334 



GEOLOGY 



of it from the calcareous secretions of plants, 1 and some of it was 

 precipitated from solution. 2 While still soft, such deposits are 

 called marl. Salt and iron-ore deposits are also sometimes made 

 in lakes. 



Extinct lakes. The former existence of lakes where none now 

 exist may be known in various ways. If a lake basin was filled, 

 its former area is a flat which bears evidence of its origin in its com- 

 position, its structure, and often in its fossils. Such a flat is com- 

 monly so situated topographically that the basin would be repro- 



Fig. 269. Shore terraces of Lake Bonneville, Wellsville, Utah. (Thomp- 

 son and Holmes.) 



duced if the lacustrine deposits were removed. To this general 

 rule there are exceptions, as where a glacier formed one side 'of the 

 basin when it was filled. If, on the other hand, the lake was 

 destroyed by the lowering of its outlet, or by the removal of some 

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

 such as beaches, terraces (Fig. 269), spits, etc., may be found, even 

 though there is no well developed flat 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 becomes extinct. 



1 C. A. Davis, Jour, of Geol., Vol. VIII, pp. 485-497, and 498-503, and 

 Vol. IX, pp. 491-506. 



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

 Third Ann. Kept., pp. 211-221. Gilbert, Lake Bonneville, Mono. I, U. S. 

 Geol. Surv., p. 167. 



