The South Staffordshire Coalfield/Chapter 8
CHAPTER VIII.
Description of the Rocks—continued.
Igneous Rocks.
There are two varieties of igneous rock in the district; namely, Basalt and Greenstone, and another apparent variety, differing greatly from both in mere external characters. All three, however, are different parts of the same great mass of molten matter, their variety depending probably more on the conditions under which they have been placed than on any difference in the origin or mineral constitution of the rocks themselves.
The "Rowley rag" is a Basalt, a bard, heavy, black, close-grained rock, weathering brown outside, having a tendency to form spheroids that envelope with several concentric coats a solid ball in the middle, and consequently often assuming a columnar structure, which in some instances becomes nearly as regular as that of the Giant's Causeway. This is the stone of the Rowley Hills. Barrow Hill at Pensnett. Pouk Hill at Bentley, and other spots.
Of this rock an analysis has lately been made by Mr. Henry of London, for Mr. S. H. Blackwell, of Dudley, who has kindly placed it at my disposal.
Composition of Rowley Rag (specific gravity 2.907).
| Silica | 49.860 |
| Alumina | 12.750 |
| Lime | 8.710 |
| Magnesia | 4.395 |
| Protox, iron | 11.380 |
| Perox. iron, with manganese | 3.360 |
| Soda | 5.250 |
| Potash | 0.570 |
| Titanic acid | 1.330 |
| Phosphoric acid | 0.580 |
| Water | 2.560 |
| 100.745 |
Burrowing in the Coal-measures, and here and there coming out to the present surface, is another igneous rock called " green rock" by the colliers. This is generally, if not always, the true "Greenstone" of mineralogists, composed of orthoclase and hornblende,[1] sometimes fine-grained or compact, sometimes largely crystalline. It contains sometimes fibrous radiated masses and plates of some zeolitic minerals.
From this Greenstone proceed dykes and veins of "White rock" trap, which at first sight might be mistaken for a compact sandstone, but when closely examined bears a greater resemblance to a partly decomposed, white, compact, and somewhat earthy felstone. In some places little shining facets of feldspar may be detected in it. The colliers all unite in stating that this "white rock" proceeds from the "green rock," and though I have never myself seen the junction of the two rocks. I have no doubt of the fact. That the "white rock" is an igneous rock, as well as the "green rock," is proved by its cutting through the coal and other beds in the same way that the green rock does, and by its producing the same amount of alteration in them.
A specimen of this trap was analysed by Mr. Henry, which gave the following results:—
| Silica | 38.830 |
| Alumina | 13.250 |
| Lime | 3.925 |
| Magnesia | 4.180 |
| Soda | 0.971 |
| Potash | 0.422 |
| Protox. iron | 13.830 |
| Perox. iron | 4.335 |
| Carbonic acid | 9.320 |
| Water | 11.010 |
| 100.073 |
This composition shows us at once that the rock cannot belong to the siliceous class of the Felstones or Trachytes, but is one of those basic compounds to which the Greenstones, Diorites, and Dolerites belong. Moreover, the presence of so large a percentage of carbonic acid and water shows that the rock has now a different composition from that which it possessed before the period of injection, when it was in a molten condition. It is, in fact, an altered rock, altered by having had so much carbon, oxygen, and hydrogen added to it as to give it a percentage of 20.330 of carbonic acid and water. If we deduct those materials from Mr. Henry's analysis, and reduce the remainder to percentages, we get—
| Silica | 48.8 | ||
| Alumina | 16.7 | 22.1 | |
| Peroxide of iron | 54 | ||
| Lime | 4.8 | 29.0 | |
| Magnesia | 5.2 | ||
| Soda | 1.2 | ||
| Potash | 0.5 | ||
| Protox. iron | 17.3 | ||
| 99.9 | |||
A composition quite sufficiently near to that of a basic kind of greenstone or basalt for us to look upon it as one.[2]
I shall not venture to intrude into the province of the chemist so far as to speculate on the way in which these additions were given to the rock, except by observing that the rock penetrates coal, and that the three ingredients acquired, namely, carbon, hydrogen and oxygen, are the essential constituents of coal.[3] Neither will I attempt to decide whether the infusion of these materials into the mass of the rock took place immediately on its injection into the coal, and while it was cooling and consolidating, or whether it was the result of the slow and gradual process of purely aqueous infiltration during long subsequent periods.
Basalt.
Rowley Hills—Of the basaltic rocks, the largest exhibition is that on the summit of the Rowley Hills. A capping of columnar basalt here rests upon the Coal-measures, over an irregular space two miles long by more than a mile in width at one part. This capping seems to be of irregular thickness, as it is largely quarried at some places on the side of the hills at least 100 feet below their summits; while on the line of the new Netherton tunnel of the Old Birmingham Canal no basalt was met with in any of the shafts, not even in the highest, which is very little below the crest of the ridge.
