Quarterly Journal of the Geological Society of London/Volume 33/On the Strata and their Fossil Contents between the Borrowdale Series of the North of England and the Coniston Flags
Introduction.
In the following communication we propose shortly to consider the various groups of strata which intervene between the great volcanic series (Borrowdale Rocks) of the Lake-district and the well-marked band of sedimentary rocks to which Prof. Sedgwick applied the name of the "Coniston Flags." In so doing we shall have occasion to note, in a general way, the physical characters and relations of the successive deposits in question; but we shall have to draw attention more especially to the organic remains which they contain and to some indications thus afforded as to their precise age and position in the geological scale.
The base of the great Silurian series of the north of England is constituted, as is well known, by the "Skiddaw Slates," a thick mass of sediments, originally in the condition of black mud, clearly proved by their fossil contents to be of the age of the "Arenig group" of Wales. Succeeding the Skiddaw Slates there occurs a great series of volcanic products, termed by Prof. Sedgwick the "green slates and porphyries," to which we have elsewhere given the name of "the Borrowdale series." These consist of ashes and breccias, alternating with ancient lavas, a portion of the series being subaerial, whilst part is of submarine formation.
Throughout the greater part of this extent, for a thickness of some thousand feet, the Borrowdale series has hitherto proved unfossiliferous; fossils, however, make their appearance in a thin band of calcareous ashes near the summit of the group (Harkness and Nicholson, Quart. Journ. Geol. Soc. vol. xxii. p. 480).
In some places, as at Style-End Grassing, between Long Sleddale and Kentmere, this band consists of brownish or bluish grey shales, and it is separated from the Coniston Limestone by a bed of trap. In other spots, as at Sunny Brow, between Ambleside and Coniston, the bed is siliceous and gritty in nature. At Millom the same fossiliferous band is recognizable, and is not only ashy in its character, but is surmounted directly by strata belonging to the Coniston Limestone series without the intervention of traps. Lastly, there are places, such as the east side of Long Sleddale and the southern declivity of Wansfell, where the same beds can be readily recognized, having the same relation to Coniston Limestone, similar to the ordinary ashes of the Borrowdale series in their ordinary character, and exhibiting no traces of fossils beyond the presence of innumerable cavities filled, or partially filled, with rusty peroxide of iron, apparently due to the decomposition of organic remains.
It is not our purpose to discuss here the position and age of these "Style-End Grassing beds," as they may be called. It may, however, be noted that the fossils which they have yielded are Bala types, such as Calymene Blumenbachii, Brongn., Orthis vespertilio, Sow., and Petraia æquisulcata, M'Coy. This is of interest as indicating that the volcanic activity of which the Lake-district became the theatre subsequent to the deposition of the Skiddaw Slates, continued to prevail, at any rate, up to the later portion of the Bala period.
Resting, apparently with perfect conformity, upon the Borrowdale rocks a series of deposits occur which we wish more especially to discuss here.
These deposits may be grouped in the following ascending order:—
1. Dufton Shales.
2. Coniston Limestone and Shales.
3. Graptolitic Mudstones, or Skelgill beds.
4. Knock beds.
1. Dufton Shales.
The "Dufton Shales" are but locally distributed, though they constitute a well-marked group of muddy sediments underlying the Coniston Limestone proper and its associated shales. They do not appear in any recognizable form beneath the main hue of the Coniston Limestone in the Lake-district itself. They do not seem to occur in the Sedbergh district; nor have they been recognized in Ravenstonedale or Ribblesdale; but they are very well developed in the Silurian area which includes the Cross-Fell range.
Here they are seen in four principal exposures, owing to the folding and faulting of the strata, viz.:—Swindale Beck, near Knock; Pusgill and Dufton-Town dykes, near Dufton; Harthwaite Gill, near Keisley; and at the Smelt Mill, near Hilton.
The thickness of the Dufton Shales in these localities probably exceeds 300 feet. They consist of dark flaggy shales with a rough cleavage, sometimes (as in Swindale) having brownish or greenish ashy beds intercalated among them. These shales are readily distinguished from those associated with the Coniston Limestone, being usually darker in colour, and more flaggy, and having a less perfect cleavage. Near their base they have two bands of nodular limestones in them; these are well seen in Swindale Beck.
The Dufton Shales are richly fossiliferous throughout, this being the case with the ashy beds before referred to, as well as others of the series.
The fossils are, for the most part, characteristic Bala types; and the entire deposit may be regarded as forming, palaeontologically, the base of the Coniston Limestone.
The following is a list of the principal fossils which have been collected in these beds:—
Fossils of the Dufton Shales.
Actinozoa.
1. Petraia, sp. Swindale.
2. Chætctes, sp. An incrusting, subniassive species, commonly growing on Orthocerata. Thin sections show that the tubes are furnished with a few remote tabulæ. Surface characters unknown. Common at Pusgill along with Discina corona, Salt., sp., Bellerophon bilobatus, Sow., &c.
Annelida.
1. Conchicolites gregarius, Nich. This singular form is found abundantly along with the incrusting species of Chætetes just spoken of, attached to the outer surface of Orthocerata, in the Discina-corona bed.
Brachiopoda.
1. Leptæna transversalis, Wahl. Smelt Mill, Hilton.
2. Strophomena expansa, Sow. Harthwaite Gill, Dufton.
3. Leptæna sericea, Sow. A small variety of this shell with fewer principal radii than the normal form, and of about half its size, occurs very abundantly in Pusgill, and less commonly in all the other localities.
4. Orthis testudinaria, Dalm. Swindale and Pusgill.
5. Discina (Trematis) corona, Salt. This fine Brachiopod occurs abundantly, though in a more or less fragmentary condition, in a single stratum in Pusgill, along with Orthocerata, Conchicolites gregarius, Bellerophon bilobatus, &c.
6. Lingula tennigranulata, M'Coy. Pusgill.
7. ovata, M'Coy. Pusgill. Both these Lingulæ seem to be confined to the stratum with Discina corona.
8. Orthis biforata, Schloth. Dufton-Town dykes.
9. Strophomena rhomboidalis, Wilckens. Pusgill.
10. Orthis vespertilio, Sow. Dufton-Town dykes.
Heteropoda.
1. Bellerophon bilobatus, Sow. Pusgill.
Cephalopoda.
1. Orthoceras, sp. A smooth form resembling O. baculiferme, M'Coy. Common at Pusgill, in the Discina-corona stratum.
2. Oncoceras, sp. Not uncommon at Pusgill, in the same bed as the preceding.
Crustacea.
