the greater part of its course, is cut upon the junction of the stratified and the unstratified rocks; in consequence of which those junctions which form so material a part of the interest of this valley have been exposed. It is easy to conceive that a longer continuance of the same action may expose a greater portion of this line, and ultimately lay bare the granite which doubtless lies below the stratified rocks beyond Glen Criny or Gow's bridge down the course of the Tilt. If from future probabilities we ascend to past ones, we may imagine the river once flowing at a higher elevation, and gradually making its way on the surface of the granite and against the edges of the soft strata; the former offering a constant resistance, while the latter, giving way to its action, have formed those precipitous faces the displaced fragments of which have during the progress of time been rolled along the valley to the course of the Garry; being destined in their ultimate progress to assist in forming the immense beds of alluvium which mark the course of the Tumel and the Tay, and to exclude the sea which once flowed high up into the present estuary of the latter river.
The actual corrosion produced by the mountain torrents is most strongly marked in those which fall from the skirts of Ben Gloe to join the Tilt. It is easy to perceive in these, that the fortuitous inclination of the strata has not produced the intervals now occupied by the streams, but that beds once continuous have been cut through often at right angles to their direction, leaving their sections fairly exposed in the deep beds of these powerful agents. The depth of these sections is always striking and frequently enormous, exceeding an hundred feet; with sides almost perpendicular, while the dark and turbulent water roars, seldom visible, and scarcely audible, beneath. In the progress of waste and ruin the falling of the upper parts gradually produces a more open chasm, destined perhaps in the progress of time to form a glen such as those narrow and prolonged ones which constitute a frequent feature throughout this district.
To what extent these agents actually operate in changing the present surface of the globe, is an enquiry beyond the objects of this brief notice, but we have, in this valley as in numerous other parts of this country, abundant proof, that many of the most conspicuous and extensive alluvial deposits have had their rise in causes of a much more general and extensive nature.
Of this a very remarkable example is to be seen near the farm of Auchgowall in the lower part of the valley, extending to Gilbert's bridge. It is most conspicuous on the left bank, where it is seen covering all the hills to a considerable height and a great depth, its thickness being distinctly shown by the sections of the streams which descend to join the Tilt, and of which the courses are marked by extensive ruin: the bare schistose foundation has also been in many places exposed. The alluvium in question consists of the materials of the surrounding country, but all of them water worn and mixed with sand and gravel as from considerable attrition. In this, as in numerous other examples, there is no assignable mode in which the materials could have been either formed, or deposited, by the slow action of the existing streams, while the total absence of a similar deposit in other parts of the valley equally subjected to similar causes, bespeak a different origin. The cause appears to have been of a diluvian nature, and the determination of the deposit to this point, may perhaps be found in the form of the valley at this place and the obstruction which it has offered to a mass of matter impelled through the upper parts where no such obstruction to its deposition existed. Such an obstruction as that which I have suggested might formerly perhaps have been rendered more complete by the continuity of the strata beyond this point, which has since been gradually destroyed by the slow action of the Tilt: in consequence of this the deposit of alluvial matter has itself gradually diminished, sliding successively down the hill as it has been undermined, and together with the more gradual abrasions of the land in the upper parts of the valley, hurried by the daily course of the river to the Garry.
The probability of this supposition is strengthened by the occurrence of a similar circumstance at no great distance. This may be seen near the junction of the Garry and the Tumel in the hills above Fascally; and the deposit is here also of enormous thickness and of similar materials. Like that in Glen Tilt, it occurs where the valley appears to have favoured the accumulation of a diluvian deposit, and where the subsequent action of the river in its rapid and corroding progress downwards as it runs through the narrow openings at Killicrankie, has hitherto proved insufficient to undermine and remove it; in consequence as it would seem of the lateral direction of its course produced by the fall of those portions of alluvium which have formed the small holm on which Fascally is situated. Thus it is for the present protected from change, and perhaps destined to remain, for a longer period than that in Glen Tilt, a monument of those revolutions which point out an extensive but transitory action of water on the surface of our globe.