These shafts, however, seem to have passed through some pale green rock, slightly schistose and ashy-looking, with a crystal of feldspar here and there in it. There are also to be seen occasionally, on the slope of the hill just below the basalt, some considerable beds of trappean breccia, or brecciated ash, containing rounded and angular fragments of igneous rock lying in a brown, rather ferruginous paste, that looks like the débris of a basaltic , rock.
Fig. 12.
Coxe's Rough, Basalt Quarry.
If these beds be connected with the basalt, which seems almost certain, it shows that the basalt was an eruptive[4] rock, poured out at the surface in the form of a sheet of lava, whether subaqueous or subaerial, immediately after the formation of the part of the Coal-measures which lie below it. These trappean breccias or conglomerates belong to and pass into the Coal-measures, and therefore the basalt of the Rowley Hills also belongs to and forms part and parcel of the Coal-measure series. That it was formed during the Coal-measure period seems also to be probable, from the fact that it has been subsequently affected by all the dislocations and other accidents that have happened to the Coal-measures.
The Netherton tunnel has lately been driven right through the base of the ridge between Oakham and Farnley Hill, running from Tividale to near the Gads Green reservoir. Eight or nine shafts were sunk from the surface along the line of this tunnel down to its level, which is that of the canal at Tividale, or 484 feet above the sea. Neither in the tunnel nor in any of the shafts was any solid basalt met with, though one or two strings or tortuous veins of trap seem to have been cut through in the tunnel.[5]
The mass of the rock passed through both in the tunnel and the shaft consisted of a reddish and greenish coloured clay or shale and sandstone, sometimes the argillaceous, sometimes the arenaceous character predominating. The sandstones often contained pebbles, and thus passed into conglomerates. It was only in the upper shafts, those numbered 5, 6, and 7, that the green ashy rock mentioned above seemed to have been got, so far as I was able to judge from an examination of the spoil banks.
The Thick coal has been worked round three sides of these hills, and it appears generally to dip under and pass beneath the basalt. On the north side only, however, has it been followed under the basalt, and that for but a very slight distance; and in every instance in which this has been attempted the coal has been found to be "blacked" and to be fractured, and frequently to pass into "rock and rig." A coal is said in this district to be "blacked" when, by its near proximity to an igneous rock, it has become so altered as to lose all its brightness, and nearly, if not quite, all its inflammability. It is not exactly coke, but is dull and earthy, and on exposure to the atmosphere is very friable. It frequently in this state contains small nodular concretions of iron pyrites. I have never had the opportunity of examining a pit where the coal was thus affected, except Mr. Percy's pit, now belonging to Dr. Percy, at the Grace Mary colliery, a little east of Rye Cross Farm. Here, at a depth of about 200 yards, they came into blackened coal, penetrated by long dykes of white rock trap, and more or less intermingled and mixed up with white sandstone. This white sandstone was full of little patches and shreds of coal, and the coal was frequently entangled in the sandstone, and the two mixed up together in a very singular way. This kind of sandstone is that called by 'the colliers "rock and rig" In one of the gate-roads of this colliery the white trap ascending from the floor, or descending from the roof, cut both into the coal and the sandstone, in the manner shown in Fig. 13.
Fig. 13.
a The white-rock trap.
b The coal.
c White sandstone (rock and rig).
This was drawn to scale with a measuring tape, and is sufficiently accurate representation of the facts. A little north-west of this part of the mine a fault was met with, beyond which the Thick coal was found uninjured either by trap or by "rock and rig."
On a subsequent visit to these pits in 1858, I examined, with Mr. Cooksey, some gate-roads that had been driven in a southerly direction, or towards the part of the ground which is capped by basalt. The sandstone called "rock and rig" came in in larger quantity in that direction, as before described, p. 50, and the veins of white rock trap became very numerous. In one of the gate-roads a horizontal vein of trap, about a foot, or a foot and a half, in thickness, ran horizontally and regularly in the middle of the coal for 40 or 50 yards, looking precisely like a deposited bed, but then suddenly cut up across two or three beds of coal, sent off a number of irregular strings and bosses into the coal, and then shot off again horizontally near the roof for some distance, see Fig. 14. The mass of this coal was a perfectly clean, bright coal, with brilliant surfaces on the face and joints. For a distance of 6 inches to 12 inches, however, from the trap the coal was "blacked," that is to say, it was dull and friable, with an earthy look, having neither the dice-like division into lumps, nor the brilliant shining surfaces which the rest of the coal had.
