404 SILICIFICATION OF SURFACE LIMESTONES 



The origin of the limestones is to be found in the 

 evaporation of water with carbonates in solution, just 

 as in the case of the calc-tufa in the Karroo ; it is more 

 abundant north of the Orange Biver because the country 

 there is flatter and more sandy than the Karroo, so 

 that the sub-surface water moves more slowly and a 

 greater proportion of it is returned to the atmosphere 

 by evaporation, leaving the dissolved lime in the 

 soil. 



The explanation of the replacement of carbonates by 

 silica, especially the peculiar distribution of the process 

 in this country, is a very difficult matter. Dr. Passarge l 

 says that the favourable conditions are an arid climate 

 in a region of internal drainage followed by a period of 

 greater rainfall ; during the dry time alkaline carbonates 

 and soluble silica accumulate in the ground, under 

 wetter conditions the silica is dissolved and replaces 

 limestone and is deposited as a cement between sand 

 grains, giving rise to silicified limestones and quartzites. 

 Deposition of silica at the surface is now taking place 

 near Mossel Bay, 2 though the process is not understood, 

 and from the fact that thin films of opal lie outside 

 chalcedony lining cracks and small cavities in the 

 northern limestones it is probable that silicification is 

 still taking place there. 



8. RAISED BEACHES. 



At many places on the coast there are beaches of 

 rolled pebbles, sand and shells at various heights above 

 the present-day shore. These deposits frequently rest 



1 Die Kalahari, p. 618. 2 G. C., x., p. 296. 



