422 Dr. Mac Cull ocH 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, w^hich 

 •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 neigh- 

 bouring 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 con- 

 ditions required for this problem, while its form and situation would 

 appear to afford sufficient facilities for measuring its solid contents. 