Fig. 14 will give the representation of these facts from a sketch made on the spot with as much care as I could draw it, the white part representing the trap, the black border round it the altered coal, and the mottled part the coal retaining its brilliancy.
Fig. 14.
SCALE FIVE FEET TO ONE INCH.
Fig. 15 is taken from another sketch made on the same occasion, showing the termination of a considerable mass of this trap, and a little isolated boss of it in the coal.
Fig. 15.
SCALE TEN FEET TO ONE INCH.
All parts of these galleries, indeed, were riddled as it were with these trap veins running here and there in the coal, and also in the sandstone, altering the coal to a greater or less extent according to the bulk of the trap, but producing very little apparent alteration in the sandstone.
The men in this colliery call this white rock the "door-case rock," because in one of the gate-roads formerly driven in the part of the coal which is generally devoid of trap, an upright vein of it crossed the gate-road like a wall, and appeared when cut through like a white door-case in the black coal.
From the fine tortuous and branching veins which this rock forms, it seems hardly possible to suppose that at the time of its injection it was not in a fluid state, and almost perfectly liquid. The distances to which these narrow veins run seem also to make it necessary to suppose that at the time of injection it had a temperature not merely just sufficient to melt it, but a much higher one, sufficiently high to allow of the loss of a considerable quantity of heat, and yet for the matter to remain still molten in its passage to very considerable distances from the volcanic focus. Doubtless the injection was a rapid one, but still some heat must be lost in the passage of these comparatively thin sheets of molten matter over such very considerable areas.[6]
Now, if masses, however comparatively small, having such an intense heat as above supposed, come in contact with a substance so easily affected as coal, we should at first expect it to be almost entirely consumed, or at all events altered to a much greater extent than it appears to have been. Our ideas, however, are derived from witnessing the effects of heat at the surface, and in contact with the atmosphere. Might not these ideas lead us astray, if we reasoned from them as to what would take place at some depth in the earth, unless we made allowance for the total or nearly total absence of air, and the effect of the covering of rock, which would greatly obstruct the dissipation of the constituent gases of the substance acted on, rendering that dissipation very difficult and therefore very small?
Is it altogether impossible that the nature of the alteration was rather the extraction of a certain portion of the hydrogen and oxygen of the coal, together with some of its carbon, and the combination of those substances with the oxides previously existing 1p the igneous matter, rather than the expulsion of those gases into the rocks above or below the coal? In this way the greatest amount of metamorphosis would be produced in the substance of the igneous rock, some part of the matter of the coal being actually absorbed into it and used up by it, and its constituents combined with its own as it passed along. The blackened margin of the coal about the igneous rock would then represent that small part of the coal that was altered by having some of its gaseous constituents sucked out of it, as it were, leaving a coating of nearly pure carbon as a non-conductor to protect the remainder of the coal.
The only explanation of the occurrence of the sandstone called "rock and rig" I can offer is, that it is part of a "rock fault," a mass of sandstone contemporaneous with the coal, as described before, p. 48. Whether its connexion with the trap be accidental or otherwise it is difficult to determine. I am inclined to think, from the frequency of the occurrence of these " rock faults" in the coal in the neighbourhood of the Rowley Hills, and their absence, so far as I know, at any great distance from them, and from "rock and rig" being found around Barrow Hill, that their connexion with the trap is not accidental. It seems possible that the volcanic focus from which was subsequently protruded the molten basalt, gave some indication of its existence, even at an early period in the formation of the Coal-measures; and that from some troubled action fracturing the rocks, and either generating springs that brought up sand from below, or causing currents, perhaps of considerable local intensity, in the water 'above and around, which might sweep in sand from a distance, sandy beds might be deposited that locally interfered with the production of the Thick coal.
We have as yet no means of ascertaining the focus or centre of eruption from which the basalt of the Rowley Hills was poured out, and from which the sheets and veins of greenstone and "white rock" trap proceeded. No central pipe or funnel has yet been met with, but, except in the Netherton tunnel, no large exploration has yet been continued beneath the basalt.
The deterioration of the Thick coal by the quantity of sandstone that seems to come into it as the hills are approached is one reason for its not having been followed as yet beneath them. The fear that the coal which may exist is partly destroyed by trap rock is probably another reason.
It may still happen that at some point round the Rowley Hills good unaltered coal may be found for some distance beneath the basalt, but at present all the known evidence is the other way.
