92 Dr. Mac Culloch's Sketch of the 



it varies considerably in its dimensions : it is sometimes white, but 

 most generally of a honey yellow colour, but I must add, that in 

 either case it is of rare occurrence. 



The last and the rarest mineral which I discovered in Sky is 

 hypersthene. This occurs at Scavlg, in that singular variety of trap 

 which I have already described in the account I gave of the Cu- 

 chullin hills : it forms veins of different dimensions and much 

 blended with the rock in which they lie, but they are neither nu- 

 merous nor large, nor are the veins simple in their composition, 

 since they resemble the containing rocks in the different substances 

 of which they are composed. The most prevailing mixture is 

 however that of hypersthene, and of a dark felspar precisely 

 resembling that of Labrador in its general aspect, but not pos- 

 sessing its iridescence. This felspar is frequently crystallized, but 

 as the crystals are always completely imbedded, nothing further 

 of their form can be discovered than the outline which is dis- 

 played by the fracture : together with the dark felspar, white and 

 glassy felspar also occurs in the mixture, and the common opake 

 white variety is sometimes, but more rarely, intermixed with all 

 the other substances. It has been already noticed that these veins 

 are traversed by veins of clinkstone and of basalt, and they are 

 also intersected by veins consisting of common white felspar and 

 quartz, sometimes confusedly intermixed, and at others disposed 

 in the graphic form. This opake felspar is sometimes distinctly 

 crystallized in cavities. Rarely mica occurs in the compound, and 

 in one specimen I observed transparent green crystals so minute 

 and so imbedded that their nature cannot be ascertained : they 

 have the aspect of olivin. Pyrites is occasionally seen, interspersed 

 among these substances, but it is also rare. 



The hypersthene presents specimens of great magnitude and 



