Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/432

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Dr. Mac Culloch on the Geology of

But to return to our subject. We have here an example of the columnar form existing in a very perfect manner in a rock, which however it may bear a considerable analogy to greenstone, and through that to those rocks which more commonly exhibit the columnar shape, is still sufficiently distinguished from them all by the great predominance of the felspar. There is no difficulty in conceiving that the same causes presided at the formation of both, when we consider the similarity of geological structure and position which pervades all the rocks distributed under the heads of basalt, greenstone, and syenite.

The insulated position of this rock precludes all possibility of tracing its connection with the contiguous ones, and the chasm which exists between Ailsa and the neighbouring shores, is such as to insulate it as much in a geological view, as it is in its geographical position.

As yet nothing resembling it in structure has been observed in the neighbouring islands, nor on the main land of Scotland, yet the syenites and greenstones of the motley island of Arran, with the porphyry of Devar, and the extensive trap formation of the neighbouring shores, point out to us its natural affinities, and enable us to guess at a connection possibly once more intimate, but long since submerged in the depths of the sea.

I cannot entirely quit the Craig of Ailsa, without remarking that it possesses in a high degree one of the conditions requisite to the solution of the interesting problem of the earth's density. This is, the absolute uniformity of its structure, which, with the exception of the few basaltic veins above described, is of one unvarying rock. Its mass is also sufficiently considerable to fulfil another of the conditions required for this problem, while its form and situation would appear to afford sufficient facilities for measuring its solid contents.