542 Profeiisor Sc/inrirz — Eaiili-JiKiveiiioit^, S. Africa. 



not have stood the action of the waves in the open ocean. The 

 southern marp;in along which the shallow-water formation, the Table 

 Mountain Sandstone, was deposited was not very far from tlie present 

 shore-line ; it was formed by the strand line of a prolongation of tlie 

 Madagascar ridge. In early Cape times between this ridge and the 

 rest of Africa there was a narrow strait, which became broader and 

 deeper in Eokkeveld (Devonian) times, and was finally enclosed in 

 Witteberg (Carboniferous) times. The succeeding Karroo rocks were 

 laid down in this lake, having for a northern margin a line running- 

 through Prieska and the Southern Transvaal and a southern shore 

 along the old Madagascar ridge. The Karroo sediments were laid 

 down throughout in shallow water, ripple-marks being common 

 tliroughout, but as they are some 15,000 feet ttiick, the floor on which 

 they rested must have been continuously sinking. 



The original floor of the Karroo Lake was thus bowed down by the 

 weight of the ever-accumulating sediments, implying a very great 

 height of and enormous denudation on the land now situated in 

 Eechuunaland and the northern part of the Transvaal, and also of the 

 old Madagascar ridge. The underlying rocks also participated in the 

 downward movement and were in tension owing to the stretching 

 due to the sinking. Into the cracks formed from this cause molten 

 rock began to creep and gradually forced its way upwards, riddling the 

 Karroo sediments with a plexus of dykes and sills of dolerite of so 

 close a texture that one may regard the whole series of intrusions as 

 constituting a single gigantic laccolite of the cedar-tree type. 



The heat of the lower layers of the earth's crust had now become 

 transfei'red to the upper layers by these intrusinns of dolerite. The 

 Karroo sediments, becoming heated, expanded, and in doing so the 

 curvature of the basin of deposition would tend to straighten out. 

 Whereas before the expansion there was a deep basin with a straight 

 upper surface representing the last-deposited sediments, after the 

 expansion the basin became less deep and the upper surface was 

 a doubly inclined plane meeting in a crest in the middle where the 

 thickest sediments had been deposited. This crest is the primary 

 watershed of the country which originally stretched in an unbroken 

 line from Cape Town to Delagoa Bay, but which is now broken once, 

 and once only, by the Orange lliver at Aliwal North, the waters of 

 this river having been forced over the watershed by the dam of lavas 

 which were poured out in the Drakensbeig. The Drakensberg 

 volcanoes were consequently situated above sea-level and not sub- 

 marine, as is often held to have been the case. 



Besides this vertical uplift there was a lateral thrust also due to 

 expansion. This, for some I'eason or other, found relief only on the 

 south. On the nortli, perhaps because there was here the whole 

 continent of Africa behind, no movement took place, but on the south 

 there was only the Madagascar ridge, which we may suppose to have 

 yielded somewhat to the pressure, and the movement once started 

 caused the whole of the lateral thrust to become expended on this 

 side. Tlie shift of the Madagascar ridge can only have been small, 

 not enough to accommodate the large amount of expansion caused by 

 the intrusion of the Karroo dolerite. The surplus expansion was 