Barrow Hill.—Barrow Hill, two miles west of Dudley, is another mass of basalt in every respect similar to that of Rowley, except in extent and in its position with respect to the Thick coal. The Rowley columnar basalt seems to he at a height of about 600 feet above the Thick coal, that of Barrow Hill is apparently at a much less height. If they are each contemporaneous with the beds in which they lie, it follows that the Barrow Hill trap is older than that of the Rowley Hills. Immediately east of the hill, it appears, from the marks of old workings, that a piece of Thick coal cropped to the surface, and was worked in open work along it. This piece lay in an angle between the Corbyn's Hall fault and another, which is supposed to run south of Barrow Hill. In the workings to the west of Barrow Hill the coals are found to be "blacked" as they approach it, and beds of "green rock" (horizontal dykes) are found between the measures to the east for a distance of about two miles.
Pouk Hill.—Pouk Hill, near Walsall, is another mass of columnar basalt of less size than Barrow Hill, but equally, or still more, interesting.
It lies altogether below the Thick coal and between the Fireclay and the Bottom coal.
It formed a small slightly prominent mound of about 100 yards across, but this is now nearly all quarried away. The quarry, however, in 1858, exposed the structure of the rock in an admirable manner, and showed that structure to be very interesting and peculiar.
In the centre of the quarry the columns, which were often very regular, were vertical over a space of some 20 yards in diameter. In the middle part of this space the columns continued vertical till they reached the surface, but on the western side the tops of these vertical columns curved in for the space of a yard or so, and were at last quite sharply bent inwards so as to have the transverse section of the columns facing towards the centre of the quarry. The line along which this curvature took place sloped obliquely down from the top of the quarry to the floor at an angle of about 35°, and above that line all the columns lay in a position more nearly approaching the horizontal than the vertical. The ends of the columns slightly curved up along the line of junction so as to become nearly parallel to the bent tops of the vertical columns, and rose in the other direction into angle of about 20°, so as to suggest the idea of a radiation from this inclined line of junction.
Fig. 16.
Sketch of Pouk Hill Quarry as it was in October 1858.
On the eastern side of the quarry the columns likewise lay in slightly bent, but nearly horizontal lines, though here the change from vertical to horizontal was in one part more abrupt and in another seemed to have been more irregular. The state of the working of the quarry, too, obscured the relation of the columns on that side. A number of uncompressed spheroids of basalt appeared in one part along the line of junction of the vertical and horizontal columns.
On the south-east side of the quarry the black shales of the Coal-measures might be seen reposing on the basalt, which just below them consisted chiefly of uncompressed balls of basalt, bedded in a mass of decomposed basalt or basaltic "wacké" or clay. A small cutting for a tramway led from the quarry south to the canal, and following down this the Coal-measure shales containing ironstone were seen lying nearly horizontal. At one part these shales were traversed by a small dyke of white rock trap cutting across them at an angle of about 60° with the horizon, and rather more than a foot in width. Farther down this cutting, in consequence of a slight rise in the measures, the basaltic clay earth containing balls of basalt again made its appearance for a few yards, and then finally sank out of sight towards the south.
The columns of Pouk Hill, doubtless, assumed the radiating form above described in obedience to the rule they are known generally to follow, that, namely, of always striking from the cooling surfaces of any mass towards the interior. Thus, in a horizontal sheet the columns are vertical, in a vertical dyke the columns are horizontal, and in a spheroidal mass the columns seem to radiate from the centre, or in other words strike from all sides towards it.
Netherton.—A mass of basaltic trap is very well seen in the canal cutting south of Netherton church, where it is exposed by the rise of the beds below the Thick coal, and is seen to send wedge-like masses into the Coal-measure sandstones.
Greenstone.
Sheet-like masses of " green rock" (the local name for the greenstone) seem to spread almost uninterruptedly in the lower Coal-measures from the base of the Rowley Hills and Barrow Hill, through the centre of the district up to Wolverhampton. Bilston, and Bentley.
At Barrow Hill Coppice pits 64 feet of "green rock" penetrated the Gubbin-stone measures just beneath the Thick-coal. At the Birds Leasowe colliery, near Tansey Green, the Thick-coal was found to be "mingled with rock and rig;" and, below the Gubbin measures, "green rock" was found, into which they sunk 38 feet. Between Tansey Green and Shut End furnace the "green rock" is only 36 feet thick, and comes in in the place of the Heathen coal, the Thick coal itself being blacked. To the east of Barrow Hill a sheet of "green rock," which at first is more than 60 feet thick, but afterwards thins to about 30 feet, stretches for at least two miles in one direction. Its usual place is between the Heathen coal and the Whitestone measures, but between Cooper's Bank and the Fiery Holes the "green rock" cuts down under the Whitestone measures. This "green rock" crops regularly out like a bed on the rising ground west of Russell's Hall, and intrusive bosses of it rise to the surface at a spot on the western outskirts of the town of Dudley, at the Fiery Holes, at the east side of Cooper's Bank, and in the brook to the west of it.
