Page:South African Geology - Schwarz - 1912.djvu/122

 rocks were riddled with dykes and sheets as in the Karroo dolerite. The two laccolites are very similar, the northern one being older than the southern one. Dykes belonging to the Transvaal one are diabase, which is a dolerite which has become altered by extreme age, otherwise the peripheral zone of both is the same. To the south of the Transvaal laccolite lies the folded zone of the Witwatersrand, which is a mountain region planed down by long-continued denudation. To the south of the Karroo laccolite lie the great folded ranges of the coast; in both cases the folding is due to the thrust which the intrusion of so much new material into the crust has occasioned.

Metamorphism is the alteration of rocks by pressure, heat, and solvent water. The change is manifested by the substance of the rock recrystallizing in new forms, or by new minerals being introduced from outside. Time is a most essential factor also; none of the results produced in nature could be copied in the laboratory during a man's lifetime. Igneous rocks may be metamorphosed, but besides zones of crushing, which produce crumbled rocks, mylonites, the only common metamorphic igneous rock is hornblende schist, which is metamorphosed dolerite. Our attention will be, therefore, concentrated on the metamorphism of sedimentary rocks.

Regional metamorphism, often called dynamo-metamorphism, is the change produced in rocks by pressure, as contrasted with that produced by the contact of intruded molten rocks. Two kinds of pressure have to be considered: dynamic pressure, where the pressure — either the load of superincumbent rocks or the squeeze occurring when a portion of the crust is thrust in any direction and meets an obstacle — causes the rocks to spread