president's address. 11 



inolluscan life."^ Some of these mollusca are at times 

 extraordinarily abundant round small springs, such as that at 

 Stinkfontein in the Richtersveld, where the vegetation about the 

 spring, seen from a distance of a few yards, appears dark owing 

 to the enormous numbers of a small Tonitchia-like snail. "'^ The 

 snails would seem to live in damp places, seepages ix^und pans 

 rather than in them, possibly on account of the fluctuating salinity 

 of the pan water. Hitherto shelly limestones filUng a former pan 

 have not been described. The deposit of diatom earth at Bank, 

 near Amsterdam, is not in a closed pan, but in a vley fed by 

 springs, part of a valley tributary to the Compies River. 

 The accumulation of common salt in pans varies greatly, and so 

 far as this country is concerned, the subject, especially the 

 origin of the salt, has not yet received the attention it deserves 

 either from the economic or geological points of view. A few 

 pans, such as Rautenbach's pan in the Kalahari, ^^^ have a thick 

 layer of salt in them; others, like the productive pans of the 

 Orange Free State, Maraisburg and Herbert districts of the 

 Cape, and the south-western Transvaal, appear to have a large 

 store of salt in the brine obtained within a few tens of feet of the 

 surface. The source of salt in these pans seems to be chiefly the 

 Karroo sedimentary rocks on which they all lie. 



Another soluble deposit found in dry regions is gypsum, and 

 it is probably still more sensitive to change of climate towards 

 humidity. It is an abundant constituent of the sub-soil in parts 

 of Namaqualand. 



Rivers. 



South African rivers in general are characterised by their 

 steep grades, and many of the larger have waterfalls. Such 

 rivers are still far from having reached the smooth curve, a 

 slope decreasing steadily from source to mouth, which is the 

 ultimate form of a river bed whatever may be the rocks in 

 which it is cut. Before such a result is reached the system may 

 .be disturbed by earth-movements; in the simplest case, that of 

 a whole country being lifted or the level of the ocean falling, 

 the readjustment necessitates the cutting down of the bed from 

 the mouth upwards, and in the process hard rocks again find 

 expression in falls or rapids. It is too early yet to discuss 

 shortly the evidence for the part played by post-Crotaceous 

 earth-movements in the development of the river grades, or to 

 answer the questions put by W. M. Davis^^ as to the effect of a 

 probable former greater extension of the continent to the south 

 and west on the development of the interior plains and river 

 systems. Regional and secular climatic differences make them- 

 selves felt in various ways ; and a regional difference which is 

 obvious in our largest river system, that of the Orange, is the 

 greater rainfall in the catchments of the main streams, the 

 Orange in Basutoland and the Vaal further north. These two 

 streams supply almost all the water in the river below their 

 confluence, in spite of their catchment being less than half of 



