380 SKY. GEOLOGY. OVERLYING ROCKS. 



v 



by those details, I shall only notice that variety which 

 occurs in the eastern cliffs already described. 



It appears to be a mixture of compact felspar and 

 augit, differing in some respects from that variety of the 

 same rock which is the most conspicuous in Rum, since the 

 felspar in this latter is generally of the crystallized or 

 of the glassy kind. There is reason to believe that this 

 rock is of much more frequent occurrence than has com- 

 monly been supposed, and that it is often mistaken for 

 common greenstone; the difficulty of distinguishing be- 

 tween hornblende and augit, when in a state of minute 

 admixture with felspar, being very great, if not insuper- 

 able. In the example now under review the investigation 

 is more easy, from its presenting many of those veins 

 known by the name of contemporaneous, which graduate 

 into the surrounding rock and contain large though im- 

 perfect crystals of the augit. There is also a certain facility 

 in detecting the nature of the rock when fresh, which 

 is lost in the specimens that have had time to dry. The 

 finer varieties when thus broken are comparatively tender, 

 with the aspect of serpentine, the felspar being of a 

 decided green and the augit of a pitchy black. In a 

 day or two this distinction vanishes, the whole acquiring 

 great additional hardness and an uniform grey colour; 

 in which case it often happens that neither of the con- 

 stituent parts can be discerned with any degree of 

 certainty.* 



* With others, I have had frequent occasion to notice the soft nature 

 of many rocks and minerals when first taken from the earth, and the 

 changes they undergo on drying, but the case of greenstone or of other 

 rocks of the trap family so circumstanced has not, as far as I know, 

 been remarked. I have observed a similar instance to this in Bute, and 

 do not imagine it to be uncommon. It is probably overlooked, only 

 because the toughness of these rocks commonly renders the mineralo- 

 gist satisfied with the first superficial fracture that he can obtain. There 

 is no apparent reason why the rocks of this family should not contain 

 water, even admitting their igneous origin ; as it is evidently in a very 

 loose state of combination, or it could not so readily be separated by 



