LAKES 



203 



corresponding to the bed of the lake. In time, such features are 

 destroyed by subaerial erosion, so that they are most distinct soon 

 after a lake- disappears. 



Many lakes, some of them large and many of them small, are 

 known to have become extinct, 1 while many others are now in their 



afegft!R*Ha^$s 



Fig. 202. Shore terraces of extinct Lake Bonneville, Wellsville, Utah. (Thomp- 

 son and Holmes.) 



last stages, viz., marshes. Many others have been reduced in size. 

 Such reductions are obvious where deltas are built into lakes. Thus 

 the delta built by the Rhone into Lake Geneva is several miles in 

 length, and has been lengthened nearly two miles since the time 

 of the Roman occupation. The end of Seneca (N. Y.) Lake (PI. 

 XVI) has been crowded northward some two miles by deposition 

 at its head. Similar changes are common. 



Salt lakes, A few lakes, especially in arid or semi-arid regions, 

 are salt, and others are "bitter." Beside common salt, most salt 

 lakes contain magnesium chloride, and magnesium and calcium 

 sulphates, as well as other mineral substances. Most "bitter" 

 lakes contain sodium carbonate, as well as sodium chloride and 

 sulphate, and some of them borax. The degrees of saltness and 

 bitterness range up to saturation. The water of the Caspian Sea 

 (lake) contains, on the average, less mineral matter than that of 



1 Gilbert, Lake Bonneville, Mono. I, U. S. Geol. Surv.; Russell, Lake Lahontan, 

 Mono. XI, U. S. Geol. Surv.; and Mono Lake, Ki-hth Ann. Rept., U. S. Geol. 

 Surv., IM. I; I'pham, Lake Agassiz, Mono. XXV, U. S. Geol. Surv.; Salisbury and 

 Kiimmel, Lake Passaic, Rept. of the State Geologist of N. J., 1893, and Jour, of 

 Geol., Vol. Ill, pp. S33-S6o. 



