172 Penck — Climatic Features in the Land Surface. 



inflow, and establish thus a new equilibrium, then the lake 

 will overflow in the Manich valley and would be changed in the 

 course of time into a freshwater lake. On the other hand, if 

 the desert region of Mongolia should extend, the principal 

 affluent of Lake Baikal would become smaller, and a moment 

 would arrive when the quantity of water running into Lake 

 Baikal would be totally balanced by the evaporation going on 

 there. Then this lake would lose its outlet, and the salts 

 brought by its affluent would gather in its basin, and it would 

 be transformed into a salt lake. Such rather light climatic 

 changes would transform the salt lakes of the northern border 

 regions of the central Asiatic deserts into freshwater lakes, or 

 the large freshwater lake into a salt lake. We can understand 

 best the existence of these lakes by the working hypothesis 

 that their basins were formed originally in a dry climate, where 

 river action could not neutralize transformation by crustal 

 movement, and that they thence were more or less filled with 

 water, according to the actual climate of their region. In this 

 way we are disposed to recognize also in the great freshwater 

 lakes in the interior of some continents 'morphological witnesses 

 of climatic changes. 



If we now look over the whole set of evidence, we see mor- 

 phological traces of glacial action in countries which are now 

 drained and shaped by running water, and recognize in mountain 

 chains, as for example the Alps, traces of river action preceding 

 this glacial action. We follow traces of extended river action 

 into the desert, and we find forms which were probably pro- 

 duced in a dry climate, with no run-off, in regions which are 

 now subject to river action. The climatic features of the land 

 surface indicate that climatic changes are not only in one 

 direction ; we cannot say that the climate of the land is becom- 

 ing dryer and dryer, or less and less glacial ; they reveal con- 

 tinued climatic oscillations. These climatic oscillations are 

 different in the different zones of the earth. We have to deal 

 with oscillations between glacial and pluvial climate, and with 

 oscillations between pluvial and desert climate. It is now the 

 question, how these different oscillations were connected with 

 one another, if there is a contemporaneity between them ; and 

 which changes were contemporaneous, those of glacial to plu- 

 vial climate with those of pluvial to desert climate, or vice versa. 

 The evaporation of the deserts of the Far West has given one 

 answer : it was there shown that the glaciers existed at the 

 same time that the climate was moister. From this we can 

 infer that one of the climatic oscillations indicated by the sur- 

 face features of the earth consisted in a movement of the cli- 

 matic zones from the pole towards the equator and back again. 

 If this inference is right, we must find on the equatorial belt of 