I have remarked in the preceding paper, that the granite which forms the right boundary of Glen Tilt has the property of affecting the magnetic needle. This influence is far from inconsiderable, and in many cases produces not only remarkable local variations, but a disturbance of that instrument so great as to render it useless for the purposes of ascertaining any meridian whatever. It is not limited to the syenitic varieties, where from the predominance of hornblende we might reasonably expect to find it more active, but is equally to be found existing in those granites which do not contain this ingredient. Neither does it depend apparently on the micaceous ingredient, since in many of the rocks which shew it strongly the mica is in very small proportion. It seems to be equally independent of a state of decomposition in the granite, since it is here, as elsewhere, to be found inherent in fresh specimens, although on the summit of Goatfell in Arran, as I have remarked on a former occasion, it appears most conspicuously in those specimens where the iron is becoming carbonated and the rock is tending to disintegration. In the paper to which I here allude, I noticed this fact as being nearly a solitary one at that time, but I have had occasion to observe it since on various occasions. In a paper on Cruachan, drawn up three years ago, the same circumstance is described, and I may here repeat that the granite in question is not decomposed, and contains a very large proportion of flesh coloured felspar, the quartz and mica being in small quantities. As granite is a rare rock in Scotland, I have not been able to extend my observations on this substance far, but I have also found the same magnetic power in some of the granites which form the Ross of Mull. The porphyries also occasionally possess the same power. I have observed it in different places, but most remarkably in those of Cruachan, and expect that it will be found most conspicuous in those, which from their black colour contain much hornblende and approach to trap in their composition: it is not always that circumstances allow of the repetition of these observations. I have not often discovered it among the primary schists, but I must add at the same time that my trials on those rocks have been but few. Yet it occurs among the schists in the hills which border Glen Tilt in more situations than one, although seldom in an energetic degree. It is not limited to the hornblende slate, but is found in those specimens of argillaceous schist into which that mineral does not enter. I have also observed that some of the beds of dark blue limestone exert a disturbing force on the needle, and imagine that I found it greatest in a stratum which crosses the upper part of Glen Fernat at the foot of Cairn Ree. I have however examined limestones of the same apparent nature in other parts of Scotland without having discovered a similar property in them, so that I conclude it will be found but rarely either among the rocks of this family, or the schistose ones.
It has so long been known and so often remarked as a quality common to the rocks of the trap family, that instances of it must have occurred to every geologist. In these indeed it is often so conspicuous as to have become an object of popular curiosity, notwithstanding which, it has rarely attracted the attention of geologists, unless, as in cases like those here alluded to, it has been exerted in an uncommon degree. I may therefore add that it is very generally diffused among the rocks of this family, although as we might reasonably expect, in very different degrees, those variations of energy depending at times on the peculiar composition of the rock, at times on its state of decomposition, and at others on circumstances which we have not the means of ascertaining, little acquainted as we are with the laws which regulate the movements, the accumulation, or the permanence, of this mysterious power. In a paper on Canna, which I presented to the Society some years ago, I pointed out the great action exerted by the trap in several parts of this island, as well as in that spot so well known to the inhabitants and to mariners by the name of Compass-hill, and took occasion at the same time to recommend to surveyors who are in the practice of using the magnetic needle in their art, the necessity of attending to these disturbances hitherto held of no moment, or rather, I might say, unknown and unattended to by them. I also pointed out with similar views the same effects occurring in several parts of the island of Sky, where they are frequent and powerful. Among the places in that island where the needle undergoes violent disturbances the hill of Glamich is the most remarkable, but as in that paper I have entered into the details which respect this spot, I need not repeat them here. I must however add that this hill consists partly of clinkstone, together with the porphyries which are usually associated with it, and partly of that syenite which appears to belong to the same family, being, like the clinkstone, superimposed on the secondary strata. Here the blue clinkstone possesses the magnetic virtue in the most eminent degree, but it is also attached to the syenite, as well as to the grey porphyries which are derived from it by the same imperceptible gradation as are the porphyritic varieties of clinkstone from the simple ones.
