44 Transactions of the South African Philosophical Society. 



enormously developed, or rather, since they must have existed 

 equally all over the other districts in which small vestiges are still 

 retained, where they have been very little disturbed by denudation. 



Seeing these gravels, then, in their full development, I have been 

 able to get a better grasp of their meaning and origin, and I propose 

 in the present paper to work out my views in regard to them. I 

 dare say these will need some modification as our knowledge 

 advances, yet I think a useful purpose will be served in drawing 

 under one head an important body of facts. 



Stated briefly, my idea is that at no very distant time there was a 

 plain passing all over the southern part of the continent, and far into 

 the Karroo, which was elevated some 1,000 feet above the level of the 

 present rivers — that is to say, on the coast it was 1,000 feet to 2,000 

 feet above the present sea-level ; in the interior mountainous district, 

 Ladismith, Oudtshoorn, and Baviaan's Kloof, it was 3,000 feet and 

 more, and on the Karroo side of the coast-range it was 3,000 feet to 

 4,000 feet. This plain was formed when the rivers had reached a 

 quiet stage in their denudation — that is, a period when they no longer 

 cut downwards, but had time to meander backwards and forwards 

 and reduce all the country to a more or less dead flat. 



When a river has reached this stage, it is said to have reached its 

 base-level of erosion, and the plain is called a peneplain. It usually 

 occurs when the fall of the river is a very gradual one from a long 

 way inland to its debouchment into the sea. It cannot, however, be 

 said that the present rivers of South Africa have such a gentle fall ; 

 in fact, it is the steep gradients that cause all the trouble of the 

 droughts by draining away the water so rapidly ; as the plain keeps 

 approximately at the same height above these present river-courses, 

 some other cause than a gradual fall is necessary to explain their 

 existence. The other cause is afforded by the mountain chains 

 which the rivers had to traverse ; these produced bars across the 

 river channels, which had to be cut down before the river was free 

 to flow to the sea, and hence behind each mountain chain there was 

 an enforced period of base-levelling, so that the plain, instead of being 

 one continuous slope, is cut into a series of steps. I do not offer this 

 explanation from purely theoretical considerations, but, as we shall 

 see later, there are actually plains now being formed in the Long 

 Kloof from exactly such a cause. The plain-formation, however, 

 must have been aided by a condition in the equilibrium of the land 

 unlike that through which we are now passing, which is one of con- 

 tinuous uplift, so that the rivers are constantly cutting downwards. 

 Before this rise, and may be at the time of the plain -formation, there 

 must have been a great sinking, as the shallow sea-bottom which 



