Page:South African Geology - Schwarz - 1912.djvu/65

 Granite consists essentially of quartz, which is easily distinguished in hand specimens as clear, wet-looking grains; the dull-white or pink grains, with smooth cleavage faces, are felspar; hornblende is dark-green or black, but more frequently in ordinary granite its place is taken by scales of black or white mica.

Quartz-porphyry consists of the same minerals, except that the quartz only, or the quartz and felspar only, have formed crystals, which lie in a fine-grained ground mass. When large crystals thus occur among much smaller crystals, they are said to be porphyritic crystals, and the rock is called a porphyry. Quartz-porphyry is a dyke rock, but passes, as all dykes do, into a volcanic rock, when the ground mass may have no crystals at all — that is to say, is glassy. If there are no crystals of any sort, not even porphyritic ones, then we have what is called volcanic glass or obsidian.

Dolerite is the corresponding basic dyke rock, and here the crystals are so small that they very rarely can be distinguished with the naked eye. The essential mineral is one of the bases, usually iron, in the form of magnetite or titaniferous magnetite, called ilmenite, the carbon of the Kimberley diamond diggers. Felspar occurs in tiny thin crystals, with the usual smooth cleavage faces, which sparkle and reflect the light as one turns the stone about in the sunlight. The black mineral is augite, which has the same composition as hornblende namely, iron, magnesia, and silica.

Basalt is a lava of the same composition as dolerite; the crystals are usually very small, with occasionally porphyritic crystals of olivine. Very frequently it is full of blowholes, which may be empty, or they may be filled with chalcedony in concentric layers (banded agate),