ii6 PRINCIPLES OF STRATIGRAPHY 



Such is the case with Lake Baikal in Siberia, which has an eleva- 

 tion of 520 meters and a depth of 1,430 meters, its floor, therefore, 

 descending 910 meters below sea-level. 



Among the great American lakes, Ontario descends 150 meters 

 below sea-level, Huron 52 meters, Michigan 88 meters, and Superior 

 114 meters. The American Great Lakes drain their surplus waters 

 through the St. Lawrence system into the Atlantic, and Lake Baikal 

 through the Angara River into the Arctic. Lake Tanganyika, of 

 East Africa, 780 meters above sea-level, has only an interrupted 

 outward drainage, the removal of surplus water being by evapora- 

 tion ; but Lake Nyassa, 480 meters above sea-level, drains by the 

 Schire and the Zambesi into the Indian Ocean. This lake has a 

 maximum known depth of 786 meters (430 fathoms), its floor thus 

 descending 300 meters below sea-level. Its length is about 350 

 miles and its average width about 40 miles. Lake Tanganyika also 

 passes below sea-level in its deeper portion. On the American 

 continent. Lake Tahoe in the Sierras, with a depth of 1,654 feet, 

 is second only to Crater Lake, Oregon, which has a depth of 1,975 

 feet. Lake Superior has a maximum depth of only 1,008 feet, while 

 Lake Maggiore (1,004 feet) and Lake Como (1,354 feet), on the 

 south side of the Alps, compare favorably with the American lakes 

 in depth. 



Classification of Lakes and Lake Basins. A genetic classi- 

 fication of lake basins differs from a classification of lakes as a 

 whole because it deals only with the depression in which the lake 

 is situated. Depressions of exactly similar characters, but without 

 water, may exist, and would have to be taken account of in the 

 classification of basins. Such "dry lakes" are, however, of no sig- 

 nificance to the limnographer or to the bionomist. Again, the 

 character of the water, whether salt or fresh, is largely a matter of 

 climate, and has no relation to the lake basin. This is also true of 

 the outlet or eMuent, for if the number of affluents is small and 

 evaporation lowers the lake sufficiently, it will lose its outlet, just as 

 it may through a rise of the rim or other tectonic change. Such 

 excess of evaporation generally brings about the salinifying of the 

 water by the concentration of the mineral solute. (Davis-7; also 

 Salisbury-33.) 



Classification of Lake Basins. 



In a natural classification of lake basins the agent active in 

 their production is of first importance, and lake basins may, there- 

 fore, be classified in the first place as A. Lakes of Deformation, or 



