Transactions of the Geological Society, 1st series, vol. 3/On a Bed of Trap in Birch-hill Colliery

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VII. Some Observation: on a Bed of Trap occurring in the Colliery of Birch Hill, near Walsall, in Staffordshire.

By Arthur Aikin, Esq. Secretary to the Geological Society.

Read November 20th, 1812.

Altough several instances have come under my personal notice in Shropshire and the adjacent counties, of the occurrence of trap rock in connection with the coal-formation, yet in all these cases the trap either forms the basis on which the coal strata rest, or is incumbent on them; or, in the form of dykes, fills up the fractures or faults. Being informed however by Mr. James (one of the members of our Society) that a bed of trap had been pierced through in several places in a colliery, at Birch-hill near Walsall in Staffordshire, which has lately come into his possession, I took an opportunity of visiting the spot in the course of the last summer, and beg leave to lay before the Society the result of my examination. On my arrival I found that the lower part of the works (in which the bed of trap is situated) was not as yet sufficiently freed from water to admit of actual inspection; but being furnished with a plan of the colliery and with registers of the strata pierced through in sinking the three deepest pits, having also an opportunity of questioning several intelligent miners who had themselves worked in the colliery, and of examining many tons of trap rock and of the adjacent beds which had been brought to the surface, I think that there will be found no material error in the following statement.

The surface of the colliery at Birch-hill (Pl. 12. fig. 1.) somewhat exceeds 83 acres; and is itself only part of a more extensive coalfield, the portions of which adjacent to the present colliery have been worked out so long ago as to preclude the obtaining any correct information concerning them. The ground rises with a very gentle slope on the east, falling nearly flat and becoming marshy on the west. On the south-western edge of the colliery is a low ridge, from 20 to 30 feet above the level of the marshy land at its foot, from 70 to 100 yards broad, and extending along the southern edge of the colliery for 3 or 4 hundred yards, till by degrees it coalesces with the general slope of the land, and is no longer distinguishable. This ridge is named the green rock fault, and forms the separation between the Birch hill colliery and an exhausted one adjacent to it on the south.

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There have been about twenty pits sunk at different times, more than half of which are in the south-western angle of the colliery; not one of these has reached the bottom of the coal-formation, and therefore the rock upon which it rests is unknown: I apprehend however (from the general analogy of this part of the country) that the fundamental rock is flœtz-limestone.

The following is a list of the Strata in pit B, from the surface to the greatest depth that has been hitherto attained, viz. 216 feet.

Ft In.
1. Soil and gravel 4 0
2. Blue and grey clunch with balls and measures of ironstone 12 6
3. Coal (not worked) 2 0
4. Clunch mixed with ironstone, and including a bed of the same 15 inches thick 35 6
54 0

The open-work, and the six pits called by the miners Stone-pits (marked a, a in the plan) are for the purpose of procuring the ironstone contained in the beds ahead) mentioned, and therefore on an average do not exceed the depth of 54 feet.

5. Coal (not worked) 0 10
6. Fire clay 5 10
7. Rock binds (slaty clayey sandstone) 27 2
8. First coal or Yard-coal 2 2
80 2

The Yard coal is the first coal which has been worked: it is soft, and applicable only to domestic purposes or to the roasting of the ironstone: its thickness varies from 26 inches to a yard. The three pits marked d in the plan are sunk into this bed: their average depth does not exceed 90 feet.

9. Sandstone, the upper part of the bed slaty, the lower compact 38 4
10. Strong coal 4 1
This is the second workable coal; it is of good quality and is coaked for the use of the furnace.
11. Alternations of compact sandstone, slaty sandstone, slaty clay, and clay 28 2
12. Coal and bituminous slate (not worked) 1 6
13. Brown clay 11 0
14. Sandstone and indurated clay 5 6
15. Black slaty clay in closing three beds of ironstone of 9, 8, and 5 inches in thickness 10 10
16. Trap (Green rock) 12 0
200 7

The pits A & C are sunk into the trap, the one to the depth of half a yard, the other to the depth of five yards, but neither of them passes through this bed.

