414 SKY. MINERALS. 



so minute and so imbedded that their nature could not be 

 ascertained : they have the aspect of olivin. Pyrites is 

 occasionally seen, interspersed among these substances, 

 but is also rare. 



The hypersthene presents specimens of great magnitude 

 and beauty, which, although they seem to resist the 

 injuries of time far longer than the accompanying sub- 

 stances, at length also decay and fall into an ochrey 

 powder. Distinct concretions are to be found exhibit- 

 ing the primitive form, but more generally it is without 

 form, while in many cases it is intermixed with the 

 dark felspar so as to present the graphic character, 

 the crystals of felspar being defined and the hypersthene 

 occupying the interstices.* The lustre of this mineral 

 is always highly metallic, but the specific gravity of the 

 specimens which I examined did not exceed 3,342. The 

 colour is various, being in general of a purplish black, 

 sometimes steel grey, and more rarely of a whitish grey, 

 while it often assumes the hue together with the lustre 

 of polished brass when it has been long exposed to the air. 



Near the small island Oransa there is a rocky islet 

 containing actinolite, worthy also of the attention of 

 zoologists, as it abounds with a rare crab, the Por- 

 tunus corrugatus. It may be traced to the adjoining 

 shore of Sky near Camuscross, as it holds an uniform 

 and parallel course with the gneiss in which it lies. 

 As the beds of this rock are here nearly vertical, it 

 presents the appearance of a vein, the edge alone being 

 visible; and for this it seems to have been mistaken 

 both here arid on the opposite shore of the mainland 

 where it occurs in Glen Elg. Nevertheless it is not dif- 

 ficult to discover that it forms a vbed in the gneiss, 

 similar to those of hornblende schist which are of such 

 common occurrence here; and it may even be observed 



* This circumstance is interesting, as it points out another striking 

 analogy between granite and the rocks of the trap family. 