1. Calymene Blumenbachii, Brongn. Very abundant in Pusgill and Dufton-Town dykes, less so in Swindale.
2. Trinucleus concentricus, Eaton. Very abundant in Pusgill and Dufton-Town dykes, less abundant in Swindale.
3. Cybele verrucosa, Dalm. Dufton-Town dyke.
4. Ampyx nudus, Murch. (?) Rare in Pusgill.
5. Lichas laxatus, M'Coy. Pusgill.
6. Illænus Bowmanni, Salt. Swindale and Hilton.
7. Encrinurus, sp. Pusgill.
8. Beyrichia Wilckensiana, Jones. Abundant in Pusgill in the Discina-corona bed, and also in the ashy beds in Swindale.
9. Primitia semicircularis, Jones & Holl. In ashy beds. Swindale.
The predominance in the Dufton Shales of such Brachiopods as Strophomena expansa, Leptæna sericea, Orthis testudinaria, O. vespertilio, O. biforata, and Lingula ovalis, together with the abundance of such trilobites as Calymene Blumenhachii, Trinucleus concentricus, Cybele verrucosa, Illænus Bowmanni, and Lichas laxatus, can leave no doubt as to the correctness of the inference that these rocks belong to the Bala or Caradoc age. The presence of what appear to be unequivocal ashes which contain some of these fossils high up in the series at Swindale also deserves attention as showing that the volcanic forces which gave rise to the ashes and lavas of the Borrowdale group still maintained an intermittent activity during the deposition of the Dufton Shales. There would thus appear to have been no break of continuity between these shales and the underlying Borrowdale rocks—a conclusion which is further borne out by the substantial identity between the fossils of the Dufton Shales and those of the Style-End Grassing beds.
The annexed section (fig. 1) exhibits the stratigraphical relations of the Dufton Shales in Swindale Beck, where, as before stated, they are very well exposed.
Fig. 1.—Sketch Section of the Strata in Swindale Beck, near Knock.
(Length rather more than half a mile.)
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a. Ashes belonging to the Borrowdale series c. Coniston Limestone. e. Knock beds (green and purple slates).
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b. Dufton Shales. d. Graptolitic mudstones. |
f. Black flags with Monograptus colonus, Barr., probably Coniston Flags.
2. Coniston Limestone.
The "Coniston Limestone," notwithstanding its comparatively small vertical extent, has long occupied the position of being the best-defined and most universally recognized of all the divisions of the Lower Silurian series of the north of England—a position which it owes to its easily recognized lithological characters, and to the number of organic remains which it has yielded. It is unnecessary here to recapitulate the geographical range of the Coniston Limestone. Its main line of outcrop crosses the Lake-district in a direction from S.W. to X.E., running from Millom on the one hand to Snap Wells on the other. It is more or less developed in Ravenstonedale, Dentdale. the Sedbergh valley, near Ingleton, in Ribblesdale, at Ireleth, at High Haulme. in Furness. and at various points in the Lower Silurians which lie to the south-west of the Cross-Fell range.
Lithologically the term "Coniston Limestone" is somewhat misleading, as it is never wholly calcareous in its composition, and the calcareous element is occasionally almost wanting. In its most typical form, as seen in its range between Long Sleddale and Broughton Mills, in Furness, the Coniston Limestone consists of hard grey or bluish grey, cleaved, and often highly calcareous shales, containing numerous nodules or thin but distinct bands of limestone. In some places the beds of limestone are the most largely developed; and in other places the shales predominate; but the two elements of the group are usually intermixed, and alternate with one another indefinitely. At some localities, again, as at Ash Gill, near Torver, and at the head of Appletreeworth Beck, the shales are largely developed at the expense of the limestone, and are sufficiently thick to have been extensively worked for slates. At Keisley, near Dufton, again, the series is almost calcareous, and the shales appear to be entirely confined to its upper portion. Finally, at Beck and Waterblain, near Millom, the group consists of an upper series of cleaved fossiliferous shales, in the lower part of which a great thickness of limestone is developed, dis continuously, in the form of great lenticular masses of a purely calcareous nature.
In its intimate characters the limestone of the Coniston Limestone group differs greatly in different localities. Usually it presents itself as a hard, compact, greyish blue, grey, or nearly black limestone, which, in thin sections prepared for the microscope, exhibits a subgranular matrix in. which fragments of Crinoids, Corals, or Brachiopods are imbedded at intervals. At Keisley (fig. 2), where we have most carefully examined it, three principal varieties may be distinguished:—(1) a hard, compact, greyish blue limestone with the microscopic characters and general aspect of the ordinary variety of the limestone just alluded to; (2) a reddish or pink compact marble with numerous patches of white calcite, both portions of the mass appearing, in microscopic sections, to be crowded with minute organisms and fragments of larger fossils; (3) a light-coloured, whitish blue or white, coarsely crystalline limestone, which is seen, in thin sections, to be composed of innumerable fragmentary organic remains and microzoa; imbedded in transparent calcite, and having large crystals of calc-spar with their characteristic cleavage-lines shooting through the mass in various directions.
Fig. 2.—Sketch Section of the Coniston Limestone at Keisley, near Dufton. (Length rather more than one third of a mile.)
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a. Borrowdale series. |
b. Coniston Limestone. |
c. Barren ground, occupied in whole or in part by the Graptolitic mudstones.
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d. Knock beds (pale slates). |
p. Permian strata. |
At Shap Wells, close to the medicinal spring, occurs a singular band of Coniston Limestone, to which we shall have occasion to refer again.
The band in question is a calcareous breccia composed of innumerable fragments of various rocks, mostly under a quarter or half an inch in diameter, imbedded in a calcareous matrix. Thin sections of this rock are very difficult to prepare, owing to the large number of angular fragments of quartz which they contain; in addition to which there are numerous fragments of limestone, felspathic ash, and perhaps traps, the cementing matter of the whole being a granular limestone apparently devoid of fossils. That this breccia is more or less altered by the near vicinity of the Shap granite cannot be doubted, the more so as the graptolitic mudstones are seen in an unequivocal form about 200 yards to the south of the breccia, with numerous graptolites, but so highly indurated as to have become almost flinty in their fracture. In connexion with this we may briefly refer to the microscopic characters of the more highly metamorphosed Coniston Limestone, about a quarter of a mile from the mineral spring, near the top of the Blea Beck. Here the Coniston Limestone is penetrated and apparently overlain by a mass of felstone, doubtless emanating from the granite of Wastdale Crag. In the immediate vicinity of this intrusive mass, the limestone is converted into an olive-green splintery rock, which shows, in microscopic sections, particles of iron pyrites and disseminated specks of a dark green mineral (hornblende?), together with a few traces of minute fossils. At a distance of a few yards from the felstone this limestone presents itself as a dark grey crystallized rock, which is shown by thin sections to be completely granular, with hardly any indications of fossils.
About two miles west from Shap Wells, at a short distance to the south-west of the farm-house of Wastdale Head, the Coniston Limestone exhibits itself in a very highly metamorphosed condition. Here, in the course of a small stream flowing from the north, a small patch of the limestone occurs, having a white colour and a crystalline structure. A few feet to the north of this patch a small exposure of rock is seen, having a gneissic character, the particles of which are very crystalline. This gneiss is very nearly in contact with the south-west portion of the Wastdale-Crag granite. It is probably either metamorphosed shales of the Coniston Limestone, or of the representatives of the Dufton Shales.
The white crystalline limestone has peculiar features. It has a fine scaly structure, with a pearly lustre, and resembles fine-grained Schiefer spar. It contains within it Idocrase; and in the upper portion, where it has been subjected to the action of the water of the stream, the idocrase almost remains alone, the carbonate of lime having been removed[1].
