Penck — Climatic Features in the Land Surface. 173 



the great desert regions of the world traces of an old desert 

 climate, where now a pluvial climate dominates. It is possible 

 that the great central African lakes point in this direction ; 

 perhaps Lake Titicaca indicates by its content of rather fresh 

 water that once the deserts of South America extended farther 

 towards the equator, and perhaps the lakes of the plateau of 

 Mexico point to the same relation with the deserts of North 

 America, whilst Lake Baikal would indicate an opposite move- 

 ment of the climatic zones from the equator towards the poles, 

 which cannot have been contemporaneous. 



There is one fact known from the Arctic regions which is 

 in harmony with a migration of the climatic zones of the earth. 

 Neither the American expeditions which have explored the 

 neighborhood of Smith Sound, nor the Norwegian expedition 

 which studied the archipelago lying farther west, succeeded in 

 finding the traces of a former greater extent of the glaciers, 

 which are so abundant farther south. At first sight this fact 

 appears rather strange, but it can be understood in connection 

 with the others. It shows that during the glacial period glacial 

 conditions did not extend farther towards the pole into those 

 regions where now the Arctic climate, on account of its dry- 

 ness, is not very favorable to the formation of glaciers. The 

 great glaciations of the northern hemisphere were not exten- 

 sions of a polar ice-cap ; they were confined to the vicinity of 

 the Arctic circle, and they surrounded, as far as we can see, a 

 region of an Arctic desert climate similar to the existing one. 

 This fact would be consistent with an equatorial movement of 

 the climatic zones of the earth. 



If there are oscillations in the situation of the climatic belts 

 of the earth, it must be asked if they are connected with the 

 disappearance of existing climatic zones and the appearance of 

 new ones. For this reason the equatorial and polar regions 

 attract particular interest ; it can be imagined that in times of 

 an equatorial movement of the climatic belts some features of 

 the equatorial climate would totally disappear, and new climatic 

 conditions could come into existence in the polar regions. The 

 reverse would occur in times of a polar movement of the cli- 

 matic zones. There is also much interest in the study of all 

 border regions of climatic belts, for every movement of climatic 

 zones of the earth would here produce changes. 



The very extended border region of the glacial and the 

 pluvial climate has afforded a splendid occasion for the study 

 of past climatic changes ; the climatic history of the great Ice 

 Age could here be determined by a careful study of the cor- 

 responding deposits, and newer researches have utilised to great 

 advantage the glacial forms in determining the glacial climate. 

 The forms of the earth's surface indicate climatic changes also 



Am. Jour. Sci. — Fourth Series, Vol. XIX, No. 110.— February, 1905. 



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