The magnetic property of a rock of serpentine was remarked by Humboldt some time ago, and I have also found it in this rock in the only situations where I have as yet had an opportunity of examining it, namely at Portsoy, and in the island of Scalpa. In both these places the magnetic power is considerable, and in some particular spots nearly as great as in the most striking examples among the trap rocks. In hand specimens I have also found that the serpentine of Anglesea, and that known by the name of Egyptian green, exert considerable influence on the needle, so that the property is in all probability generally dispersed among the rocks of this description, as it is among the traps. I may add finally, that I have observed it, but sparingly and feebly, among some of the dark pitchstones, and that in the augite rock which I have formerly described as forming so large a part of the island of Rum; it is as vigorous and common as among the traps with which that rock has so strong an affinity both in mineralogical and geological character.
Such then is the catalogue of the rocks among which I have found the property of disturbing the magnetic needle by the extent ion of their own magnetic powers, and it will be seen that they occupy a considerable share of those rocks which constitute the surface of the earth. Geologists, hereafter perhaps more attentive to this subject, may probably add to this catalogue, or at least augment the number of examples, and add many more to the few localities which I have had opportunities of examining.
To surveyors who are more deeply interested in the fact and its consequences, I shall also look for more extensive and more accurate observations, since the possession of more delicate instruments, and the devotion of time and attention to this particular object, are required to determine the quantity and extent of these influences, as well as their practical effect in producing permanent local variations in the magnetic meridian.
I need scarcely say that an ordinary ships compass is insufficient to detect these variations, unless where they are considerable, as the instrument, either from the rudeness of its workmanship, or the intentional insensibility given it by the maker to render it steady in steering small ships or in navigating through a cross sea, is rarely alive to minute quantities of the disturbing force.
I have attempted on various occasions to discover the positions of the poles of the natural magnets which thus disturb the needle brought within the sphere of their influence. From the observation of Humboldt we are led to imagine that he conceived the rock or mountain which he describes, to have possessed but two poles, and that its effects on the needle could in consequence be easily assigned. This is not impossible, but in all the cases where I have been able to make observations, I have found that the disturbances must have been produced by a number of independent magnets, each rock or fragment being possessed of a meridian of its own, and the disturbances of the needle being in consequence of that, extremely irregular and uncertain. I have already entered into a detail of this fact where I first observed it in Sky, in the paper to which I have already alluded, and shall not therefore repeat it here. On the extent of their influences I have also spoken in the same place, where I have shown that the needle was sensibly disturbed even at the distance of four feet from the ground. There can be no doubt that influences of this power and extent will produce local variations which if not absolutely permanent, will, by affecting the other better known variations of the needle, introduce errors into those observations which assume for granted a certain local and temporary variation on the evidence of observations made at a distance from the precise point where these additional disturbing forces are in action.
It is already well known to philosophers that the ordinary variations of the needle are not steady, either through small spaces of time, or over a considerable extent of surface, and it is well known that in different parts of Scotland the quantities of the variation are subject at any given time to considerable differences. To this cause are doubtless to be attributed errors in geodesic operations which have occurred even to experienced observers, where these differences were either neglected or perhaps not suspected. The errors in General Roy's Survey of Scotland, appear to have been derived at least in part from this cause. It would be interesting to know how far these local differences depend on the magnetic influences of masses of rock, or merely on the same mysterious cause which produces the ordinary variation, acting in an unequal manner. We must look to fixture investigations for the solution of this question. In the mean time, while we are sure that such irregularities actually exist, it is plain that much caution is required in the use of surveying instruments, where magnets are concerned either in the observations, or in the adjustment. In the ordinary operation of maritime surveying by the needle and sights, the errors are often likely to be considerable; more particularly where some of the angles are determined with the instrument placed on shore, mariners having been uninformed with respect to the existence of such disturbing forces. To such causes it is very probable are owing the discrepancies found in charts of new or distant coasts, where the instruments have even been good and the observer practically expert in its use. It is even probable that the charts of the west coast of Scotland, constructed by Mackenzie with abundant leisure and experience in his art, owe many of their very glaring errors to this cause, which on many of these coasts is exceedingly active. Want of care or want of accuracy, as far as this could be obtained by the means which he possessed, cannot be imputed to him, as the soundings, distances, and enumeration of even the most minute sunk rocks, is surprisingly accurate, rendering his work such, that even the most inexperienced pilot may enter these intricate harbours and come to his anchorages with no other guide, as I have abundantly verified on numerous occasions. If, as has been said, the well known map of North Wales constructed by Evans, was also surveyed by the compass, we may perhaps attribute a portion of the errors which it comprises to this neglected cause among others. It is true that the repeated comparison of bearings will afford some correction to inaccuracies of this nature, but a medium of bad observations can never produce a good one.
