STUDIES OF SPECIAL LACUSTKAL HISTORY. 113 



carbonate thus concentrated, is indicated by the weight of the calcareous 

 tufa lining the basin. In ortlinary river Avaters, as already shown, the 

 calcium carbonate is about the same as the amount of all other salts in 

 solution. It follows, therefore, that the more soluble salts contributed to 

 Lake Lahontan must have been equal in weight to the tufa deposits just 

 described. Such a vast quantity of saline matter, if contained in the 

 present lakes, would make them concentrated brines. The question is, 

 what has become of the more soluble salts contriljuted to the waters of the 

 ancient sea ? 



A lake may occasionally evaporate to dryness, or exist as a playa lake 

 for a long ])eriod, that is, expanding during rainy seasons and becoming 

 desiccated eitlier during (hy seasons, or occasional!}' in years of unusual 

 aridity. Under such conditions its contained salts would be precipitated 

 and become buried or absorbed by mechanical sediments, so that when 

 a change of climate permitted the existence of a perennial lake in the 

 same basin, it would be fresh, or essentially so. This is what seems to 

 have occurred in the Lahontan basin. The old lake was prol)aljly evapor- 

 ated to dryness and the preci})itated salts l)uried beneath playa clays, and 

 Avhen a change to slightly more humid conditions permitted of the birth 

 of the present lakes, a new cycle was begun. 



From analyses of the waters flowing into the present lake of the 

 Lahontan l}asin, it lias been estimated that under existing conditions they 

 would acquire their present degree of salinity in about 300 years. It 

 seems to follow from this study that during a long term of years, ending 

 al)out 300 years ago, the climate of Nevada was so intensely arid that no 

 perennial lakes could exist within her borders. 



An account of the physical and chemical histories of the ancient lakes 

 of Utah and Nevada should be followed by a description of the plants and 

 animals that found a home on their shores, but unfortunately our informa- 

 tion in this connection is vague. 



The sediments of lakes. Bonneville and Lahontan, unlike many other 

 lake-beds, are extremely poor in vegetable fossils. As the conditions for 

 the preservation of such remains were favorable, and as an extended 

 search has failed to unearth so much as a single leaf or a single water- 

 logged tree-truidv from their sediments, it may reasonably be eonc liided 

 that their shores were not forested, and were probal)ly even more barren 

 and desolate than at the present day. Tliis result cannot be considered 

 as surprising in view of the great fluctuation of elimate that the Great 

 Basin experienced in Pleistocene times. 



