Page:South African Geology - Schwarz - 1912.djvu/70

 Muds may be consolidated to shale, or into more compact forms called mudstones and clay slates. Slates are hardened considerably, and when they have been subjected to pressure often have planes of cleavage which allow them to be split into thin slabs. The cleavage develops quite independently of the original bedding planes, and is due to movement in the rock as the pressure squeezes it out laterally. Flagstones are slates with more massive structure. Phyllites are slates in which the planes of cleavage are covered with fine scales of mica, and are thus slates partially metamorphosed to schists.

Limestones vary from loosely compacted fragments of marine organisms, as in chalk, to the more dense varieties, as are found round Pretoria and in Bechuanaland in the Kaap plateau. The older limestones are mostly altered to dolomite by the replacement of a certain amount of lime by magnesia. They are usually coloured blue. On weathering, limestones and dolomites assume a brown, shaggy coat, like the skin of an elephant; hence the South African name for these weathered limestones — Olifant's Klip.

Septaria are patches of shale or sandstone cemented with lime, which has been deposited, after the rock has been hardened, by the percolation of water containing the lime in solution. The lime is usually precipitated originally round some organic substance, a fragment of