At any rate where such an instrument is to be used it will always be prudent to erect it on a staff at some distance from the ground, as we shall then at least avoid those disturbances which result from the contact, or very near approach of rocks when it is placed on or near the ground. This addition is easy and of little encumbrance to the operator, and as the compass and sights is so useful and convenient for ordinary purposes, it will at least render it one degree more correct, although even in this way we shall not avoid those disturbances which arise from the more extensive influence of larger masses of magnetic matter. This caution is equally necessary to the geologist in the taking of geological bearings or angles, since the errors which may arise from its position on the ground are generally considerable, on account of the delicacy of the needle of a pocket compass, and may often be as great or greater than those which proceed from the short radius of the instrument itself, and the consequent difficulty of accurate observation.
In the use of the miner's compass the species of error which I have here noticed is always in danger of becoming serious, from the difficulty of removing the instrument to a sufficient distance from the disturbing forces. Yet even in the narrowest shaft or level an attention to this will enable the miner to take precautions, which, if they cannot entirely remove, will at least diminish in a great degree the probability of serious error.
In adjusting the position of a common theodolite it is evident that one of the sources of error will be sometimes removed, and always diminished, as it rarely happens that the local influences of small masses are felt at the distance at which that instrument is elevated from the ground. Yet I have shewn in the instance of Glamich, that the disturbing power was sensible at four feet from the surface. Such cases are however rare, although the ordinary quantity of the variation will be altered at distances infinitely greater, where the action of mountains or large tracts of magnetic matter is concerned.
In general I may remark that no confidence can be reposed in
magnetic angles until the regular action of the needle has been ascertained by repeated observations on its steadiness at different elevations
above the surface, and at different points upon it. It is not
a sufficient trial that detached specimens of the rocks in the vicinity
exert no action on it, since the variations will often be sensible from
the joint action of a considerable mass of polar matter, when small
pieces of the same substance produce no effect. The dipping needle
would give the most correct information respecting the existence of
these local causes of error, but it is an instrument unfortunately
too nice and expensive for ordinary purposes. In all cases where
such a disturbing force is suspected to be in existence, and where
accuracy is necessary, we should not be content until the ground has
been examined and the actual variation ascertained by the observation
of the magnetic azimuth, wherever it is practicable to procure
such an observation. It would however tend still more to the removal
of all possible errors arising from this cause, if surveyors
were to reject the use of the needle altogether, and depend solely on
the back angle; since although every one who lays claim to accuracy
will correct the one observation by the other, yet the temptation
arising from the facility of using the needle alone, is perhaps
too great to be always resisted.
Since this paper was prepared for the press, there has been published in the Transactions of the Royal Society of Edinburgh a description of the same place, drawn up from the joint observations of Lord Webb Seymour and Professor Playfair, Among other differences, such as may be expected to occur in the observations of different individuals, and which are not subjects for discussion, there are some which call for remark, since they involve questions of nomenclature, the discussion of which may be useful in the present uncertain state of that important preliminary to geological observation. It is much to be desired that all observers should agree in the denomination of those rocks respecting which frequent discordances arise, since without such agreement there can be no prospect of a definite application of terms.
