Passing south to the Kalahari, we find evidence of a number of moist stages separated by drier intervals, but they can apparently be grouped into two main Pluvial periods, separated by a long interpluvial with steppe-like conditions. One at least of these Pluvial periods must be correlated with the former immense extension of Lake Ngami and the Etosha Pan. From Cape Colony, there is some evidence of moister conditions in the past, but the Quaternary variations cannot be separated from those of historic times. Before leaving Africa some reference must be made to an interesting suggestion by C. W. Hobley, as to the mechanism of climatic change in tropical countries. He notes that the north-east and south-west monsoons extend to a height of only a few thousand feet. Above them are the very steady “trade winds” connected with the general circulation of the atmosphere. In Kenya Colony these blow from east or a little south of east. “Their effect is very marked on the high mountains of the interior, such as Kenya, Kilimanjaro and Elgon; in the early morning they are generally quite clear, but about 10 a.m. the clouds sweep up from the S.S.E. and collect on the mountains and blot them out from view for the rest of the day. These are believed to be clouds borne inland by the trade winds, and the moisture they carry is precipitated mainly on the south and south-east sides of the mountains.” Hobley suggests that there was formerly a nearly continuous ridge of high land extending north and south, and this caught the moisture from the trade winds, so causing the Pluvial period, the evidence for this ridge being the distribution of alpine plants on the now isolated high mountains. An alternative explanation is that the greater strength of the earth’s circulation during glacial times caused the trade winds to be much stronger and also to extend to a lower level at the expense of the monsoons, just as the west winds extended to a lower level in northern Egypt. This would bring a great deal more moisture to be precipitated on the mountains, increasing the length of the glaciers and also the volume of the rivers. BIBLIOGRAPHY Scott Elliott, G. F. “The geology of Mount Ruwenzori and some adjoining regions of tropical Africa.” _Q.J.G.S._, 51, 1895, p. 669. Hobley, C. W. “The alleged desiccation of East Africa.” _Geogr. Journ._, 44, 1914, p. 467. Freydenberg, H. “Le Tchad et le Bassin du Chari.” Diss. Paris, 1908. Passarge, H. “Die Kalahari.” CHAPTER XI AUSTRALIA AND NEW ZEALAND The continent of Australia has a relatively low relief, only rising above the snow-line in Mount Kosciusko, and glacial traces have a relatively unimportant development. The history of the region appears to be as follows: