SKY. GEOLOGY. H YPERSTHENE UOCK. 391 



persistent, and intersecting in a decided manner the 

 rock, which occasionally contains marks of fracture and 

 displacement. The veins are of various sizes, most com- 

 monly not exceeding one or two feet in breadth, some- 

 times attaining a much larger dimension, and frequently 

 descending to that of a few inches or even less, while 

 they also ramify into minute branches. They are always 

 conspicuous, from the contrast of their white and weathered 

 surfaces with the black colour of the hypersthene rock. 

 In one of them is presented a beautiful example of the 

 entanglement of the including rock, the fractured and 

 displaced parts admitting of the most accurate readapta- 

 tion ; and as it could not be fully understood by description 

 it is represented in an accompanying drawing.* 



In some places, large veins are seen, composed of a 

 very compact hard substance, as sonorous as cast iron. 

 Detached fragments of these are conspicuously strewed 

 on the borders of the Lake Coruisk, being generally honey- 

 combed into deep cavities, but without any mark of rusting 

 or decomposition. The rock is of a pale grey colour 

 and fine grain, with an unusually high specific gravity, 

 and appears to be formed of an intimate mixture of pale 

 augit and compact felspar. Fragments of similar veins 

 are also found studded with protuberant bodies resembling 

 large pedunculated fungi. 



Very rarely there occur thin veins of a black substance 

 rather resembling Lydian stone than any variety of basalt, 

 perhaps analogous to pitchstone, and scarcely distinguish- 

 able, by the eye alone, from jet. Veins of common grey 

 syenite, not to be distinguished from that which forms 

 the mountains of the middle district, are equally rare; 

 whether ramifications from this mass is a question that 

 has already been considered. 



Having thus described as far as appears necessary the 

 overlying trap rocks of Sky, I must now inquire into 

 the apparent effect which they have had in displacing 



* PI. XXVI. fig. 4. 



