184 Dr. Mac Cul loch's Supplement to the 



specting the identity of their composition throughout. This is the 

 rock on which the elements seem to make no impression, and on 

 which no soil accumulates, causes which equally determine the 

 permanent and rugged nature of their spiry outline. I have no 

 hesitation in saying that the whole of the group as far as it is 

 visible from Coruisk, as well as the opposite side of Garsven far 

 on towards Loch Brittle seaward, and the smaller mountains which 

 separate this valley from Blaven, consist of the same rock, but to 

 what extent it may reach northwards cannot be determined until 

 the whole shall have been traversed, unless the rugged outline and 

 external general characters are admitted as a proof of identity of 

 composition. In this case the whole of the Cuchullin is a mass of 

 hypersthene rock, with the exception only of the veins which it 

 contains, which consist of basalt, compact felspar, augit rock 

 syenite, felspar-porphyry, and lead-blue claystone. I have chosen 

 the term hypersthene rock to designate this new and important 

 member of the trap family, since like that of augit rock it is ex- 

 plicit, and introduces no confusion into the existing nomenclature. 

 There appear but two prevalent varieties of composition. In 

 the first the mixture consists of hypersthene with greenish com- 

 pact felspar, and in the second with crystallized white felspar 

 possessing generally a slight glassy lustre. This latter variety seems 

 to predominate, and is much more easily recognized than the for- 

 mer, which from its greenish hue and the minuteness and intimacy 

 of the admixture, is often difficult to be distinguished from com_ 

 mon greenstone. The principal variations of appearance are pro- 

 duced by the greater or less proportion of the hypersthene, by the 

 varieties of its colour, and by the unequal magnitude of the crys- 

 tals. While on the subject of Coruisk I ought to add, that the 

 sonorous rock formerly described is either a compact felspar or a 



