Crystalline Schists are also developed in the Isle of Anglesey, and on the coasts of Devon (Start), and Cornwall (Lizard).
The younger group of pre-Cambrian rocks is represented in N.W. Scotland by the Torridonian rocks, red felspathic sand stones and conglomerates, usually held to have accumulated under arid conditions, and believed to show considerable resem blance to the Keweenawan sandstones of America and the Jotnian sandstones of Fennoscandia.
In southern Britain a wider range of rock types is found; in W. Shropshire rocks of Torridonian type are found overlying with apparent unconformity a series of sandstones, grits, flags and shales predominantly grey in colour, the whole being referred to the Longmyndian Series. Elsewhere the pre-Cambrian rocks are mainly volcanic in origin, and include a series of lavas and tuffs of varied composition referred collectively to the Uriconian Series.
The relation of the Uriconian to the Longmyndian is difficult of determination ; it possibly intervenes between the "red" and the "grey" Longmyndian.
J. J. Sederholm bases his classification of pre-Cambrian rocks in Fennoscandia (i.e., Scandinavia and Fin land regarded as a geological unit) on the relations of sedimentary and other rocks to four great intrusions of granites, named (com mencing with the youngest) Rapakivi, post-Kalevian, post Bothnian and Katarchean. The oldest rocks, called Svionian, consist in part of schisted lavas, tuffs and crystalline limestones and are penetrated by the Katarchean granites. Unconformably overlying the Svionian and Katarchean granites is the Bothnian, the coarse conglomerates of which are reminiscent of the Tim iskamian conglomerates of Ontario. The Bothnian is penetrated by the post-Bothnian granites. The Jatulian-Kalevian forma tions, which resemble the Huronian in Ontario, rest with great unconformity on the older rocks referred to above. The J,qulian Kalevian sediments have been metamorphosed by post-Kalevian granites which are said to be analogous to the Algoman granites of Ontario. The Rapakivi granites are younger than all of the rocks just described. The youngest series of pre-Cambrian sediments belong to the Jotnian, the beds of which rest uncon f ormably and generally almost horizontally on the older rocks.
The Jotnian red sandstones have been compared with the Ke weenawan of N. America.
Cambrian fauna have not as yet been recog nized in South Africa ; but, although the oldest fossils known are Devonian, it is believed that the pre-Devonian rocks are certainly largely pre-Cambrian. The pre-Devonian formations occupy an enormous stretch of country but in the central part they are almost entirely covered by vast areas of sand. Du Toit classifies them as follows :— The oldest rocks have been named Primitive by Du Toit, and they are also known as the Swaziland system. In Rhodesia, the basal rocks—the Rhodesian schists—of the Primitive system are altered basic lava flows, commonly showing pillow structure, associated with bands of brilliant red jaspilite and banded iron stones; they are intruded by "old granite," and resting uncon f ormably on them and "old granite" is a series of folded sediments known as the Eldorado series. The latter is intruded by younger granite. In Barberton and neighbouring portions of Swaziland the Jamestown and Onverwacht series of the Primitive system consists of basic lavas with subordinate cherty bands. The Moodies series of sediments is apparently 20,000 to 30,00o ft. thick; its age relation to the lavas is not definitely known. The Jamestown, Onverwacht and Moodies series are intruded by the "old granite." In the Transvaal the Witwatersrand series rests with marked unconformity on the Primitive system. The Witwatersrand has a maximum thickness of almost 25,000 f t. and consists of quartz ites, slates, and conglomerates. Certain conglomerate beds, re garded as placer in origin, contain gold, and these constitute the greatest known gold deposits in the world.
The Ventersdorp system, made up of lavas and minor amounts of sediments, rests unconformably on the Witwatersrand system; the Transvaal-Nama system of sediments, including a great dolomite formation, overlies this unconformably. The felsites of the Rooiberg series are regarded as the upper part of the Transvaal system.