388 GEOLOGY. 



Lacustrine deposits. — The beds of sediment deposited in lakes are 

 similar in kind, in structure, and in disposition to beds of sediment laid 

 down in the sea, but river-borne sediment is more commonly concen- 

 trated into deltas, since waves and shore-currents are less effective. 

 Even the limestone of the sea has its correlative in some lakes. Some 

 of it was made of the shells of fresh-water animals which throve where 

 the inwash of terrigenous sediment was shght, some of it from the 

 calcareous secretions of plants,^ and some of it was precipitated from 

 solution.^ Salt and iron-ore ^ deposits are also sometimes made in lakes. 



Extinct lakes. — The former existence of lakes where none now exist 

 inhy be known in various ways. If the lake basin was filled, its former 

 area is a flat, the beds of which bear evidence, in their composition, 

 their structure, and often in their fossil contents, of their origin in stand- 

 ing water. Such a flat is commonly so situated topographically that 

 the basin would be reproduced if the lacustrine deposits were removed. 

 To this general rule there might be exceptions, as where a glacier 

 formed one side of the basin when it was filled. If the lake was destroyed 

 by the reduction of its outlet, or by the removal of some other barrier, 

 such as glacier ice, or by desiccation, shore phenomena, such as beaches, 

 spits, etc., may be found. In time such evidences are destroyed by 

 subaerial erosion^ so that they are most distinct soon after the lake 

 becomes extinct. 



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

 to have become extinct, while many others are now in their last stages, 

 namely, marshes. Many others have been greatly reduced in size. 

 Such reductions are often 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 has been 

 crowded northward some two miles by deposition at its head. Similar 



1 C. A. Da^ds, Journ. of Geol., Vol. VIII, pp. 485-97, and 498-503, and Vol. IX, pp. 



491-506. 



2 Russell, Lake Lahontan, Mono. XI, U. S. Geol. Surv., Chap. V; also Third Ann. 

 Rept., pp. 211-221. Gilbert, Lake Bonneville, Mono. I, U. S. Geol. Surv., p. 167. 



2 Stapff, Zeit. deut. geol. GeseU., Vol. XVIII, pp. 86-173. 



^Upham, Lake Agassiz, Mono. XXV, U. S. Geol. Surv.; Salisburj^ and Kiimmel, 

 Lake Passaic, Rept. of the State Geologist of N. J., 1893, and Jour, of Geol., Vol. Ill, 

 pp. 533-560; Gilbert, Lake Bonneville, Mono. I, U. S. Geol. Surv.; Russell, Lake 

 Lahontan, Mono. XI, U. S. Geol. Surv.; and Mono Lake, Eighth Ann. Rept., U. S. 

 Geol. Sur^^, Pt. I. 



