188 E. II I. Schwarz — Plains in Gave Colon;/. 



natural cause for its existence. Strong currents wash over the 

 bank and drag the sand to and fro over it, but the sand here 

 is mostly — I might almost say entirely — composed of com- 

 minuted shells, and even if the grains were quartz, the erosive 

 action of such sand suspended in water would hardly be 

 greater than that of hailstones on land. Beyond the Agulhas 

 Bank the ocean deposit is glauconitic. 



It has been too much the fashion lately to ascribe every 

 plane surface on land to the action of rivers, but every plain 

 must be considered apart, and the causes that led to its forma- 

 tion examined with no preconceived ideas, for there are many 

 ways of explaining such surfaces. 



(1) A plain may result from the long emergence of a rug- 

 ged land, when the rocks piled high, in some cases above the 

 crushing strength of the rocks at the base, will in time flatten 

 out. Such a plain is hardly likely to arise on the earth under 

 present conditions, as denudation works more rapidly than such 

 action could, but in planets without an atmosphere it might 

 become operative. At all events, it is a curious coincidence 

 that in a former paper,* when estimating the maximum height 

 of a rock mass which could be self-supporting, the figure came 

 out approximately the same as the height of the highest 

 mountain in the world ; that is to say, if my figures are correct, 

 any mountain thrust up to a higher altitude than Mount 

 Everest would slowly subside by mass flowage to a mean of 

 approximately 30,000 feet. 



(2) A plain may arise by an inland sea being filled up, and 

 then the whole being elevated by block uplift. Supposing the 

 southern portion of the Caribbean Sea, or parts of the Yellow 

 Sea, were elevated, a natural plain would result. Such a plain 

 I believe once existed in South Africa. There is an old Per- 

 mian shore-line running approximately northeast-southwest, 

 through the Southern Transvaalf; on the southward side of 

 this there are found Karroo sediments filling in a wide basin- 

 whose southern border is now beneath the sea. Towards the 

 close of the Jurassic period, elevation of the whole of South 

 Africa took place, and this plain was steadily raised 2,000 feet. 

 At this time there w T as a simple watershed extending from 

 what is now Cape Town to Delagoa Bay, lying parallel to the 

 old shore line, and from which the rivers came off in orderly 

 succession, some to the northwest, others to the southeast. 

 These rivers are still the dominant ones in South Africa, and 

 are only obscured in the east by a gigantic dam of lavas, which 



* An unrecognized agent in the deformation of rocks. Trans. S. A. Phil. 

 Soc, vol. xiv, p. 387. Capetown-, 1903. 



f Schwarz, E. H. L., The Volcanoes of Griqualand East, S. A. Phil. Soc, 

 Capetown, vol. xiv, p. 108, 1903 ; see also Passarge, S., Die Kalahari, p. 63. 

 Berlin, 1904. 



