Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/316

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Dr. Mac Culloch on the Junction of Trap.

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.