Gneiss is described in that paper as of frequent occurrence. With a few trifling exceptions the most remarkable one of which I have described, I have no where observed any real gneiss among the schistose rocks which form the left ridge of the valley or the gear mass of stratified matter. These schistose rocks, as far as my observations go, are clay slate, mica slate, hornblende slate, and quartz rock. Gneiss is a rare rock in Scotland, and is principally to be found in the Long island, in Tirey, Coll, Rona, Iona, and Isla; as well as on the mainland, in Glen Elg, and Morven, I do not consider the rock which is of such frequent occurrence between Blair and Loch Spey, already described in this paper, as gneiss, since although the form is laminar the structure is not foliated: nor have I observed that the gneiss y of Scotland any where alternates with mica slate, although it does with clay slate, nor that it has any immediate connection with granite, much as it resembles it in composition. But in mica slate, as well as in the micaceous quartz rock which occurs in the vicinity of granite, a real gneiss is not infrequently seen near the points of junction; the schistose rock which in other parts consists of quartz and mica in various proportions, having felspar superadded, and preserving the same parallelism in the disposition of the mica which it possessed before the addition of that ingredient. This change is of various extent, in some cases not reaching many inches beyond the line of contact with the granite, and gradually disappearing even in the lateral progress of the laminæ, while in other instances it occupies a more considerable space. But in no case that I have witnessed is it prolonged through the whole extent of a bed so as to allow us to say that beds of gneiss alternate with or precede the mica slate. I conceive the appearance of gneiss which I have described in Glen Tilt to be of this partial nature, and in all probability owing to similar causes. I would also remark that as quartz rock is evidently recomposed from the ruins of ancient granites, it often contains all the ingredients of that substance. In the extensive remarks which I have made on this rock, as yet so little observed, I have noticed instances in which it has been mistaken for granite. In the same way it may be mistaken for gneiss, since with a stratified structure it may contain all the ingredients of that substance. Nay, even the mica may be disposed in the same parallel form in the triple compound as it is in the more ordinary mixtures of quartz and mica. Even here however an experienced eye may detect differences which words are inadequate to define, and the true connections and nature of the rock may be known, by tracing its connections with, and gradation into, the more characteristic beds of quartz rock with which it is associated.
The next remark I shall make is on the term syenite, which in the paper alluded to is applied to that rock which I have called granite. The difference here is merely a question of expediency, and I shall content myself, in addition to what I have already said on that subject, with a brief statement of my reasons, as I shall have occasion to discuss this point at some length in giving an account of the Western islands.
Werner has described syenite as an overlying formation, and incumbent on granite. With a rock of these characters I have no acquaintance, and his description is therefore unintelligible to me in a practical view.
But there are in Scotland (and elsewhere) two compound rocks formed of the ingredients described in his definition, namely, quartz, felspar, and hornblende, but occupying two positions, in a geological view most distinct, and neither of them agreeing with that which he has assigned for his syenite. That to which I have thought it expedient to limit the term, is incumbent upon the secondary strata and at the same time interferes with them in the same way that certain varieties of trap and porphyry, with which it is intimately connected, are known to do; examples of this are to be found in many of the Western islands. It is therefore not only posterior to these strata and distinct from the syenite of Werner in its geological relations, but it differs from it in composition, inasmuch as it does not contain mica, unless, as some also of the traps do, accidentally.
The other rock which agrees with the mineralogical definition of syenite, is found connected with granite and consequently subjacent to the most ancient stratified rocks. I have attempted to shew in this paper, that it passes into granite, and is associated with it in an intimate manner, having no further the appearance of stratification or of an overlying character than granite has, and bearing precisely the same relation to the stratified rocks which that does. In aspect it has generally a considerable resemblance to granite; often indeed it cannot be distinguished without a careful inspection, while the syenite which lies above the secondary socks rarely has a granitic aspect. This variety frequently contains mica as well as hornblende, and it is indeed not rare to find the former ingredient far exceeding the latter. There appears therefore a sufficient mineralogical distinction between these two rocks to justify us in distinguishing them by two names, without infringing the rule which I have suggested on another occasion, that we were not at liberty to vary our principles of nomenclature, by drawing them sometimes from mineralogical character, and sometimes from geological position. But for this purpose it is necessary that the whole of any mass of rock should be considered together, and that its mineralogical character should be defined by its prevailing, not by its occasional composition. Whatever more experienced geologists may determine on this question, it is indubitably necessary that the conclusion which I have here pointed out should be removed by some expedient, and I shall gladly conform to any better suggestion; but it is too plain that the indiscriminate application of the term syenite to two rocks so essentially different in connections, would tend to produce a most incurable confusion in geological description.
