ORIGIN OF LAKES IN UPHEAVAL. 135 



then the lowland of Southern Russia, peopled by the Kirghis and the 

 Cossacks of the Don, would "be overflowed, and the Caspian Sea would 

 become continuous with the Sea of Azov and the Black Sea. Since 

 depression would cause the Caspian to cease to be a lake, it is an 

 obvious inference that elevation severed it from the sea, of which it 

 formerly formed part, by laying bare the shallow sea-bed, which is 

 now largely occupied by salt marshes, and in part liable to inundations. 

 In the same way it may be noticed that the Baltic is almost enclosed 

 by land, and if elevation instead of depression of the country were 

 now going on in the south of Sweden, the Baltic would inevitably be 

 converted into a lake by a process of change in level, which need not 

 necessarily affect a wide area. The great Bussian lakes Ladoga and 

 Onega are merely prolongations of the Gulf of Finland, leading north- 

 ward to the White Sea, and are remains of a channel partly dried up. 



Lakes Formed by Evaporation and Upheaval. Consideration 

 of the lakes of Central Asia, many of which are salt, and of broad 

 areas occupied in Persia, Turkestan, and Northern Jndia by salt 

 marshes, helps to show how old the main outlines of the rocks are 

 which form the physical features of the surface, relatively tp the lakes 

 which fill depressions among them ', and also demonstrates how recent 

 the last elevation of the country from the sea has been. All over 

 the plains between the Caspian and the Sea of Aral, and between that 

 sea and Lake Balkash, and thence on to Lake Baikal, deposits of shells 

 are found, recent in aspect, some of which are similar to those which 

 now live in the Caspian Sea. Lake Baikal, however, is a freshwater 

 lake, yet it contains a large number of saltwater types of animals. 

 Seals abound there similar to those which live in the Adriatic and 

 the Caspian. Many saltwater types of fishes have representatives 

 which thrive in its waters. Hence it may be concluded that Lake 

 Baikal was originally a portion of the great Central Asian Sea, and 

 was one of the deepest pools in its bed ; and that it became, by eleva- 

 tion of the mountain axis of the old world, converted first into a salt- 

 water lake, and afterwards into a freshwater lake. Some evidence 

 in favour of this view exists in the fact that in the deep waters of Lake 

 Superior some types of animals have been found which are otherwise 

 only known in the sea. The question of a lake being salt or fresh 

 depends entirely on the rainfall. When the amount of rainfall is in 

 excess of the evaporation in the district, then the fresh water drains 

 into the lake basin and dilutes the salt water; this diluted water 

 overflows, and the stream carries some of the salt from the lake down 

 to the sea. Thus, little by little, the salt is entirely removed, 

 and what was originally a saltwater lake becomes a freshwater lake. 

 It is probable that in most cases the change goes on more rapidly 

 than happened in the case of Lake Baikal, and in consequence the 

 marine life, unable to adapt itself to the altered conditions, perished ; 

 and it is only conceivable that the change to freshwater conditions 

 in that lake occupied so many generations as to have had little influ- 

 ence on the lives of individual animals. 



Lakes Formed by Folding of the Rocks. A second group of 



