As the impurities increase in amount the clay rocks pass gradually into argillaceous sands and sandstones, argillaceous limestones and dolomites, shaly coals and clay ironstones.
It is composed of red and variegated sandstones, dolomites and marls, traversed in some places by ophitic rocks, and containing deposits of gypsum, aragonite and rocksalt, It thus resembles the Trias of England and Germany.
Some unfossiliferous conglomerates, sandstones and dolomites in South Africa and on the west coast are considered to belong to the Cambrian, Ordovician and Silurian formations, but merely from their occurrence beneath strata yielding Devonian fossils.
The chief rocks of the Campbell Rand series are limestones and dolomites, with some interbedded quartzites.
Besides limestones and dolomites, the only building stone of much commercial importance is the Potsdam sandstone, extensive beds of which lie in the north part of the upper peninsula.