94 Prof. Forbes on the Topography and Geology of the 



that half the solid is visible, and their diameter reaching to 

 50, and even to 100, feet. These are the only modifications 

 of external form visible in this rock, which in no case pre- 

 sents either the flat stratified disposition, or the tendency to 

 vertical fracture, so common in members of the trap family ; 

 being in every respect, except that of mineral structure, en- 

 tirely different from the ordinary varieties of this substance. 

 Its granitic aspect is still farther displayed in a most striking 

 manner in the spiry forms of the summits in their hard ser- 

 rated outline, and their overhanging masses, by which they 

 are rendered inaccessible even to the stags and the wild goats 

 that roam over this region of solitude and rocks. To this 

 they owe their highly picturesque aspect, which bears a 

 striking resemblance to the granite hills of Arran, or the 

 more stupendous gi'anitic masses of the Alps." * 



Now, in this extract, we find a suitable attention paid to 

 the marvellous spheroidal forms of which we have spoken ; 

 but the notion of their being due to mechanical action (na- 

 turally enough) never entered the mind of the writer ; the 

 force of the mechanism it required seemed beyond the limits 

 of even a geological possibility. He therefore falls back on 

 the idea of natural exfoliations and concretionary surfaces ; 

 in doing which, he not only overlooks the true geological 

 character of the rock he is describing, but almost demonstrates 

 the fallacy of his own supposition. For it has been shewn in 

 this paper, that the structure of the Cuchullin hypersthene 

 is pre-eminently of a tabular kind, traversed by more or 

 less regular systems of cleavage. The view from Coruisk 

 itself might have satisfied the Doctor of his error, in suppos- 

 ing that it, " in no case, presents either a flat stratified dis- 

 position, or a tendency to vertical fracture." The gleaming 

 surfaces of the cloven mountains above him, streaming with 

 wet, as they are most commonly seen, and reflecting a lurid 

 purple hue, might have demonstrated the existence of exten- 

 sive planes of cleavage, and the " spiry forms of the sum- 

 mits," the " hard serrated outline, and their overhanging 



* MacCuUoch, Desmption, &c., i., 387-8.. 