At one of Lord Ward's pits at Tividale (about a quarter of a mile north by west of Coxes Rough) they found, at a depth of 500 feet, the following beds:—
| FT. | ||
| 1. | Coal mingled with rock, representing Thick coal | 24 |
| 2. | White rock, sandstone | 33 |
| 3. | Strong rock | 9 |
| 4. | Binds | 6 |
| 5. | Gubbin measures | 2 |
| 6. | Coal mingled with rock (represents Heathen coal) | 4 |
| 7. | Green rock, sank into for | 27 |
The usual distance between the Thick coal and the Heathen coal is only 12 or 14 feet, instead of 54, as here; Nos. 2 and 3 being quite unusual measures.
At Dudley Port, in Mr. Bagnal's limestone pits, they passed through 15 feet of "green rock" in the lowest sandstone of the Coal-measures, just before entering the Silurian shale.
At Tipton Moat colliery and at Tipton there were 34 feet of "green rock" at a distance of 36 feet below the Bottom coal. At Deepfields they got 20 feet of 'green rock" at a depth of 39 feet below the Bottom coal. At Highfields they found the " green rock" at a depth of 66 feet below the same coal, and sank into it for 15 feet only. At Bradley, in Mr. Addenhook's colliery, they found the "green rock" 55 ft. 6in, thick at a depth below the Bottom coal of 24 ft. 6 in.
Near Bilston, in a pit in a field called Crabtree piece. Messrs. W, and J. W. Sparrow found 15 feet of "green rock" 22 feet below the Bottom coal; and at the Wallbutts colliery the " green rock" was struck 27 feet below the Bottom coal, and sank into for 10 feet.
No "green rock" has been seen cropping to the surface on the rise of the lower measures on the north-east flank of the Dudley and Sedgley ridge; nor, so far as I am aware, has any been met with east of Darlaston or Wednesbury, or about West Bromwich, either at the surface or under ground. A considerable boss of it is seen, however, in the canal bank between Moxley and the Broadwater furnaces, which must either belong to another mass or must cut up through the Thick coal and the beds above it to reach the surface. It may also have been met with in other situations in the workings of which the record is now lost.
In the district between Wolverhampton and Walsall "green rock" is frequently met with in sheets in the lower measures, varying in thickness from 15 feet to 80 and 90 feet. In the southern part of this tract it lies below the Bottom coal, but between Wolverhampton and Willenhall it cuts up through that coal) and to the north of that is always found above the Bottom coal, between it and the Fire-clay coal. A boss of it rises to the surface, cutting up through the New mine coal at the Heath colliery north-east of Wolverhampton, and a little north of that a large mass of it rises broadly out and forms the surface of the ground all around Wednesfield, as delineated in the map. In the Bentley district the basalt of Pouk Hill seams merely to be an irregular swelling of the bed of "green rock" that crops out a little further east along the bank of the canal. This bed is found in the underground workings for about half a mile to the westward of Pouk Hill, and for the same distance to the northward, varying in thickness from 20 feet to 40 feet, lying always just above the Bottom coal, which is often greatly injured by it. It does not extend as far east as the Birch Hills,[7] nor far north towards Bloxwich, nor beyond Clark's-lane to the westward, neither was any found in Messrs. Bates's pits at the Trentham colliery between Mumber-lane and Wednesfield.
Near Wolverhampton none has been found in the Rough Hills. Cockshutts, or Parkfields collieries. At the Chillington colliery it is found, however, sometimes above, sometimes below the Bottom coal, and varying from 15 feet to 30 fect in thickness.
I was assured by Evan Lloyd, the ground-bailiff of the Chillington colliery, that there were two distinct beds of "green rock" there, as in the two following sections:—
| Western part of the Chillington Colliery. | Eastern part of the Chillington Colliery. | ||||
| FT. | FT. | ||||
| 1. | Black and white ironstone | 3 | 1. | Black and white ironstone | 3 |
| 2. | Bottom coal | 12 | 2. | Green rock, about | 15 |
| 3. | Clunch | 24 | 3. | Bottom coal | 12 |
| 4. | Gubbin and ballsironstone | 8 | 4. | Clunch | 24 |
| 5. | Slums | 3 | 5. | Gubbin and balls ironstone | 8 |
| 6. | Green rock, about | 35 | 6. | Slums | 3 |
| 7. | Hard rock | 12 | 7. | Hard rock | 12 |
Notwithstanding these facts the Gubbin and balls ironstone was worked continuously over the whole colliery without meeting any green rock. In this locality, therefore, there could have been no cutting up of the green rock through the measures, though it is by no means sure that these two sheets may not have a connexion elsewhere.