The metamorphic limestone of Wastdale Head has above it rocks which also exhibit changes, the effect of the influence of the Wastdale-Crag granite. These appear in the form of rocks banded white and black with a pseudo-gneissic structure, the black bands beginning to prevail higher up in the series until hard dark-grey strata prevail, having lighter beds associated with them. These light-grey rocks are very compact, showing no traces of lamination, and possessing a conchoidal fracture. They are translucent on the edges, and, but for their colour, would be referred to the Helleflint of the Swedes, or to some form of felstone, were it not for their bedded character, which is very manifest. These rocks are intersected by an Elvan dyke emanating from the granite. This Elvan dyke effervesces with acids, from the presence of carbonate of lime derived from the metamorphic limestone, which has infiltrated itself into the interstices of this dike.
The fossils of the Coniston Limestone are numerous, but usually not well preserved. Sometimes they occur in great numbers, and in a state of good preservation, in the limestone itself, as is the case at Keisley. Usually the limestone is nearly destitute of fossils, and the palæontologist is obliged to have recourse to the shales associated with the limestone, in which the fossils, though numerous, are greatly distorted by cleavage. Without quoting here the lists of fossils from the Coniston Limestone given by M'Coy (Palæozoic Fossils) and by Salter (Cat. Cambr. and Sil. Fossils), we subjoin a list of the more important in our own collections, which have been mostly obtained from the limestone of Keisley, near Dufton.
Fossils of the Coniston Limestone.
Actinozoa.
1. Petraia, sp. Keisley.
2. Chætetes, sp. A small dendroid form, such as is usually called Stenopora fibrosa, var. ramulosa: the surface is unknown; but it is perhaps referable to Chætetes Fletcheri, E. & H. Keisley.
3. Heliolites insterstincta, Linn. Pool Wyke.
4. Halysites catenularis, Linn. Keisley.
5. Petraia æquisulcata, M'Coy. Millom and Long Sleddale.
Polyzoa.
1. Fenestella (?) assimilis, Lonsd. Keisley.
2. Ptilodictya costellata, M'Coy. Millom.
3. Ptilodictya, sp. Keisley.
Brachiopoda.
1. Strophomena corrugatella, Dav. Keisley.
2. rhomboidulis, Wilckens. Keisley.
3. deltoidea, Conrad. Keisley.
4. expansa, Sow. Keisley.
5. Orthis biforata, Scbloth. Keisley.
6. calligramma, Dalm. Keisley.
7. flabellulum, Sow. Keisley.
8. Actoniæ, Sow. Keisley.
9. vespertilio, Sow. Keisley.
10. porcata, M'Coy. Keisley.
11. elegantula, Dalm. Skelgill Beck.
12. Discina? corrugata?, M'Coy. Keisley.
13. Triplesia? monilifera, M'Coy. Keisley.
14. Atrypa imbricata, Sow. Keisley.
15. Leptæna tenuicincta, M'Coy. Keisley.
Cephalopoda.
1. Orthoceras vagans, Salt. Keisley. Abundant.
Crustacea.
1. Sphærexochus mirus, Beyrich. Keisley.
2. Cheirurus juvenis, Salt. Keisley.
3. bimucronatus, Murch. Keisley.
4. gelasinosus, Portl. Keisley.
5. cancrurus, Salt. Keisley.
6. octolobatus, M'Coy. Keisley.
7. Lichas laxatus, M'Coy. Keisley.
8. hibernicus, Portl. Millom.
9. Illænus Davisii, Salt. Keisley.
10. Bowmanni, Salt. Keisley.
11. Rosenbergi, Eichwald. Appletreeworth Beck.
12. Calymene Blumenbachii, Brongn. Keisley. Rare.
13. Agnostus, sp. A form somewhat resembling A. trinodus, Salt. (= Trinodus agnostiformis, M'Coy), in shape and size, but having its whole surface tuberculated. Not uncommon at Keisley.
14. Phacops (Chasmops) macrourus, Sjögren. This characteristic Bala species is the Odontochile obtusicaudata of M'Coy; but it wants the surface-granules of the true P. obtusicaudatus, Salt. Fine tails, though somewhat distorted by cleavage, are not uncommon at Appletreeworth Beck.
15. Phacops apiculatus, Salt. Appletreeworth Beck.
16. Ampyx Sarsii, Portl. Keisley.
17. Remopleurides, sp. Alai-ge form allied to, but apparently distinct from, R. longicapitatus, Portl. Not uncommon at Keisley.
18. Proëtus, sp. The free cheeks only are known, and resemble P. latifrons, M'Coy. Keisley.
19. Bronteus, sp. Pygidia only known. Keisley.
20. Beyrichia impendens, Jones. Appletreeworth Beck.
21. Primitia protenta, Jones. Appletreeworth Beck.
No one can analyze the preceding list of fossils and entertain any doubt as to the geological horizon of the Coniston Limestone. Whether or not it be in the precise position of the Bala Limestone of Wales is a point which may admit of doubt; but it is the precise equivalent of the limestone of the Lower Silurian of Portraine, co. Dublin, and also of that of the Chair of Kildare (see Appendix), regarded as unquestionably of Bala age. This is shown conclusively by the Brachiopods, and especially by Trilobites of the following forms—Sphærexochus mirus, Cheirurus juvenis, C. cancrurus, C. gelasinosus, C. octolobatus, Lichas laxatus, L. hibernicus, Illænus Bowmanni, I. Davisii, Phacops macrourus, P. apiculatus, constituting amongst the latter a characteristic assemblage of Caradoc or Bala types. It is somewhat curious that of the two commonest and most characteristic Trilobites of the Dufton shales, Calymene Blumenbachii is hardly known in the Coniston Limestone, and Trinucleus concentricus is generally absent. The latter occurs plentifully in the "Trinucleus-shales" of Prof. Hughes, a locally developed group at the summit of the Coniston Limestone in the Sedbergh district. The evidence to be derived from the Brachiopods is equally conclusive with that of the Trilobites, all the common forms being characteristic species of the Bala formation; but it is not necessary to enter into further details on this point.
It remains briefly to consider the conditions under which the Coniston Limestone series was deposited. On this matter we wish especially to indicate that there seems to be good evidence that the volcanic activity which produced the vast mass of ash, breccias, and lavas constituting the Borrowdale series, though greatly mitigated in intensity, had not entirely died out during the period of the deposition of the Coniston Limestone and Dufton Shales, but continued to operate at occasional intervals. It would seem probable that the Lake-district was not entirely submerged at the time when the Coniston series began to be laid down, but that a portion of the volcanic region remained above the level of the sea, its vents occasionally giving exit to showers of volcanic ashes or even currents of lava. If this hypothesis be established, it would follow that there was no breach of continuity between the close of the Borrowdale series and the commencement of the Coniston series, but that the two groups of rocks are intimately related, and in point of fact actually overlap one another in time. The principal grounds which at present appear to indicate the correctness of this hypothesis may be briefly stated as follows:—
(1) The intercalation in the Borrowdale series, close to its summit, of a band of fossiliferous shales containing Bala species, proves that the volcanic energy of this period still continued in force at a time when the sea was peopled with well-known Bala Brachiopods and Trilobites. The shales in question (Style-End Grassing beds) are usually separated from the Coniston Limestone proper by a mass of lava; but they are undoubtedly to be regarded as, palæontologically, a portion of the Coniston series.
