president's address. 15 



"thick growth of reeds which project a few feet out of the water 

 ■even where the pan is well filled. These Eastern Transvaal pans 

 are entirely out of harmony with present conditions. The 

 surrounding country, made of sandstones and shales, furnishes 

 sand and mud to the pans, and the cover of vegetation must 

 prohibit or greatly hinder wind erosion on the pan floor. The 

 average rainfall at Lake Chrissie is, according to Mr. Stewart, 

 about 33 inches, and it is very difficult to conceive how the pans 

 ■could have been formed in such a climate. The pans near 

 Heidelberg in the Southern Transvaal^" are almost as hard to 

 account for as those of Ermelo ; during my residence of over 

 three years at that place the larger pans held water continuously, 

 and the smaller when dry were thickly covered with vegetation. 

 Penck's hypothesis of their formation during an arid period in that 

 region is at least well founded. 



A Pluvial Period? 



Passarge has laid stress on a former Pluvial period in South 

 Africa contemporary with the Pleistocene Ice Age,''" and in fact 

 the notion of a Pluvial period is closely bound up with a theory of 

 the Ice Age, according to which the lowering of the snow-line and 

 advance of glaciers were coincident with and in part due to 

 heavier precipitation. But Penck has shown that the recorded 

 depressions of the snow-line do not indicate that in regions of 

 heavy precipitation at the present time there was a greater 

 lowering of the snow-line than elsewhere during the Ice Age, as 

 there should have been had heavier precipitation been connected 

 with the extension of the snowfields.'^' He has also shown that 

 in North America the Great Salt Lake, in the neighbourhood of a 

 formerly glaciated region, bears evidence of a contemporary high 

 water level and an outflow, while further south amongst the 

 bolsons or pans of Nevada and California corresponding high level 

 and deserted shore lines do not exist. ^- He concludes that the 

 limit between the arid region and that of greater precipitation to 

 "the north was further south than it is to-day by about 5° of 

 latitude; that there was no world-wide Pluvial period, but that 

 the great climatic regions were in different positions ; that the 

 desert belts were nearer the Equator. He points out that 

 evidence for this shifting of the belts is to be gathered from North 

 Africa, and lately E. Chudeau has demonstrated that the North 

 African desert in Pleistocene times lay over the Northern Sudan 

 and has since then moved northwards into the Sahara. ^^ Penck 

 emphasised the fact that temperature controls humidity; that, 

 for instance, under similar conditions of rainfall and topography, 

 lower temperature means less evaporation and greater run-off. 

 The contemporaneity of lowered snow-lines in Europe and Central 

 Africa is not proved, but it is probable that the period when the 

 glaciers of Kenya descended 5,000 feet lower than they do now 

 coincided with part of the Pleistocene Ice Age of Europe ;" in 

 any case, the lowering of temperature in Central Africa probably 

 implies a contemporary lower temperature in South Africa, even 