The last remark which I shall make on the apparent discrepancy between my observations and those of the philosophers above mentioned, is also little else than a question of nomenclature. It consists in the use of the term greenstone, which has been applied by them to some of the dark varieties of the syenitic granite. It has appeared to me, as I have remarked in the body of the paper, that these varieties of syenitic granite were distinguishable from true greenstone by the presence of quartz, and that however minute their texture, and apparently uniform the white ingredient, it always contained that mineral in conspicuous proportion, while in greenstone, a common member of the trap family, it was rarely present, and might, when present, be considered as accidental. If I have been mistaken in this observation, I shall consider it unfortunate that no criterion of a mineralogical nature can be found to distinguish these rocks, if indeed the presence of the occasional minerals, mica, epidote, or sphene, minerals in general of a more ancient origin, cannot lend us some assistance. It is so necessary to prevent rocks distinct in their geological relations from being confounded, that the terms which are assigned to them, ought if possible to be equally distinct, since the mere effect of terms, as we all experience, has in every science a powerful effect in misleading or influencing our judgments. If there really be no such distinction as I have here thought probable between certain syenitic granites and the greenstones of the trap family, it will then be necessary to have recourse to the same expedient as we use in distinguishing the limestones, the addition of the terms, primary, and secondary; an expedient however, which in this case it would be desirable to avoid, since many geologists are inclined already to distinguish by the addition of these very terms the real trap rocks which are found connected with primary or with secondary strata.
- ↑ Vide Plates 14, 15, 16, 17, 18.
- ↑ Vide Plate 15
- ↑ Vide Plates 15, 16, 17.
- ↑ Vide Plates 16, 17.
- ↑ Vide Plate 14.
- ↑ Vide Plate 19.
- ↑ Vide Plate 21. fig. 1.
- ↑ Vide Section, Plate 20. fig. 1.
- ↑ Vide Plate 22.
- ↑ Having pointed out the circumstance to the Duke of Atholl bro years ago, quarries are now opened in them, by which the numerous varieties which they contain have been more completely brought to light than they could have been by the operation of a mineralogist hammer.
- ↑ I may here add that I have found this mineral in Glen Elg, and in a similar situation, but exhibiting different colours, the predominant variety being snow-white, and the more rare one green: I have also found it in great variety in Tiree and Harris.
- ↑ Vide Pl. 20. fig. 3.
- ↑ See the first paper in this volume.
- ↑ Rocks thus situated have by some mineralogists been divided into principal and subordinate beds, but however this distinction may occasionally be found to hold good with regard to some of the rocks so enumerated, yet as it is by no means a distinction universally existing, it ought not to be erected into a general rule, since it increases the number of artificial divisions, and offers a convenient and unmeaning phraseology instead of the language of accurate description. Subordination implying inferiority or dependence, it should follow that the one rock was in all cases either necessarily less in quantity, or in some way dependent on the others The term interstratified, involves neither obscurity nor hypothesis, and is amply sufficient for the purposes of description, when combined with the relative proportions and positions of the rocks in question.
- ↑ I may here add that I have also observed rutile in quartz in the Scarsough mountain, and accompanied by oxidulons iron in the micaceous schist of Rannoch, so that it is not a rare mineral in Scotland.
- ↑ Vide Map, Plate 13.
- ↑ Plate 21, fig. 2 & 3.
- ↑ Vide Plates 14, 15, 16, 17, 18, & 20 fig. 4.
- ↑ Vide Plates 13 & 20.
- ↑ I am indebted for this Map to the Duke of Atholl, it being the survey of part of his estate, and executed by Stobie, the author of the General Map of Perthshire.
- ↑ Geological Transactions, vol. ii.
- ↑ This paper does not appear, having been retained for the purpose of connecting it with the history of the neighbouring and similar islands.
- ↑ Printed in this volume.
- ↑ To distinguish this Scalp; from other islands of the same name, I must say that it lies at the eastern side of Harris, and is remarkable for its lighthouses
- ↑ After this paper has been prepared for the Society, The voyage of Captain Flinders was published. It gives me great pleasure to find so thorough a confirmation of my observations and suspicious on this subject, in the original remarks of that indefatigable and unfortunate navigator. I was also pleased to see that he had frequently observed the polarity granite; more frequently I doubt not than it has occurred to me, had he published the details of all his observations. There seems to me also some reason to think, from a remark on which however he does not lay much stress, that he imagined that property to be more conspicuous on the summits of hills than elsewhere.
- ↑ Vide Miscellaneous Paper, Vol. II. of Geological Transactions.