(2) The presence of beds of ashes, containing numerous fossils, high up in the Dufton Shales at Swindale, near Knock, proves similarly that the volcanic eruptions of the preceding period had not entirely ceased at the time when the Dufton shales were in course of formation, and these shales, as we have seen, belong palæontologically to the Lower Coniston group, and are only a local development of the base of the Coniston Limestone itself.
(3) The presence, near the summit of the Coniston Limestone itself, at Shap Wells, of a calcareous breccia containing numerous fragments of ash proves that an eruption took place towards the close of the period during which the Coniston Limestone was deposited. Nor does the site of this eruption appear to have been very far removed from Shap Wells itself; for the microscopic investigation of this breccia indicates that some of the older and previously formed beds of the Coniston Limestones were broken up by this outburst, and were thrown over the sea-bottom along with innumerable fragments of ash, the whole being subsequently cemented together by calcareous ooze to constitute the singular stratum in question.
(4) The presence in the Coniston Limestone in the Sedbergh district, as shown by Prof. Hughes, of interbedded felstones, proves conclusively the occurrence of volcanic eruptions contemporaneous with the deposition of the limestone, in this area at any rate, if not elsewhere.
(5) By the supposition we have brought forward a satisfactory explanation is obtained of a certain amount of apparent discordance between the Coniston Limestone and the underlying volcanic Borrowdale rocks. This discordance, so far as it exists, might be set down as due to a want of conformity; but we do not think that this is its true explanation.
If the views we entertain be correct, it is rather due to the fact that the Coniston Limestone was deposited round the shores of the volcanic nucleus of the Lake-district very much after the fashion that a modern limestone might have been in process of formation for thousands of years off the coasts of Sicily. In such a case beds of limestone would wrap round sheets of lava so as to be apparently transgressive thereon, or might be interstratified with strata of tuff, of ash, or of volcanic rocks. Any seeming discordance between the calcareous series and the volcanic products would be due, not to absolute unconformability, indicating a lapse of time, but simply to the difference in the method by which the two groups were formed. In spite of any apparent discordance, both groups would belong to the same period, and in part they would be actually contemporaneous.
If these inferences be confirmed by further researches, it will follow that the Borrowdale rocks must be regarded as being of Lower-Bala age. As the Skiddaw slates are unquestionably Arenig, and the Coniston Limestone equally unquestionably Bala, the only other view which could be taken as to the Borrowdale series would be to refer them to the Llandeilo. Apart, however, from the considerations just mentioned, this view is rendered unlikely by the fact that the great series of Llandeilo rocks developed in the south of Scotland appears to be wholly free from intermixture with contemporaneous igneous matter. We can hardly suppose that such could possibly have been the case, if the Llandeilo strata of the southern uplands of Scotland had been in process of formation at a time when the closely adjoining region of the Lake-district was the scene of the intense volcanic activity which gave birth to the Borrowdale series.
3. Graptolitic Mudstones.
Resting directly upon the Coniston Limestone, in every locality where the summit of the latter has been recognized, is a series of dark-coloured Mudstones and grey Shales, which, though of small vertical thickness, are of especial interest from the organic remains which they contain. These beds were originally described by us (Quart. Journ. Geol. Soc. vol. xxiv. p. 296); and to them was applied the name "Graptolitic Mudstones," as indicating their general mineral character and the predominant fossils which they yield. Subsequently, as an alternative local name, the title "Skelgill Beds" was proposed by one of us for these strata, from the farm of High Skelgill, near Ambleside, where they are typically developed (Nicholson and Lapworth, Rep. Brit. Assoc. Bristol, 1875). As before stated, immediately above the highest member of the Coniston Limestone these Graptolitic Mudstones come on. The finest sections of the series, however, are to be found on both sides of Long Sleddale, in Skelgill Beck, near Ambleside, in Appletreeworth Beck, and in Swindale Beck, near Knock.
Many other localities exhibit the same beds; but they are generally badly exposed, and, from their comparatively soft nature, they often become so far worn down as to be indicated simply by a depression immediately following the outcrop of the limestone.
In their mineral characters the Graptolitic Mudstones are so well marked that they may be recognized by this alone, apart from the detection of their characteristic fossils. They consist of dark-coloured, often nearly or quite black Mudstones, which are sometimes anthracitic, the joints of which are characteristically iron-stained. These dark Mudstones are replete with beautifully preserved Graptolites, and alternate in successive bands with lighter and darker grey or even sometimes greenish shales, both the light and dark bands being more or less cleaved. Annexed is a diagrammatic sketch (fig. 3) showing the succession of the strata at Skelgill, where the Graptolitic Mud-stones are admirably exhibited.
In our earlier researches in the Graptolitic Mudstones we devoted ourselves principally to the black bands containing Graptolites, a large number of species of these organisms being described by one of the present writers from this group (Nicholson, Quart. Journ. Geol. Soc. vol. xxiv. p. 521). More recently an examination has been made, in greater detail, of the grey bands which interstratify the dark-coloured graptolitic layers, with the gratifying result that these have been proved to contain a considerable number of fossils of higher organization than Graptolites. We append a list of the more important species of Graptolites, and of the other fossils at present known as occurring in this series.
Fig. 3.—Diagram showing the vertical Succession of the Silurian Strata in Skelgill Beck, near Ambleside.
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Base of the Coniston Flags (Broughton-Moor Flags), with Monograptus colonus, Retiolites Geinitzianus, &c. |
| Knock beds. Greenish slates with dendrites, casts of a small Orthis, &c. | |
| Graptolitic Mudstones, with numerous Graptolites and other fossils. At a is the lowest Graptolitic band resting directly upon the limestone; and immediately above it (b) is a grey band full of Brachiopods. | |
| Coniston Limestome proper, consisting of shales alternating with bands of limestone and layers of calcareous concretions. The shales are highly fossiliferous. | |
| Summit of the Borrowdale series, c is a thin band of ashes representing the Style-End Grassing beds. It is full of spots of peroxide of iron and rusty cavities, apparently due to the decomposition of fossils. |
Fossils of the Graptolitic Mudstones.
Graptolitidæ.
1. Climacograptus teretiusculus, His. Everywhere.
2. Diplograptus pristis, His. Long Sleddale.
3. palmeus, Barr. Skelgill.
4. tamariscus, Nich. Skelgill.
5. confertus, Nich. Skelgill.
6. Monograptus Sedgwickii, Portl. Skelgill &c,
7. Nilssoni, Barr. Skelgill &c.
8. spinigerus, Kick. Long Sleddale.
9. intermedius, Carr. Skelgill.
10. gregarius, Lapw. Skelgill.
11. discretus, Nich. Long Sleddale.
12. sagittarius, His. Mosedale &c.
