16 president's address. 



though there were no glaciers here. The reason for the absence 

 of Pleistocene glaciers in South Africa has not been found ; 

 perhaps the precipitation was too small and took place, as now, 

 chiefly during the summer months, but the effect of the lowered 

 temperature must have been an increase of flowing water in what 

 are now occasional rivers ; possibly some of those in the Karroo 

 and Southern Kalahari became permanent streams for a long 

 period. There is some evidence of a shifting of the dry belt south- 

 wards in times not remote in a geological sense both in the 

 distribution of pans, as noted above, and in the fact that the 

 sand region of the Kalahari is now covered with vegetation, which 

 has been taken as evidence of former drier conditions in that 

 country. ^^ This presumed southerly shift of the dry belt implies 

 that the region of the Great Karroo is drier now than it was in 

 Pleistocene times, and there may be evidence of this in the 

 imperfectly developed pans and in the depth of alluvium which 

 is a striking feature in many poorts of the Karroo rivers. ^'^ The 

 great difficulty here is to get adequate evidence of the time 

 relation of the various events in different regions, but this will 

 eventually be overcome ; and there is an element of uncertainty 

 in attributing the silting up of river beds to climatic change, for 

 lowering of grade due to tilting or the erosion of the bed above a 

 given place will have much the same effect by reducing the 

 velocity of the current, and possible alterations in the conditions 

 such as these can only be eliminated from the argument by 

 detailed consideration of groups of streams. ^'^ 



In the Salt Pan north of Pretoria there is preserved a record 

 of the Bushveld climate for a long period. The pan occupies a 

 caldera, a pit of volcanic origin, and its floor lies 200 feet below" 

 the level of the surrounding Bushveld. The great interest of the 

 caldera consists in its being a depression formed during post- 

 Karroo times, probably at a much later date than the pipes filled 

 with kimberlite, and in its never having been filled with water to 

 overflowing; thus the record of the history of the caldera con- 

 sidered as a large basin holding a solution can be read by the 

 apphcation of aknowledge of physical chemistry. Dr. Wagner 

 has lately made a detailed investigation of the pan and has 

 arrived at some conclusions important to our subject. He found 

 that the mud separating layers of trona contains diatom shells, 

 and that the water, at times fresh enough to allow diatoms to live 

 in it, deposited at intervals the layers of trona which indicate the 

 concentration of the lake waters at those times, while the inter- 

 calated diatomaceous muds were formed during more humid 

 periods. The final desiccation of the pan led to the deposition 

 of a crust of common salt and to the drying out of the muds and 

 clays to a depth of at least 29 feet; the more humid conditions 

 now prevailing keep the muds and clays underlying the floor 

 saturated with water, the dry clays mentioned being apparently 

 sealed off by impervious layers, and the common salt is for the 

 most part held in solution, for the water now rarely or never 

 entirely disappears by evaporation.^* 



