42 Reports & Proceedings — Edinhurgli Geological Society. 



II. — Edinbuegh Geological Society. 

 1. Octoler 17, 1917. — Professor Jehu, President, in the Chair. 



" Sketches of South African Geology." By Professor S. J. Shand, 

 D.Sc, Ph.D. 



The major physiographic divisions of South Africa are the Coastal 

 Plain, the Mountain Barrier, and the Interior Plateaux of the Karroo 

 and the High Yeld. The Coastal Plain on the west has a width of 

 some 30 miles, rising gently from the sea-level to the foot of the 

 Tuountains. Geologically it is formed of rocks of the jS'ama System 

 (Cambrian or Pre-Cambrian), resting upon old metamorphic rocks, 

 with numerous great granite intrusions. There is no continuous 

 plain on the south coast, but there are many narrow shelves and 

 patches of raised beach, testifying to recent movements of elevation. 

 On the east side the Coastal Plain again becomes a continuous 

 feature, reaching a great width in Portuguese East Africa, where, 

 however, little is known of its geological structure. The Mountain 

 Barrier of the west and south is the folded margin of the Interior 

 Plateau, but on the east it is the fractured edge of the plateau; that 

 is to say, the west and south coast ranges are fold mountains, while 

 those of the east are fault mountains. The former are largely 

 composed of the hard Table Mountain Sandstone, resting with a 

 strong unconformity upon the Nama rocks. The age of the Table 

 Mountain Sandstone is deduced from the fact that the overlying 

 Bokkeveld beds yield trilobites, brachiopods, and lamellibranchs of 

 Lower Devonian age. On the inner side of the Mountain Barrier the 

 rocks of the Karroo System make their appearance and cover the 

 greater part of the interior of the Sub-Continent. The lowest 

 member of this system is the Dwyka Glacial Conglomerate, which 

 extends from the South-West African Protectorate to Natal, and 

 from the Transvaal to the south-west corner of the Cape Province. 

 The higher members of the Karroo System have a more limited 

 distribution than this, and the uppermost or Stormberg Series is 

 restricted to the eastern districts, where it caps the Drakensberg 

 Range. The close of the Karroo period was signalized by great 

 igneous activity and diastrophism. The folded ranges were elevated 

 at this time ; the Karroo sediments were invaded by basic dykes and 

 sills, and great outpourings of basaltic lavas took place in the regions 

 of the Drakensbergen, the Yictoria Falls, the Bushveld, and the 

 Kaokoveld. The concluding event seems to have been the drilling 

 of the kimberlite pipes, from which diamonds are now obtained. 

 The deposition of the rocks of the Karroo Sj'stem occupied the whole 

 period from Carboniferous to early Jurassic time. Bocks younger 

 than the Jurassic are only found in restricted areas along the south 

 and east coasts, where there are patches of marine Cretaceous and 

 Tertiary rocks. 



As regards climate, South Africa shows four fairly distinct natural 

 regions — one of summer rains in the east and central districts, one 

 of winter rains in the south-west, a semi-arid region in the north and 

 north-west, and an entirely desert area extending along the west 

 coast roughly from the Orange Biver to Walvis Bay. This Coastal 



