Page:South African Geology - Schwarz - 1912.djvu/125

 Round the plutonic bosses the metamorphism is sometimes more intense, at others entirely absent. The granite of Cape Town has produced a feeble spotting or knotting of the slates near the contact, that is, small, lightish, hard lumps have been caused to form. At Stellenbosch the slates have been changed to biotite schists at the contact.

Sometimes substances are transferred from the igneous magma to the sedimentary rock; felspar, for instance, may be so conveyed, as along the dolerite dykes at Cradock, or along the granite dykes of Stellenbosch. In the latter case the crystals grow to an inch or more, and a similar occurrence of andalusite in the schist at George, near the granite contact, is of the same nature. The granite, being a muscovite granite, required all the potash for itself, and therefore, instead of handing over potash aluminium silicate to the sedimentary rock to form potash felspar, it transferred only the aluminium silicate, which is the composition of andalusite.

In the Transvaal, round the Bushveld granites, the spots and knots at the contact grow into small crystals of chiastolite or staurolite, while at other places the whole rock is turned into a flinty hornstone by being impregnated with new silicates of the same nature.

Pneumatolitic Action. — This is alteration of rocks by the passage through them of mineralizing vapours. The commonest example is the passage through granite of fluorine and boron compounds in the condition of vapour, which causes the development in the rock of fluorspar, tourmaline, and topaz. Tin and tungstic chloride may come up in a similar way, and in meeting with water decompose, forming crystals of tinstone and wolfram. Corundum, or emery, is deposited in the same way, but