Page:South African Geology - Schwarz - 1912.djvu/68

 proportion of sands are made up of grains of quartz, weathered out of granite, or derived from the disintegration of sandstones. The grains are small, and are therefore buoyed up when they are immersed in water, hence sands laid down under water are not completely rounded by grinding one against the other; they nearly always are covered with a certain amount of dirt lodged in the crevices of the grain, and usually become covered with a skin of colouring matter, red or yellow due to iron oxide, blue due to iron suphide, or green due to iron silicate, and the sandstones resulting from the consolidation of the sands is coloured accordingly.

Sand blown about in the desert or on the seashore is characterized by the complete rounding of the grains, whereas sand derived from the disintegration of rocks by frost, and occurring in glacier deposits, is characterized by the splintery and angular shapes of the grains.

Sand may be consolidated into sandstone merely by pressure, when the grains adhere by interlocking, or cement may be added by precipitation from water; and this cement, which may be compounds of iron, calcite, or silica, binds the grains into a firm rock, usually in such cases called quartzite. If the grains are sufficiently separated to allow the secondary silica to form crystals, these are formed round the grains, which appear in the centre of the crystals in their original forms, dirt and all. The glittering sandstones of the Molteno Beds are of this nature, the name being derived from the sparkling of the rock from the reflections from the innumerable facets of the crystals.

Ordinarily sands are laid down in successive horizontal layers or strata, but where there is a strong current the grains are heaped up in one place, and then, on the