242 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



Ltibontan fossils are smaller and exhibit greater variation than the same 

 species when living under normal conditions. On compai'ing the size of 

 Pompholjjx from Anaho Island (lower Lahontan), " white terrace" (upper 

 Lahontaii), and White Pine, Nevada, (living), the ratio of 63: 88: 100 was 

 obtained." 



The investigations of conchologists have proven that there are al least 

 three variations of environment Avhich may cause depauperation in fresh 

 water niollusks, viz., salinity, low temperature, and scarcity of food. 



As regards salinity, it has been shown that a sudden change from 

 fresh to saline water, i. e., to water resembling that of the ocean, is fatal 

 to fresh-water moUusks. When the change is gradual the life of the spe- 

 cies may be maintained until a considerable degree of salinity is reached, 

 but the limit has not been determined, and is known to vary widely with 

 difl'erent species. To make the expei'iments in this direction definite and 

 comparable with the gradual changes which take place in the waters of 

 inclosed lakes, would I'equire a much greater length of time than has yet 

 been devoted to the subject. Enough has been determined, however, to 

 show that a gradual increase in the salinity of a lake would be accompanied 

 with the depauperation and decrease of its moUuscan life; should the salinity 

 continue to increase until a condition approximating that of the ocean was 

 reached, the molluscan life would become nearly if not completely extinct. 

 We may reasonably conclude, therefore, that the waters of Lake Lahontan 

 were not strongly alkaline or saline during the time the sediments and 

 tufas so richly charged with fossil shells were deposited. It is perhaps 

 well to .mention in this connection that there is no reason to doubt that the 

 niollusks whose shells are found in such abundance actually inhabited the 

 ancient lake. They could not have been contributed by inflowing streams 

 and are not found in exceptional abundance where springs entered the 

 lake. The degree of salinity attained by the ancient lake cannot be deter- 

 mined, but, as we have seen, must have been low, at least during the high- 

 water stages. This conclusion is most definite in the case of the upper 

 lacustral marls which are frequently charged with the shells oi' Anodoida, a 

 genus which, at the present time, is confined to waters of exceptional purity. 



" These uieasurenieuts refer to the length of the ahells. 