17. Indurated sandstone 3 0
18. Carbonaceous slaty clay and blind coal 4 4
19. Slaty clay 1 6
20. Strong hard bituminous coal 4 6

The above two beds of coal, with the thin interposed bed of slaty clay, form the third coal which is worked.

21. Fire clay 2 0
218 21

The Engine pit, the Bye pit, and the pit marked B, terminate in this bed.

Besides the green-rock fault, already mentioned, there is another which passes through the colliery longitudinally in a direction E. by S. and W. by N. This fault is a mere dislocation of the strata causing a depression of 72 feet to the S.: a smaller fault or Rider branches off nearly due W. from the larger one causing a further depression of 4 feet in the strata which lie immediately to the S. of it. The junction of these two faults takes place precisely under the Furnace.

The beds on each side of the central fault rise nearly S. at an angle of about 6°; but, in the immediate vicinity of the green-rock fault, the second coal and all the beds lying above it (as far as they have been explored) are thrown up at an angle so rapidly increasing as, in the space of about 100 yards, to amount to 25°.

None of the beds are known to vary materially in thickness except the trap. The thickness of this latter in the Engine pit and in the Bye pit amounts to 24 feet, but in the pit B it is diminished to 12 feet, and in the pit D which is sunk down to the third coal the trap is wholly wanting. The miners themselves conclude from these facts, and apparently with reason, that the bed of trap is merely a great wedge from the green-rock fault which has intruded itself between the proper coal strata, but is by no means co-extensive with them.

To the geologist the circumstances connected with the relative situation of the trap and with the state of the beds that lie immediately above and below it are of singular interest; and it happens fortunately that the two most interesting of these beds, namely the indurated sandstone and the blind coal, have been explored to the distance of several yards. This was effected in driving a heading for ventilation from pit A to the pit B, see section Pl. 12, fig. 2.: this heading was begun in the first or yard coal, but on passing the great fault (which was then first discovered) the miners found themselves unexpectedly in the upper bed of the third coal (i.e. the blind coal), and in this they continued to drive forwards the heading to pit B, a distance of about 100 yards.

I now proceed to a more particular description of the trap and of the adjacent beds.

The colour of the trap is a dark bluish green; it has a glimmering lustre from the intermixture of minute, shining, crystalline laminæ; its fracture is uneven, and it breaks into irregular wedge shaped blunt edged fragments; it is tough, acquiring a kind of polish under the hammer, moderately hard and rather heavy. It attracts very strongly the magnetic needle, but does not exhibit any signs of polarity: it effervesces moderately on being immersed in cold dilute muriatic acid, and on examination with a lens appears to consist of felspar, of calcareous spar, of minute shining black grains, and of a brownish, blackish, and bluish green substance, which I suppose would generally be considered as amorphous hornblende: it is to be remarked however of this latter substance that it exhibits no appearance of crystalline laminæ, and that if a piece of the entire rock be digested in boiling dilute nitro-muriatic acid this green matter is almost entirely dissolved with considerable effervescence; and the stone assumes a greyish white colour consisting almost wholly of crystalline laminæ of felspar.

This trap or greenstone is penetrated by contemporaneous nearly vertical veins of calcareous spar, from the size of a mere thread to about half an inch in thickness. The rock, although very compact in its recent state, after a few weeks exposure to the air, acquires a liver-brown colour and crumbles to pieces.

The slaty clay, with subordinate beds of ironstone, which lies above the greenstone, differs but little from the common slaty clay of the coal formation. This however is by no means the case with the three beds that lie immediately below the greenstone, namely, the indurated sandstone, the carbonaceous slaty clay, and the blind coal.