13. lobiferus, M'Coy. Skelgill.
14. turriculatus, Barr. Long Sleddale.
15. fimbriatus, Nich. Skelgill.
16. triangulatus, Harkn. Skelgill.
17. Rastrites peregrinus, Barr. Skelgill &e.
18. distans, Lapw. Long Sleddale.
19. Retiolites perlatus, Nich. Long Sleddale.
Actinozoa.
2. Favosites, sp. A form with smaller corallites than the preceding, and having the walls perforated with numerous minute irregular mural pores; corallum pyriform. Known from casts only. Skelgill.
Brachiopoda.
1. Strophomena expansa, Sow. Skelgill.
2. Orthis vespertilio, Sow. Skelgill.
3. flabellulum, Sow.? Skelgill.
4. elegantula, Dalm.? Skelgill.
5. , sp. A small indeterminable form. Skelgill.
Cephalopoda.
1. Endoceras proteiforme, Hull. Skelgill,
2. Orthoceras angulatum, Wahl, Skelgill,
Crustacea.
1. Agnostus trinodus, Salt. Skelgill.
2. Phacops apiculatus, Salt. Skelgill.
3. Cheirurus bimucronatus, Murch. Skelgill.
4. Harpes Flanagani, Portl. Skelgill.
5. Calymene senaria, Conr. (?). A single imperfect example only. Skelgill.
6. Trinucleus fimbriatus, Murch. Skelgill.
7. Discinocaris, sp. A Phyllopod with a concentrically striated, nearly circular carapace, nearly resembling D. Brouniana, H. Woodward, from the Llandeilo rocks near Moffat. Not uncommon in the Mudstones at Skelgill and Poolwyke.
All the more highly organized fossils of the above list were obtained from beds unequivocally belonging to the Graptolitic Mudstones. Some of them, such as Endoceras proteiforme and the species of Discinocaris, are found in the darkest and most highly fossiliferous graptolitic zones. The Trilobites were obtained exclusively from a dark grey band lying between two graptolitiferous bands, about ten feet above the highest bed of the Coniston Limestone.
Lastly, all the Brachiopods, with the exception of a common but undetermined Orthis of small size, were procured from a single band, the position of which is shown with absolute clearness at several points in the course of Skelgill Beck.
The highest bed of the Coniston Limestone in this locality is fortunately actually a limestone, and not merely a calcareous shale; and it is seen at various points to be directly overlain by a thin band of the characteristic black Mudstone, not more than six or eight inches in thickness, containing numerous Graptolites (Climacograptus &c). The direct contact and juncture of the limestone and the above-mentioned graptolitiferous band can be observed at various spots; and the latter is at once surmounted by a dark grey shale, about eighteen inches in thickness, from which Graptolites are almost entirely absent, but in which the great majority of the Brachiopods were obtained; and this shale is immediately succeeded by a second graptoliferous band. The shale affording the Brachiopods is highly cleaved; and these shells are, unfortunately, so much distorted as to render their determination a matter of considerable difficulty. The Trilobites, on the other hand, occur in a zone several feet higher in the series, surmounted in turn by other graptolitiferous beds, and they are so well preserved as to admit of ready and complete identification.
Any doubt which might have been previously entertained as to the precise age of the Graptolitic Mudstones seems to be removed by the fossils recently obtained from these beds.
It has been already mentioned that these Mudstones were to be regarded as of Lower-Silurian age; and the materials at present in our hands appear entirely to confirm this view. Leaving the evidence afforded by the Graptolites to be considered separately, it is impossible to doubt that the fauna of the Mudstones is essentially a Lower-Silurian fauna. Amongst the Trilobites Agnostus trinodus, Phacops apiculatus, Calymene senaria, Trinucleus fimbriatus, and Cheirurus bimucronatus are characteristic Bala forms ; and all of them, with the exception of the last mentioned, are exclusively confined to rocks of Lower-Silurian age. Harpes Flanagani, the sole remaining form that we have been able to determine with certainty, is a Bala type from Tyrone and Desertecreat. The Discinocaris is closely allied to if not absolutely identical with D. Brouniana, a well-known fossil from the graptolitic Lower Silurians near Moffat.
Of the Cephalopoda the singular Endoceras proteiforme is a characteristic fossil of the Trenton Limestone (Llandeilo-Caradoc) of North America, and Orthoceras angulatum ranges from the Bala to the Ludlow group. The Brachiopoda, even when so much distorted as to be specifically indeterminable, have nevertheless a distinct Lower-Silurian facies; and Orthis vespertilio with Strophomena expansa are characteristic types of the Bala rocks. Lastly, the few corals which have been found are not such as to give any guide to the age of the strata, all that can be said being that they belong to a type not hitherto recognized in the underlying Coniston Limestone.
The clear evidence borne by the Crustacea, Brachiopoda, and Cephalopoda as to the Lower-Silurian age of the Graptolitic Mudstones, is still further substantiated when we consider the characters of the Graptolites themselves, by far the most abundant fossils in this series. Without entering into any minute analysis of the Graptolites found in these rocks, it may at once be stated that they constitute an assemblage of forms of an unequivocally Lower-Silurian aspect. This is proved by the presence, in abundance, of representatives of the Diprionidian genera Diplograptus and Climacograptus, by the remarkable variety of the species of Monograptus, and by the presence of the genus Rastrites. At the same time, that the Graptolitic Mudstones are not low down in the Lower Silurian series is equally clearly shown by the total absence of the genera Didymograptus and Dicranograptus.
As regards the species of Graptolites, Climacograptus teretiusculus, Diplograptus pristis, D. palmeus, D. tamariscus, Monograptus Sedgwickii, M. triangulattis, M. spinigerus, M. intermedius, M. gregarius, M. Sagittarius, M. fimbriatus, M. lobiferus, M. Nilssoni, Rastrites peregrinus, R. distans are all found in the Moffat shales of the south of Scotland, and are more especially characteristic of that division of the Moffat shales to which Mr. Lapworth has given the name of the "Birkhill Group." Thus nearly three fourths, or seventy-five per cent., of the total number of Graptolites known in the Mudstones, including all the common and characteristic species of the group, can be specifically identified with forms which serve to mark the Lower Silurian rocks of the southern uplands of Scotland, the position of which has never been questioned.
Taking all the various fossils now known from the Graptolitic Mudstones together, it is impossible to doubt that the balance of the palæontological evidence is overwhelmingly in favour of the view that this formation is of Lower-Silurian age.
Such being the case, the Graptolitic Mudstones must correspond in position with the highest beds of the Bala series or with the lower portion of the Llandovery group; and this is the direction in which we believe all the evidence tends. As to the precise physical relations between the Graptolitic Mudstones and the subjacent Coniston Limestone, we are of opinion that the two groups are strictly conformable to one another. Not only are the Mudstones invariably found in their proper position, resting upon the limestone, as seen when there are sections of these groups, but no discordance can be detected, as regards the dip and strike, between the two series where they cannot be seen in actual contact.
Moreover (and this appears to us to be an argument of the greatest weight) it cannot be shown that there is any overlap of the Graptolitic Mudstones upon the Coniston Limestone, the former always resting, so far as we have seen, upon the highest bed of the latter.
