Transactions of the Geological Society, 1st series, vol. 2/On the Junction of Trap and Sandstone at Stirling Castle Rock

On the Junction of Trap and Sandstone at Stirling Castle Rock
by John MacCulloch

XIII. On the Junction of Trap and Sandstone, at Stirling Castle.

By J. Mac Culloch, M.D. Chemist to the Ordnance, and Lecturer on Chemistry at the Royal Military Academy at Woolwich.
V. Pr. Geo. Soc.

I HAVE herewith transmitted a sketch of one of those circumstances in the mutual relation of greenstone and sandstone, from which the Huttonian theory is presumed by its advocates to derive so material a support. The particular instance, of which this sketch is intended to give a general notion, has been lately brought to light, and has not, as far as I know, been observed by the geologists of Edinburgh. It does not indeed differ so greatly from the same class of facts in the neighbourhood of that city, as to require very particular attention; but I have been induced to preserve this notice, and drawings of it, (Pl. 12, 13) partly on account of its decided and clear disposition, and partly, lest the same operations by which it was first exposed, may, at no distant period, again remove or overwhelm it. Nevertheless, it may have its use in extending the analogy between those classes of rocks in which this appearance is found, and in rendering it probable that the same cause, whatever it was, presided at the formation of all similar phenomena.

The rock on which Stirling Castle is built, and on which the town also is founded, resembles so strongly that on which Edinburgh stands, that it would be superfluous to describe it very particularly.

It forms a large stair, having its escapement towards the west, and its inclination similar to that of the great rocks which are not only found in the vicinity of Edinburgh, but are to be seen rising above the general level through the whole interval between Edinburgh and Stirling. The hill of Stirling, and that of Craig Forth, are, I believe, the most western of this class of rocks, which are connected here, as at Edinburgh, with the coal district.

It was in cutting a new road through the castle hill that the appearance in question was laid bare.

The trap stratum consists of a dark blueish black compact greenstone, varying to umber brown, and it is accompanied by tufo; but the former alone is in contact with that part of the sandstone stratum which is exposed.

On inspecting the drawing, it will be seen that the sandstone stratum has been split into two parts in the direction of its stratification. The upper portion is then separated by a perpendicular fracture, and bent upwards, terminating abruptly. It is in this position involved, supported, and covered by the greenstone. The broken end is irregularly fractured, but all its cavities are perfectly filled up with greenstone. The different laminæ of which the sandstone stratum is composed, are not broken to accommodate themselves to this new position, but are irregularly waved and bent, preserving their continuity every where.

A little additional disturbance appears on the lower side of the bent portion, as if formed by the separation of fragments; but the drawing will render the appearance more intelligible than any description could do. The state of the rock did not, when I was there, allow me to examine the other portion of the upper part of the bed from which the broken end had been disrupted: future operations on the rock may hereafter lay bare further portions to illustrate this interesting appearance.

The sandstone bed itself consists of different laminæ, varying in colour, but generally slate-coloured, yellowish, and grey. The two lowermost and thickest beds differ little or nothing in texture, colour, or quality, from ordinary calcareous sandstones, and they are (as the drawing shews) of considerable thickness compared with those which immediately follow them.

These thinner beds are separated from each other by laminæ, of a clay slate, very much confounded with the sandstone, and those two substances alternate frequently and irregularly, till they approach the thicker beds which lie in contact with the greenstone. As they approximate to it they become more indurated, and assume a texture approaching to that of hornstone. The lowermost of the two upper beds thus exhibits a kind of hornstone of which the fracture is occasionally granular, and occasionally passes into the splintery and conchoidal. The uppermost bed exhibits the characters of perfect hornstone. It is of an ochre-brown colour, its fracture is conchoidal, or splintery, with an even shining surface, and the thin fragments transmit light.

As the specimens are before the Society, I need not enter into more minute details, particularly as, if taken together with the drawing, they will shew the regular gradation from the ordinary calcareous sandstone to the perfectly characterized hornstone. I may just be allowed to point out the resemblance which this change bears to that gradation from sandstone to a jaspideous rock, which occurs in a similar situation at Salisbury Craig. Considering merely the mechanical position of the sandstone, I confess myself unable to comprehend how by any combination of accidental abrasion or fracture, and successive deposition by precipitation, the present, and similar appearances can be produced. Although the projecting portion of the sandstone might have been able to maintain its place during the deposition of the greenstone bed, yet some other expedient must be resorted to before the curvature of the sandstone stratum can be accounted for. The explanation afforded by the Huttonian hypothesis is too apparent to require notice.

Whether this hypothesis be esteemed well founded or not, it must rest on a much wider basis than that of the mere phenomena which accompany the trap rocks. But if we consider those rocks with the appearances which I have now described, and which they so often exhibit, we may safely conclude that no hypothesis is competent to explain geological phenomena at large, which does not admit of the forcible displacement of the strata which accompany them, and on which the marks of violence are so evidently impressed.