The indurated sandstone is of a yellowish grey colour, dense, compact and ponderous; it consists essentially of fine grains of quartzy sand and silvery mica, and contains very slender imperfect veins of calcareous spar, and small irregular granular concretions of ferriferous carbonate of lime: these concretions decompose by exposure to the air, and give the external surface of the stone a brownish yellow ochry tinge. In the specimens that have come under my notice, the surface of the greenstone where it is in contact with the sandstone, is of a brown colour, and decomposed to the depth of an inch or more; but I have not observed any direct mutual penetration of the two beds. The lower part of the sandstone, however, is considerably broken, and angular pieces of the carbonaceous slaty clay (technically called batt) occur, completely involved in the sandstone.

The characters of the batt, if taken from that part of the bed where it is not covered by the greenstone, are those of common bituminous shale or soft slaty clay, inclosing seams of pitch coal about one-fifth of an inch in thickness: but wherever this bed is covered by the greenstone, the slaty clay is considerably indurated, the coal is also harder and more compact, and possesses a brilliant semi-metallic lustre; and both the clay and the coal are wholly destitute of bitumen.

Below the batt lies a coal four feet thick, which, where covered by the greenstone, has a shining somewhat iridescent lustre; it is entirely destitute of bitumen, and when put in the fire burns rapidly, like common cinder, differing materially in this respect from the Culm of South Wales, or the blind coal of Kilkenny; but where this coal is not covered by the greenstone, it exhibits the usual characters of common bituminous stone coal.

The beds below this coal differ in no material respect from similar beds in other collieries.

Hence it appears that the Birch-hill colliery presents the following important facts: First, the existence of a bed of greenstone interposed between the usual strata of the coal formation, but not co-extensive with them; and secondly, that the coal and bituminous shale, where they are covered by the greenstone but protected from actual contact with it by an indurated sandstone a yard in thickness, differ materially in many respects, but chiefly in being deprived of bitumen, from those parts of the same beds where they are not covered by the greenstone. The works have not indeed been sufficiently opened to demonstrate that the changes just mentioned are strictly co-extensive with the greenstone, yet I think we may infer by fair analogy that such is the case; and that the greenstone is necessarily concerned in bringing about these changes.

Professor Jameson, in his mineralogical account of Dumfriesshire, mentions beds of greenstone occurring in the independent coal formation; and I at first took for granted, that the greenstone above described was also a true bed. On further consideration, however, I am rather inclined to adopt a contrary opinion, for the following reasons:

In the first place the green rock fault being composed of precisely the same kind of greenstone as the bed is, renders it probable that the one is a continuation of the other; and this is still further confirmed by the gradual thinning out of this bed as it recedes from the fault; for in the pit B, the most distant one that has been as yet sunk through the greenstone, this bed is 12 feet thick, while in the engine and bye pits, which are the nearest to the fault, the same bed is 27 feet thick. The actual junction of the greenstone of the bed with that of the fault has not indeed been proved, but from the very little unexplored space at present between them, there can exist but little doubt I imagine as to the fact. Now, though it is possible that the outburst of the greenstone (supposing it to be really a bed) may so coincide with the elevation and curvature of the ridge as, with a thickness of no more than about 80 feet, to give the appearance of a rock 60 or 100 yards across, yet this hypothesis is scarcely consistent with the rapid increase of the angle at which the strata above the greenstone bed are elevated in the vicinity of the green rock fault, advancing in a few yards from 6° to 25°. The absence of the greenstone bed in the pit D. though it is full as near the line of bearing of the fault as the engine and bye pits are, is a further support of the opinion that I have hazarded. Upon the whole, then, I am inclined to consider the green rock fault as a fissure in the coal field, filled up by greenstone, and the supposed greenstone bed as a wedge-shaped prolongation of the same.

With regard to the mode in which this and similar fissures have been filled, whether by deposition of the constituents of greenstone from solution or suspension in a superincumbent aqueous fluid, or by the bursting from below upwards of earthy matter, either melted or in the state of boiling hot mud, like the mud-volcanoes of Mexico and of the island of Taman in the sea of Asof, I shall not pretend to decide.