Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1042

912 projecting, as happens with the gabbro of the Cuchullin Hills, Skye. I have had slides cut from one of the dark bluish unweathered specimens, and from one of the exterior white and green. The former shows the rock to be an olivine gabbro. There is plagioclase felspar, generally in rather short irregularly oblong crystals, showing bright colours and twinning. Some of these exhibit a peculiar strongly marked cleavage (or minute twinning, with one set of crystals dominating), which gives them a general resemblance to the structure of diallage. The felspar is in places rather decomposed. There is a fair amount of diallage, and a few crystals of common augite. These with ordinary light are as nearly as possible colourless, and are in good preservation. The olivine is rather rough in texture and much cracked; the cracks are marked out by a deposit of granular opaque mineral, probably magnetite, which in some cases appears to penetrate the intermediate spaces (which are commonly fairly translucent), rendering them almost opaque; now and then it assumes a browner tinge, as from hæmatite or limonite, and the grains are slightly stained with brown or green. Most of the olivine grains have a finely granulated aspect at the edges, and are sometimes bordered by a finely fibrous mineral, probably serpentinous and a secondary product; the grains, however, show very little trace of conversion into serpentine. Except the minute granules described above, there is very little magnetite or other iron-oxide visible. The above appearance would lead us to conclude that the olivine is a rather ferruginous variety. The other slide (fig. 7), cut from a somewhat weathered mass, which in appearance closely resembled that of the veins, exhibits plagioclase felspar beginning to pass into the saussuritic mineral, and diallage, more or less converted into minute hornblende, but no olivine or serpentine that can be recognized—the slide on the whole being remarkably like one cut from a vein on the shore at Coverack. On closely examining