Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/100

 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 Scavig, in that singular variety of trap which I have already described in the account I gave of the Cuchullin hills: it forms veins of different dimensions and much blended with the rock in which they lie, but they are neither numerous 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 possessing 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 displayed 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