CLIMATE OF HISTORIC TIMPJS 541 



of the Kalahari are for the most part dead, aud so stationary are they that 

 the traveler can find his bearings from the number of dunes which he passes. 

 The circumstances which have led here to the formation of dunes no longer 

 obtain on the equatorial side of the desert zone, but prevail solely at the pres- 

 ent day on the polar border. 



"All this leads us to assume that the area of extreme aridity in Africa once 

 lay much nearer the equator than it does today, exactly as was the case in both 

 Americas, and guided by the phenomena of the Great Basin we may fix this 

 period in the Ice Age, The great Ice Age presents itself, then, neither as a 

 period of extreme cold — as was originally held — nor as a peiiod of excessive 

 humidity over the whole earth, but as a period during which the climate belts 

 of the world lay some 4 or 5 degrees nearer the equator, while the snow-line 

 was found more than 3,300' feet loAver, . . . The shifting of the climate 

 belts, however, during the Ice Age has never gone so far that one belt has 

 entirely usurped the position of another." 



In explanation of this supposed shifting of the climatic belts Penck 

 states his belief in a lowering of temperature 



"which would bring about not only an advance in the snow-line, but at the 

 same time a shifting of all the climatic belts equatorwards. If the heat supply 

 of the earth decreases, the atmospheric circulation will become less intense. 

 The great areas of high pressure will become weaker and the horse-latitudes 

 must move toward the equator. And it is they that determine the position of 

 the arid areas upon the land-masses. Thus everything points to the fact that 

 the . . . climate of the Ice Age was a period of reduced heat supply." 



This last conclusion does not agree with those to which we seem to be 

 ]ed by a study of solar activity. Cold years, it will be remembered, are 

 times not only of intense solar activity, but of intense movement of the 

 air, as evidenced by an excess of tropical hurricanes and temperate cyclonic 

 storms. This, however, need not be discussed further at this point. For 

 our present purpose the importance of Penck's conclusion lies in two con- 

 siderations. In the first place, it is somewhat remarkable that by lines 

 of reasoning which are apparently wholly separate, students engaged in 

 the study of glacial climates and of historic climates respectively should 

 be led to almost identically the same hypothesis at approximately the 

 same time.^^ In the second place, Penck has rendered an important serv- 

 ice by calling the attention of geologists to the fact that during the Ice 

 Age the same kind of climatic change did not take place everywhere, nor 

 did all places suffer from a change of such a character as to produce any 

 important results. Thus, to use Penck's terms, we have to do with rela- 



^ In this connection see C. E. P. Brooks : The meteorological conditions of an ice- 

 sheet and their bearing on the desiccation of the globe. Quart. Jour. Royal Meteoro- 

 logical Soc, vol. 40, 1914, pp. 53-70; and also W. F. Hume and J. I. Craig: The Glacial 

 period and climatic change in northeastern Africa. Rept. Brit. Asso., 1911. p. 382, 

 XXXIX— Bull, Geol. Soc. Am., Vol. 25, 1913 



