OVERLYING ROCKS. 507 



of augit with common felspar, but as yet they 

 have not fallen under my observation, and I have 

 therefore forborne to reserve a place ibr such a 

 variety. It appears further that both augit and 

 hornblende exist together in the same rock ; but 

 I also forbear to make a variety of this at present, 

 as the distinction between these two minerals 

 when in a state of mixture is so difficult. 



H. Hypersthene with compact felsar. 



I. Hypersthene with common felspar. 



K. Hypersthene with glassy felspar, and occa- 

 sionally with common and glassy felspar both. Hy- 

 persthene rock. 



These form three varieties of a rock hi- 

 therto unknown, and to which I have given the 

 name of hypersthene rock. It occurs in large 

 granitiform beds, in mountain masses ; and is 

 found, as already noticed, in Sky, and in Aird- 

 namurchan. As yet I have never observed it 

 either in a columnar form or in that of veins, 



It occasionally exfoliates like the granites, in 

 a solid form, by the action of the weather. In 

 other cases, it possesses a foliated structure like 

 gneiss, arising from the parallel disposition of 

 the hypersthene. More generally, the structure 



