Page:South African Geology - Schwarz - 1912.djvu/119

 stand out as a mountain, as is the case in the Paarl; but if the intruded rocks are harder, then it is the granite that forms the low ground, as is the case between Johannesburg and Pretoria.

The plutonic rocks — granite most commonly — when they work up into the cavities they finally occupy, exert considerable pressure, and the sedimentary rocks are pushed apart, so that their bedding planes lie parallel to the sides of the bosses. The edge of the granite frequently exhibits a kind of bedding: the minerals quartz, felspar, mica, and so forth are arranged in bands, and this banded granite is called an igneous gneiss. It is due to the pressure causing the minerals to separate out in zones, or it has also been explained as being due to movement when the granite was not quite solid, which dragged the minerals out into trains. At some places, as at Robertson, the pressure has squeezed the igneous magma in between the bedding planes of the surrounding sedimentary rocks in very fine layers, so that one can hardly tell where the granite ends and the sedimentary rocks begin; this is called lit-par-lit, or bed-by-bed injection. It is possible that this illustrates the way in which the granite makes room for itself; that is to say, the molten magma invades and gradually absorbs the surrounding rocks.

A still further case is when the granite breaks off slabs of the sedimentary rocks, which then sink into the liquid magma. All stages of absorption of these may be observed in South African granites. At Sea Point, on the shore, one may see slabs of slate at the contact lying almost unaltered in the granite. On the road above, however, the granite merely shows dark patches crowded with black mica, which are the