I was assured also by almost every one engaged in the works of this neighbourhood that, notwithstanding the variations in thickness of the "green rock," there was no change in the total thickness of the measures; that, for instance, the thickness between the New mine coal and the Blue flats ironstone remained the same, whatever might be the variation in the thickness of the "green rock." In other words, it was affirmed almost universally that the "green rock" not only intruded between the measures, but obliterated a mass of beds equal to its own thickness. This assertion was so confidently made by almost every practical man in the neighbourhood, that, however incomprehensible. I should have received it as true, had not an analysis of the materials received from them enabled me to disprove it. It is no doubt founded in fact; the greater the thickness of the intruded trap rock, the more intense, probably, would be the squeeze, and the consequent contraction suffered by the adjacent beds. Beds of coal, too, might certainly be nearly or altogether annihilated by the intrusion of molten rock, but we cannot conceive sandstone or clunch being thus destroyed. The truth is, that the original thickness of the measures was itself very variable, and it probably happened that, in one or two of the places where the facts were first observed, 2 partial thickening or thinning of the trap rock compensated for the reverse in the original beds. It may also have happened that the very fact of there being a local thinning of some of the upper measures gave occasion for a corresponding thickening of the intruded trap rock. That, however, the assertion before mentioned is really not borne out by the facts as to all places, may be shown by the following deductions from the pit sections furnished me in the district.
In six shafts in the Stow Heath colliery, partly in the occupation of Messrs. Sparrow, partly of Messrs. Ward, we find the following values for the thickness of the "green rock," and for the thickness of the whole measures, including the " green rock," from the top of the New mine coal to the top of the Blue flats ironstone:-—
| Pit. | Green rock. | Total beds from New mine coal to Blue flats. |
| FT. | FT. | |
| 1. | 30 | 164 |
| 2. | 33 | 157 |
| 3. | 51 | 205 |
| 4. | 55 | 177 |
| 5. | 66 | 204 |
| 6. | 66 | 214 |
which shows that the total thickness does increase with the increase of the thickness of "green rock," although not regularly or in strict proportion, owing to the original irregularity in the thickness of the other measures.
In three pits at the Portobello colliery, just where the "hole" of "Moseley Hole," is engraved in the map, all three pits being within a distance of 264 yards, we get the following thicknesses:—
| Pit. | Green rock. | Total beds from New mine coal to Blue flats. |
| FT. | FT. | |
| 1. | 59 | 168 |
| 2. | 56 | 104 |
| 3. | 84 | 190 |
In which we find the increase in the total thickness to be very nearly in direct proportion to that of the green rock.[8]
In an open work in the New mine coal, some years ago, on Wednesfield Heath, a dyke of the white feldspathic-looking trap was seen cutting up into the coal from below, and ending in some black shale. In some quarries north of Willenhall the same rock may still be seen in veins cutting through the Coal-measures.
In the northern portion of the coal-field, north of Wednesfield, and Walsall, no trap rock is known to show itself at the surface of the ground, with the exception of a little spot of hard dark hornblendic trap at the Essington wood brick-kiln, only to be seen in a small quarry partially concealed by underwood, and surrounded by the red clays of the upper Coal-measures.
Much "green rock" was found in some old sinkings between Pool Hayes and the New Invention, obliging the works to be abandoned. I was informed that in some sinkings made by Colonel Vernon two or three miles north of Bloxwich on Essington Wood, the measures were found to be disturbed and altered by "white rock trap," to such an extent as to oblige them to abandon the undertaking. Large intrusive masses of this white trap also are found in the pits near Birch Hills Hall, north of Walsall. At Union colliery, north of that, the Bottom coal is cut entirely out by "green rock;" and at Goscott, still farther north, there are six yards of "green rock" resting directly on the Bottom coal. White rock trap just shows itself in the cutting of the Cannock railway south of the bridge in the lane leading from Landywood to the turnpike road. In the rest of the field I have found no trace of the rock having been met with, and it certainly has not been seen in any of the Brereton workings at its northern apex.
Time and Mode of Formation of the Trap Rocks.—There is nothing in the mineralogical constitution of, these igneous rocks that will give us any assistance in determining the geological period during which they were formed. I am not aware that the basalts of the Rowley Hills differ in any essential particular from those of the county Antrim (the Giant's Causeway, &c.), from those poured forth by existing volcanoes, or from basalt of any other period. The greenstone does not appear to differ -essentially from the rock so called, which was formed during any period, from the Lower Silurian down to the most recent.