If it be remembered that along one line of outcrop alone, from Appletreeworth Beck to Shap Wells, the Graptolitic Mudstones can be traced almost continuously succeeding the limestone for a distance of about twenty-four miles, it will be seen to be almost impossible that any want of conformity, however slight, could exist without there being, at the same time, a transgression of the Mudstones over the Limestones. It is true that, owing to the circumstance that the calcareous matter of the Coniston Limestone series is disposed in the form of irregular lenticular masses or concretionary layers, the lithological character of the bed immediately below the Mudstones is not invariably the same, being at one time a limestone and at another a calcareous shale. This, however, is due to an irregularity of deposition which obtains throughout the entire limestone series; and we have failed to find any evidence that the Graptolitic Mudstones ever rest upon any of the lower beds of the Coniston Limestone. As the latter group is of small thickness, and as its main line of outcrop is a very long one, such an overlap must occur, supposing unconformity to exist; and in all probability we should even find the Mudstones passing across the limestones and resting upon the older Borrowdale series.
The absence therefore of any unobserved overlap is, under the circumstances, the strongest possible proof that the Mudstones are entirely conformable to the Coniston Limestone.
That the Graptolitic Mudstones constitute a geological horizon of a definite character, and of much more than mere local importance, is shown by the fact that they can be recognized in Ireland in circumstances similar to those under which they occur in the northwest of England (see Appendix). They have also been recognized in Sweden, in Carinthia, and in Bohemia, while future researches will doubtless bring corresponding strata to light in other Lower-Silurian regions.
In Sweden Dr. Linnarsson has shown (Geol. Mag. June, 1876) that the so-called "Upper Graptolitic Schists" are the equivalents in that country of the Graptolitic Mudstones of the north of England. These Upper Graptolitic Schists, as seen in Westrogothia and Ostrogothia, are the highest Silurian rocks exposed to view, so that they add nothing to the evidence as to the age of the Coniston Mudstones.
In Scania, however, they are overlain by undoubted Upper Silurian beds; and in Dalecarlia they are surmounted by a locally developed limestone (the "Leptsena Limestone" of Tornquist), which appears to form either the summit of the Lower Silurian or the base of the Upper Silurian, being in turn covered by the undoubted Upper Silurian "Encrinurus-beds." Upon the whole, therefore, the evidence to be derived from the Swedish area entirely corroborates the view that the Graptolitic Mudstones are of Lower-Silurian age.
In Carinthia, beds corresponding precisely with the Graptolitic Mudstones have been described by Dr. Guido Stache (Die Graptolithen-Schiefer am Osternig-Berge in Kärnten); and the parallelism of the two deposits has been fully noticed by this distinguished observer. Unfortunately the succession of the Silurian strata in the Osternig is still so obscure as not to permit of any safe con- clusions being drawn as to the precise stratigraphical horizon of these beds, though Dr. Stache concludes that they stand on the borderland between the Lower and Upper Silurians.
In Bohemia we find the representatives of the Graptolitic Mudstones in the "Colonies" of Barrande's Étage D and in the lower portion of the Étage E of the same eminent palæontologist.
These beds, as is well known, are referred by M. Barrande to the base of the Upper Silurian.
Without, however, entering fully into this question here, and disregarding the strong evidence which we now hold as to the Lower-Silurian age of our Graptolitic Mudstones, we would simply point out that M. Barrande himself fully admits that the succession of Silurian life in the Bohemian area was later, stage by stage, than in the northern European and British areas. Thus he supposed that the Bohemian area was peopled with a general Lower-Silurian fauna at a time when the North European and British areas were peopled with a general Upper-Silurian fauna, and he employs this supposition to explain the phenomena of the "Colonies." Admitting therefore that the "Colonies" and the lower portion of Étage E are Upper Silurian, it would in no way follow that the corresponding Graptolitic Mudstones of the north of England are also Upper Silurian. On the contrary, by M. Barrande's own theory, the Graptolitic Mudstones ought to be Lower Silurian, being thus "homotaxeous," but not "contemporaneous" with the lower part of Étage E of Bohemia. We thus see that the evidence to be derived from Bohemia, though apparently at conflict with our views as to the age of the Graptolitic Mudstones, is, when fully analyzed, an additional argument in favour of our conclusions.
4. Knock Beds.
The Graptolitic Mudstones are succeeded by a series of strata for which the name of "Knock beds" has been proposed (Nicholson & Lapworth, Rep. Brit. Assoc. Bristol, 1875), on account of their excellent development in Swindale Beck, near Knock. Lithologically the Knock beds present a singular uniformity wherever they are found, enabling them to be recognized with the greatest readiness. They consist principally of pale green, fine-grained slates, extremely ashy in their appearance, exhibiting numerous dendrites, and very commonly containing crystals of cubic pyrites. Along with these greenish slates are often well-marked bands of red and purple slates of the same grain and texture as the preceding; and occasionally there is met with a thin band having a grey or even a nearly black colour, though this is exceptional. The general strike and dip of the Knock beds conform with those of the underlying Graptolitic Mudstones; and there is, at present, no clear evidence of any want of conformity between the two groups.
The Knock beds have been subjected to a varying but always a high amount of cleavage; and fossils, though by no means wholly unknown in them, are almost always more or less distorted. At Knock they have yielded only two Graptolites, viz. Monograptus priodon, Bronn, and M. broughtonensis, Nich. & Lapw. At Skelgill, and on the high ground between this place and the vale of Troutbeck, they have afforded many specimens of small Brachiopods (Orthis and Discina) and minute Lamellibranchs. These, however, occur only in the form of external and internal casts; and, as yet, no conclusion has been arrived at concerning their specific forms.
The Knock beds are directly surmounted by the well-known and well-marked group the "Coniston Flags," a series of strata corresponding to the "Denbighshire Flags" of North Wales, and which have been clearly shown, more especially by the researches of Prof. Hughes, to be of Upper-Silurian age. As to the exact age of the Knock beds, it is not to be denied that sufficient evidence is yet wanting on which to found any positive or final opinion. They rest upon the Graptolitic Mudstones, which we have shown to be placed nearly or quite at the summit of the Lower Silurian; and they are overlain by the Coniston Flags, which are quite or nearly the base of the Upper Silurian. It is therefore clear, from their physical position, that the Knock beds must be either the basement series of the Upper Silurian, or the summit series of the Lower Silurian, or a group of passage-beds between these two.
The palæontologieal evidence at present obtained is not enough to justify us in adopting definitely any one of these hypotheses. So far as it goes, the evidence tends to favour the view which regards them as the base of the Upper Silurians—the only two species of Graptolites observed being forms common to the overlying Coniston Flags, whilst there appears to be a complete absence of the genera and species characteristic of the Graptolitic Mudstones.
Further researches, however, will undoubtedly add to the fauna of this group of beds, and enable its position to be determined with greater precision.
