SKY. GEOLOGY. HYPERSTHENE ROCK. 387 



evidently resulted from the wasting of the soft veins, not 

 from that of the fundamental rock. It is partly owing 

 to this cause, partly to the rapid declivity of the mountains 

 and to the absence of shelves and fissures, that so little 

 vegetation is found in this place. A patch of grass may 

 occasionally be seen where some favourable circumstance 

 has permitted a little gravel to accumulate, but in general 

 it presents one continuous brown surface, not even di- 

 versified by a lichen. 



The roughness of the surface of this rock is "scarcely 

 less remarkable than its nakedness, being comparable to 

 nothing more properly than to a coarse rasp ; in conse- 

 quence of which it is easy to walk or run over those steep 

 declivities which would otherwise be impracticable. This 

 roughness arises from the crystals of hypersthene that 

 project from the compound in consequence of their supe- 

 rior durability to the felspar ; undergoing no waste, and 

 often indeed gaining rather than losing brilliancy by 

 the exposure. 



If the overlying position of this rock, and its connexion 

 with the other members of the trap family, did not deter- 

 mine its place among these, we should, from its external 

 features and disposition, assign it one with granite, so 

 much does it resemble the rocks of this tribe. It is dis- 

 posed in huge beds with a convex surface, separated from 

 each other, not so much by the actual presence as by the 

 indications of future fissures although, in a few instances 

 these have become sufficiently complete to allow masses to 

 be detached and rolled from their places. Besides the more 

 extended convex beds, large spheroidal concretions are of 

 frequent occurrence ; being in some cases so protuberant 

 that half the solid is visible, and their diameters reaching 

 to 50 or even to 100 feet. These are the only modifi- 

 cations of external form visible in this rock, which in no 

 case presents either the flat stratified disposition or the 

 tendency to vertical fracture so common in the members 

 of the trap family ; being in every respect, except that 