There appears, however, to be this difference in the circumstances under which basalt and greenstone were at any time formed: that while basalt is found in the lower part of the lava streams of existing and extinct volcanoes, and is therefore capable of being produced by the cooling of a molten mass on the surface of the earth, greenstone is not known to have been ever found to be so circumstanced, but always in such situations as either prove it to be an intrusive and comparatively deep-seated, or, at all events, not a superficially formed rock, or at least render it probable or possible that it was so formed.
Whether the difference between basalt and greenstone can be accounted for solely by the difference of the circumstances under which they cooled and consolidated from a molten mass into a solid rock, is a question I do not pretend to decide. My own belief is, that those circumstances exercised a preponderating 4nfluence on the distinction between the two rocks, and this belief is supported rather than opposed by the facts to be observed in the South Staffordshire coal-field.
If the ashy-looking beds associated with the Rowley basalt be really of the nature of "ash" or "tuff,"[9] then it follows, as a consequence, that the Rowley basalt is part of an actual lava stream poured out at the surface, either into the air or into the water. The time of this ejection was apparently after the formation of 600 or 700 feet of Coal-measures over the Thick coal, probably after the deposition of the red coal-measure clays, and about the commencement of that of the Halesowen sandstone group.
The intrusive sheets, dykes, and veins of "green rock" and "white rock" trap which burrow in the lower measures about the Thick coal and beneath it on the northern side of the Rowley Fills, and seem to spread through the centre of the district as far north as Wednesfield and Bentley, and thereabouts, may have commenced their intrusions some time before the outpouring of the Rowley basalt, which, giving a sensible and large relief to the struggles of the volcanic matter below, may have terminated the igneous action.
There may also be seen, in the neighbourhood of Barrow Hill, rocks having the character of ash or trappean débris, and it ma be that that outburst of basalt likewise reached the surface, though from its position above the Thick coal it would appear in that case to have taken place at a rather earlier period than that of Rowley.
About Pouk Hill, there is no appearance of ash, and it seems to be so intimately associated with the horizontal sheet of greenstone, a short distance below, that I am more inclined to look upon that as a boss proceeding from it, not to the surface, but only towards the surface, and consolidating in an isolated mass at no great distance perhaps, below the surface of the rocks that existed at the period of its formation.
In all cases the isolated masses that are found in the higher parts of the Coal-measures are. I believe, basalt, while the wide spread horizontal sheets of igneous rock that spread over such large areas below, and occasionally make their appearance at the surface where the lower measures crop out, as about Wednesfield and to the east of Pouk Hill, are greenstone.
it is not by any means intended to insist too strongly on the superficial formation of the Rowley or Barrow Hill basalt, since except the occurrence of the ashy-looking beds, and the 'basalt being always above the greenstone, "there 3 is no conclusive evidence to show that these basaltic masses were not also intrusive sheets of igneous matter injected in between the beds of the Coal-measures.
If the basalts were outpoured lava streams, it of course follows that all the igneous rocks, taken az a whole, were contemporaneous with the Coal-measures, taken as a whole.
Even on the supposition of the igneous rocks being all intrusive, and therefore formed subsequently to the formation of the beds between which they now lie. I still think that we cannot assign a much later age to them, and that we shall be compelled to consider them as older than the Permian rocks.
The first argument in favour of this conclusion is a negative one, namely, that in this district no igneous rocks of any kind are found in any formation newer than the Coal-measures.
So long as the red clays of Essington Wood were considered to be of Permian age, this argument failed us as regards that formation, since there is a little boss of a peculiar kind of greenstone to be seen in a small quarry just west of the brickpits of that place. Now that these red clays are proved to be measures, however, this occurrence of igneous rock in them becomes no exception to the general statement given above.
The second argument is of a more positive character, and is this, that at whatever period these igneous rocks were produced, they were all existent before the production of the faults and dislocations that have traversed the Coal-measures, and before any great denudation had been effected on the country.
The northern end of the Rowley Hill basalt is distinctly cut off by the extension of the northern of the pair of Dudley Port Trough faults and the southern of those faults seem also to have affected it.
The sheets of greenstone that spread from below the Rowley Hills, through the centre of the district, up to Wednesfield and Bentley, always run pretty nearly in the same beds, at whatever depth those beds may be found, and however they may be broken by faults. This shows that the "green rock" is itself cut through by the faults and thrown up or down by them, as the case may be, exactly as the Coal-measures are affected by them.
The same may be said in general of the sheets of "green rock" proceeding at a higher level from Barrow Hill. In each case the "green rock" crops out to the surface, along with the beds in which it lies; although as its thickness is very irregular, and as it shifts its place now and then in the beds, cutting up or down within certain narrow limits, above or below a particular set of beds, that outcrop has a corresponding irregularity and want of continuity.
