Page:South African Geology - Schwarz - 1912.djvu/145

 found to be the case in several other areas, as at Vogel Vley, the granite dome is not yet properly exposed, but only the ends of the dykes that surround the domes appear at the surface of the ground.

The granite is usually moderately coarse grained, consisting of quartz, felspar, and biotite. At Sea Point porphyritic varieties occur with felspars up to 2 in. in length. At this last place there are inclusions of slate in the granite, which at the periphery are unaltered, but, further in the granite, change to bands of biotite. At Kuil's River, Stellenbosch, and at George the granite becomes a white mica granite, and wherever this occurs there is an abundance of tourmaline with, at least at Kuil's River, the usual accompaniment of tinstone and wolfram. Graphic granite occurs at Camp's Bay, and coarse pegmatite with felspars up to a foot in length at Malmesbury; the dykes at the latter place pass into porphyries. At Platte Klip, above Cape Town, the granite contains cordierite, usually altered to a micaceous mineral, pinite, which is the same as the more massive mineral agalmatolite, found in Bechuanaland, which was used by the Bushmen for making pipes, &c.

The contact phenomena of the granite are confined to the simpler manifestations. At Cape Town there is a slight spotting of the slate, and at Stellenbosch a certain increase of biotite near the contact. At the latter place, also, felspar has come from the granite and crystallized in the chinks in the slate, so that good Carlsbad twins may be obtained up to 1 in. in length. In the case of the contact of the muscovite granite of George there are andalusite crystals instead of felspar, sometimes imperfectly developed as hard knots in the slate, at other places forming well-defined crystals 1½ in. in length.