O ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



Paleozoic seas, and the student of paleogeography may well pause before 

 he declares that no abyssal deposits occur upon the continental blocks. 

 Chamberlin has shown that the low temperature of the deep water of the 

 recent oceans is possibly the result of recent changes, and totally different 

 conditions may have prevailed in more normal conditions of the earth.^ 



CUT-OFF ARMS OF THE SEA. 



Areas of water partly or completely separated from bodies of marine or 

 brackish water, but periodically or continuously supplied with less saline 

 water, become concentrated by evaporation until they are finally desiccated 

 or reach the point of saturation and begin to deposit their salts. Deposits 

 from such bodies of marine water are readily distinguished from salt lakes 

 or playa lakes by the character of the contained fossils. In a typical de- 

 velopment, the lower beds should contain a normal marine fauna which 

 would gradually become pauperized, losing many of its members and having 

 others dwarfed or developing extreme specializations. 



The Gulf of Kara Bagaz, on the eastern side of the Caspian Sea, though 

 not marine, illustrates one case in which the process is in active development. 

 The Salina deposits of the upper Silurian in New York and adjacent States 

 is a good example of such action in past time. 



NON-MARINE SALINE DEPOSITS. 



Salt lakes. — Lakes without an outlet which have endured some time, 

 or playa lakes, subject to periodical desiccation, leave evidence of their 

 former existence in deposits of soluble salts. Gypsum and salt are the 

 most common, but borax, niter, and other less soluble substances, as the 

 calcareous tufas of the dying lakes of Nevada, tell the same story. Indica- 

 tions of such temporary bodies of water are also seen in the presence of non- 

 marine fossils, which show in their structure the effects of the stress of 

 adverse conditions. (See below, page 25.) The story of such lakes 

 may be in part interpreted from the succession of the deposits laid down 

 in the order of their solubilities; here the worker should turn to Clark's 

 Data of Geochemistry for invaluable information. Gilbert's History of 

 Lake Bonneville^ may be taken as a type study of such conditions and a 

 guide for future work. 



Salt swamps. — By this is not meant the great salt marshes of the sea- 

 coast, where the flux of the tides inundates and drains the land, bringing 

 an abundant marine life and supporting the growth of a typical and luxuriant 

 vegetation, but rather the great areas where depression of the surface 

 permits the accumulation of waters from salt springs. 



^ Chamberlin, T. C, On a Possible Reversal of Deep-sea Circulation and Its Influence on 

 Geological Climates, Journal of Geology, vol. 14, pp. 363-373, 1906. Dacqu6, 

 Grundlagen u. Methoden der Palaogeographie, pp. 172 and 213. 



^ Clark, C. W., Data of Geochemistry, Bull. 616, U. S. Geological Survey, 1916. 



' Gilbert, G. K., Lake Bonneville, Monograph i, U. S. Geological Survey, 1890. 