This proves the igneous rocks to have been equally and similarly affected with the Coal-measures by the two great actions of "dislocation" and "denudation."
It seems quite impossible to suppose that if the faults existed before the "green rock" was injected into the measures, that it would not have taken advantage of those fissures to have made its way to the surface along them, rather than have forced itself in among and lifted up and floated a thickness of several hundred feet of beds over an area of several square miles. But since it is known for a fact that it is itself dislocated by these faults, as shown in the Horizontal (Longitudinal) Sections (Sheets 23, 24, and 25), that fact is conclusive in favour of the rock having been cooled and consolidated before those faults were formed.
My friend, Mr. S. H. Blackwell, indeed, informs me, that in the district west of Russell's Hall, south of Lower Gornal, where there are a number of step faults close together, the green rock there was found to go up into those faults. It certainly did not go up very far along them, since I believe it did not injure or intrude into the Thick coal, which there lies at no great distance above the "green rock." It may have sent veins up into the superincumbent measures along certain lines of slight resistance, which afterwards were converted into faults; or it 1s possible perhaps, that small dislocations were then caused before the time when the larger and more general dislocation of the coal-field was produced. Or, lastly, with all respect to my friend Mr. Blackwell's acuteness of observation, (which no one will be more ready to acknowledge than I, who have so often profited by it,) he may have been led away by appearances which in the imperfect light of underground workings might have deceived any one. It seems to me quite possible that portions of the consolidated basalt may have been squeezed for some distance up into the fissures, and still more likely that some of the more decomposed parts (the clay or wacke, which often forms the covering of the hard rock) may have been so squeezed, at least as far as it was possible to follow it in underground explorations.
Whatever may be the exact state of the case with regard to this particular instance, the larger and more general fact remains undoubted, that there is no relation whatever either of cause or effect between the igneous rocks of the coal-field and the principal dislocations that have affected it; and that the igneous rocks all existed in and among the Coal-measures very much in their present condition before any of the great dislocations were commenced.
- ↑ On the authority of Sir H. De la Beche.
- ↑ M. Durocher, in his Essay on Comparative Petrology, gives the following as the mean composition of Basalt and Diorite, of which latter term we may consider Greenstone as the English synonym:—
Diorite. Basalt. Silica, with trace of Titanic Acid 53.2 48.0 Alumina 16.0 13.8 Potash 1.3 1.5 Soda 2.2 3.0 Lime 6.3 10.2 Magnesia 6.0 6.5 Oxides of Iron and Manganese 14.90 13.8 Loss by ignition 1.0 3.2 100.0 100.0 (Essay on Comparative Petrology by M. J. Durocher, translated from the Annales des Mines, vol. xi., 1857, by Rev. S. Haughton. Dublin, McGlashan and Gill.)
- ↑
According to Bischof, bituminous coal may be taken as essentially composed of—
Carbon 82.1 Hydrogen 5.5 Oxygen and Nitrogen 12.4 100.0 independent of a variable admixture of earthy matter forming the ash.
- ↑ This term of "eruptive" is sometimes applied to granitic rocks, which, however intrusive they may have been, clearly cannot have been eruptive, that is, can never have burst out to the surface. I would, therefore, by an eruptive rock mean only such a rock as could have been ejected out of the earth and poured over its surface, whether that surface was there covered by air or by water.
- ↑ Mr. Walker, jun., who superintended the works of this tunnel, was good enough to let me see a large section constructed from the workings, from which the above statements were taken.
- ↑ M. Delesse, in his "Etudes de Metamorphism," supposes in these cases that the intrusive rock had not such an intense temperature, but that it was mixed with water into a state resembling mortar. It does not appear to me, however, possible for a mere paste to be injected into fine crevices to such great distances as we find in the above and other cases.
- ↑ Beyond the Birch Hills furnaces, however, near Birch Hills Hall, trap is again found in the Coal-measures, about or above the place of the Bottom coal.
- ↑ I have insisted a little more strongly on this point than its real importance deserves, because it is a good illustration of the error into which purely practical men are so apt to fall, that, namely, of over-hasty generalisation from insufficient data. The charge of "theorising," as it is called, is so often brought as a criminal charge against scientific men, that it is but fair to show those instances in which theory necessarily leads to truth, and therefore to safety, in order to counterbalance those in which it may occasionally have led to danger or expense.
- ↑ By "ash" or "tuff" I would understand the débris of an igneous rock, either produced by the action of water at the time of its being poured out, or immediately after, or by the action of steam or other gases rushing from the volcanic focus and ejecting the debris into the water, or into the air so that it ultimately fell into the water.