In the meanwhile it can only be said that the conclusion to which the few known fossils point is corroborated by the strong lithological resemblance between the "Knock beds" and the "Tarannon Slates" of Wales. We cannot, therefore, be far wrong in provisionally regarding the Knock beds as the base of the Upper Silurian series of the Lake-district, in which case the Graptolitic Mudstones will constitute the highest portion of the Lower Silurians of the same area.
Appendix.—The Irish Representatives of the Coniston Limestone and its associated Rocks.
Lambay Island and Portraine, co. Dublin.
On referring to a geological map of the British Isles it will be seen, from the strike of the Coniston Limestone in the southern portion of the Lake-district, and after it disappears under the newer rocks west of Millom, that its occurrence might be looked for in Ireland, where rocks appertaining to the Bala series are seen.
On the coast of the co. Dublin, the nearest locality where such rocks could be expected to appear, they are absolutely seen in the direct line of strike of the Coniston Limestone of the north-west of England.
On Lambay Island, two and a half miles east from the mainland, in the south-east portion of the island, we have part of the Coniston series forming a synclinal, though exhibiting many of the same features and the same fossils which this group of rocks in the Lake-district affords. Here are grey limestones succeeded by concretionary bands called by Mr. Du Noyer "coarse Conglomerates" (Explanations of Sheets 102 & 112 of the Maps of the Geological Survey of Ireland). This "Conglomerate" is described as "composed chiefly of rounded pebbles and boulders of grey Silurian Limestone, either fossiliferous or not, with fragments of dark cleaved state, grey grit and greenish grey greenstone, and ash: in one instance a boulder of an older Silurian Conglomerate was discovered, in which were rolled pebbles of a dark green close-grained greenstone, the base being a grey limestone containing Silurian fossils." (Note by Mr. Jukes:—"Some of the Silurian corals were attached by their bases to the pebbles, showing that they had grown on them, just as corals may now be seen growing on pebbles or fragments of rocks along a tropical shore.") "The matrix of the Kiln-Point Conglomerate is a black mud; and throughout the deposit are irregular slaty layers." "When we get lower into the mass we lose the conglomerate, and find nothing but pure dark grey slates, which, near Raven's Well, are found to contain Graptolites and thin calcareous fossiliferous bands" (p. 48).
It is also stated that at Kiln-Point the "grey Silurian Limestone is a wedge-shaped mass of lumpy layers, with thin bands of dark grey earthy shales between them, all very much contorted and resting on the porphyritic greenstone, which has evidently come up under them while in a pasty condition from heat, as it sends veins and strings into the lower beds of the limestone, and often enclosing fragments derived from it." Other circumstances indicate volcanic activity during the deposition of these limestones, even to a greater extent than has been recognized in the Lake-district.
These limestones have below them rocks intimately related to those of the Borrowdale series of the north of England, which are doubtless the equivalents of the latter.
At Portraine, on the mainland, immediately opposite Lambay, there is seen, on the coast, one of the finest sections of the Coniston Limestone and its associated rocks which occurs in the British Isles.
Portraine is about two miles east of Don abate Station, on the Dublin and Drogheda Railway. On reaching the coast an exposure of rock is seen a little east of the coast-guard station. This consists of a purple conglomerate, largely made up of quartz pebbles, which has been designated Old Red Sandstone; but it is more probably the basement conglomerate of the Carboniferous formation. As such it has its representatives well exposed under the Carboniferous Limestones along the eastern margin of the Lake-district, and also under the Pennine escarpment.
A short distance east of the conglomerate a porphyry, having a delicate purple tint and containing well-developed crystals of a greenish white felspar, occurs. This porphyry is similar to that which forms the bulk of Lambay. Its purple tint is doubtless due to staining from the purple conglomerate which, in some places, overlies it.
Fragments of a similar purple porphyry are met with in the Lake- district near Caldbeck; and in some spots this porphyry is seen in situ overlain by purple conglomerates, when it manifests the same tint as the Portraine porphyry.
The Portraine porphyry is succeeded by traps and ashes; and these are seen to occur to beyond the farm-house on the coast.
At the farm-house the coast trends for a short distance south-east. A little beyond this the ash-beds begin to exhibit trappean fragments in them, the ashes assuming the nature of ash-breccias. The trap fragments, however, are not the only substances which the ashes contain; calcareous nodules also make their appearance in them.
Patches of black shale, which show no traces of volcanic origin, are also associated with the ash-beds; and these black shales afford Graptolites (Climacograptus teretiusculus). These ash-breccias with calcareous nodules and graptolitic shales prevail in greater abundance higher in the series; and in the cliffs under the Martello tower they are seen to be succeeded by fine green-coloured shales so much affected by cleavage that their bedding can scarcely be made out. There are some small faults indicated by Mr. Du Noyer as occurring between where the porphyry is seen and where the green shales make their appearance. These, however, are not of sufficient importance to render the section difficult to interpret. The green shales contain limestone nodules in bands which, though much contorted, indicate distinctly the lines of bedding of the shales, these shales being, no doubt, of ashy origin. Fossils occur in these nodules and also in those which are found in the ash-breccia; and these fossils are distinctly of a Bala type.
The "conglomerates," before referred to as seen on the south-east side of Lambay Island, are a portion of the ash-breccia series. They afford the same fossils, and they have the same graptolitic shales accompanying them.
As regards the horizon in which the nodular ash-breccias occur, as compared with the rocks of the north of England between the Borrowdale group and the Coniston Flags, it would seem that they must be placed on a parallel with those of Style-End Grassing and with the more amply developed Dufton shales. To the latter they are in part allied by their black shales, and to the former by their ashy nature. They indicate more violent volcanic activity in the locality of their deposition than the Dufton shales, but less powerful igneous influences than in the case of the Style-End Grassing ash-beds, where no black shales occur.
Nodular limestone bands have been referred to as making their appearance in the highly cleaved and contorted green shales. Near the top of these shales the limestone bands become more predominant, and the nodules gradually change into continuous layers succeeded by thicker strata, the whole forming a finely developed mass of Coniston Limestone, in some parts very fossiliferous. The higher beds of the series, however, again assume a nodular form; and at the top of the group a very singular deposit is seen. This bears some resemblance to the nodular ash-breccias; the nodules, however, prevail to a much greater extent.
This bed is so conglomeratic in its aspect as to appear, at first sight, a mass of this nature overlapping the Coniston Limestone. This, however, is clearly not the case, as there occurs underneath this curious nodular bed a mass of a somewhat similar kind, the two being separated from each other by apparently ashy shales in which nodules of limestone are seen.
This singular nodular mass is probably near the horizon of the band of limestone which occurs at Shap Wells as a calcareous breccia.
Dark-coloured rocks are seen resting conformably on the nodular limestone. In their nature these rocks have a great affinity to the Graptolitic Mudstones of the Lake-district. They occur in cliffs which are very inaccessible, and therefore could not be well searched for fossils. Judging from their mineral aspect (and this is a well- marked feature in the Graptolitic Mudstones), we are disposed to refer them to this position.
The rocks south of the Coniston Limestone of Portraine form a synclinal trough. Here the strata are more accessible than where the black shales occur. They consist of fine-grained greenish-coloured shales, having dark grey rocks intercalated with them. The former, as regards their nature, are identical with the green rocks of the Knock beds. Their position also allies them with the latter strata; and they are succeeded by hard grey rocks having a close resemblance to the Coniston Flags. Taking the Portraine section collectively, the nature, the arrangement, and the fossils contained in the rocks here are such as to justify us in referring them to positions lying between the Borrowdale group and the Coniston Flags of the north of England.
To the south of the synclinal alluded to, the continuity of the Portraine section is broken, the coast being for a short distance sandy.
Beyond this sandy area the Bala rocks again appear in the form of ashes and traps, the latter being the most abundant.
Grange Hill and the Chair of Kildare.
The line of strike before alluded to, if continued south-west from Portraine, would bring us to another area where the Coniston Limestone and its associated rocks are seen. This area is in the co. Kildare, about three miles north of the town of Kildare; and here the rocks are, in part, well exhibited. Their nature and arrangement have been described by the officers of the Irish Geological Survey (description of Map 35, N.E.). The hill of Grange, where these rocks are best seen, consists principally of porphyries, traps, and ashes, the latter at one spot affording abundance of fossils of Bala age. The porphyries have a great affinity to those of Portraine and Lambay; and the traps and ashes are intimately related to those of the east coast of Ireland.
The western side of the hill of Grange consists of Coniston Limestone, well seen in the portion of the hill known as "the Chair of Kildare" and its immediate surroundings. Judging from its strike, this limestone would appear to be brought against the underlying igneous rocks by means of a fault. Although the limestone and igneous rocks are well exposed, the strata which overlie the former cannot be recognized in the hill of Grange.
Though provided with the six-inch Map, on which Mr. Du Noyer recorded his observations, through the kindness of Prof. Hull and Mr. O'Kelly of the Irish Geological Survey, the rocks succeeding the limestone could not be determined by us. The occurrence of dark shales about this position was indicated on the Map, but the places where they are represented no longer show them.
The exposures seem to have been in ditches; and they are now covered up. On the west the Coniston Limestone is cut off by a fault, "the last traces of it being seen on the south-west brow of the Chair hill."
A short distance south-west of the Chair, a hill called Dunmury occurs. The composition of this hill is altogether different from that of the hill of Grange, neither limestone nor volcanic products being found in connexion with it.
On the eastern side of this hill, and on the western side of the road to Kildare, near a well, a very dark-coloured compact rock, exceedingly like the Graptolitic Mudstones, is seen; and rocks of a like nature occur westward.
The exposures of rock on this hill, however, are very poor; Mr. Du Noyer records, on the map, the appearance of purple and greeu rocks on the west of the road south of the well above referred to. These cannot now be seen; and it is probable they were exposed when the surface of the road was levelled. Judging from Mr. Du Noyer's observations, these purple and green rocks appear to be nearly akin to the Knock beds. Should this be the case, and taken in connexion with the occurrence near them of strata resembling the Graptolitic Mudstones. it would seem that the rocks of Dunmury hill represent these two series.
It is also to be remarked that the outline of this hill differs greatly from that of the hill of Grange.
The contour of Dunmury bears a much greater affinity to the hills of the north-west of England composed of rocks above the Borrowdale series than to such as are made up of members of this portion of the Bala group.
Discussion.
Prof. Hughes regretted that the more important fossils were not upon the table, as he thought an examination of them, or of the matrix, might suggest some explanation of the difficulties. He questioned the discovery of Orthis vespertilio, Trinucleus, &c. in the Graptolitic Mudstones; and with regard to the sections drawn by the authors, he said he had not carried away quite the same impression of the stratigraphical position of the beds in the area described. He thought that the Coniston Limestone could seldom be considered as one distinct mass of limestone, but that concretionary bands of varying thickness and number appeared at various horizons in a mass of shale in which different fossils locally prevailed at different horizons. By this kind of evidence it was almost but not quite certain that the Graptolitic Mudstones and their basement-beds did rest on different parts of the Lower Series, e.g. at Skelgill, on the limestone bands, and near Coniston, on the Ash-Gill Flags. In the Craven district he had found a conglomerate at the base of the Coniston-Ulag Series, but no Graptolitic Mudstones. In the Sedbergh district a similar conglomerate seemed distinctly to underlie the Graptolitic Mudstones. He allowed that the facies of the Graptolites was very like that of the Lower Series, but pointed out that Barrande had got the very same group in his E, e, i at the base of his Upper Series.
He further pointed out that there were in North Wales two sets of pale slates, one near the top of the Lower Series, the other near the base of the Upper, and probably derived at second hand from older volcanic rocks. Only the Upper were well marked in the Lake-district; and these were the Knock beds of the authors.
Mr. De Rance stated that he had spent two years in mapping the rocks of the Volcanic (Borrowdale) series underlying those under consideration. He agreed with Prof. Hughes, that the fossiliferous calcareous band referred to by the authors belonged to the Coniston Limestone, and not to the underlying volcanic rocks, as stated by them. And he remarked that in tracing the outcrop of the Coniston Limestone across country, it was found to rest upon different and successive members of the underlying volcanic series, which plunge under it with varying direction of strike and amount of dip, the unconformity being so marked between the two sets of rocks that occasionally the volcanic series appear to have obtained a dip, been denuded, and faulted before the deposition of the overlying Coniston series.
Mr. Hicks differed from Prof. Hughes as to the value of the paper, which he regarded as at all events opening a question on which other observers might be induced to bring forward their views. He inquired whether Mr. De Rance's statements showing unconformity at the base of the Coniston series did not conflict with the views of Prof. Hughes. He thought that there was no occasion to be surprised at repetitions of beds in such a district. In some sections there is no visible unconformity between the base of the Upper and the top of the Lower Silurians, whilst in others a real or apparent unconformity occurs. He expressed much surprise at some of the fossils from the Mudstones, and remarked that Trinucleus fimbriatus is a true Lower-Silurian form.
Mr. Etheridge remarked that, as many geologists have worked over the area referred to in the paper, and have differed so much in their results, it is very difficult to come to any definite conclusion. The Mudstones possess a fauna belonging to the Caradoc beds and low down in the Bala series. He agreed with Prof. Hughes that there are more than one series of pale slates. It occurred to him that there must be some mistake in the determination of the species in the Graptolitic Mudstones; and yet all the interest of the paper hinges upon them. The Coniston Limestones have a well-recognized position.
- ↑ Idocrase or Vesuvian, as the latter name implies, is a rather common mineral in connexion with the volcanic products of Vesuvius, being often found in a crystalline limestone ejected from this volcano. It also occurs in the metamorphic Lower Silurian Limestone in Glen Laion, Aberdeenshire, and likewise in the metamorphic Liassic Limestone in the Isle of Skye. In Ireland it is met with among the metamorphic Lower Silurian Limestones of Donegal, at Derrylougharn, Barnes Gap, and elsewhere in that comity. Its occurrence at Wastdale Head is the first instance of this mineral having been found in England. It consists of silicates of lime and alumina, with small portions of oxide of iron and magnesia—a composition very likely to result from metamorphic action on a limestone such as the Coniston Limestone.