220 GEOLOGICAL niSTOUY OF LAKE LAOONTAK 



cium; yet all of these lakes are fed by waters that hold about the normal 

 amount (0.0088 per cent.) of calcium carbonate found in river waters. 

 When streams and springs enter these highly concentrated lakes, the cal- 

 cium carbonate they liold in solution is at once precipitated, either in an 

 amorphous or a cr^-stalline condition, and accumulates at the bottom in 

 the form of marl or, in some instances, as oolitic sand. In order that 

 tufa may be deposited about the borders of a large lake it is evident 

 that the calcium carbonate must remain in solution for a considerable time 

 so that it may be carried to distant parts of tlie lake; hence the lake waters 

 cannot be highly concentrated or else the calcium would be precipitated 

 before reaching points situated at a distance from the mouths of the inflow- 

 ing streams. From observation we learn that compact lithoid tufa is now 

 being deposited in Pyramid and Walker lakes, which contain about three- 

 tenths of one per cent, of total solids in solution. In the more highly con- 

 centrated lakes mentioned in this paragraph, no deposits of tufa have been 

 observed in process of formation. This evidently indicates that a lake in 

 which heavy deposits of calcium carbonate were accumulated could not 

 have been a concentrated solution during the tufa-forming stages. The 

 deposition of marl in lakes of concentrated water has not been observed, 

 but it appears probable that the highly calcareous beds found in the sedi- 

 ments of some of the Quaternary lakes of the Great Basin were precipitated 

 from saline waters. The precise chemical conditions which determine 

 whether the calcium carbonate precipitated from lake waters shall be 

 incoherent and form marl, or whether it shall crystallize on coming in 

 contact with foreign bodies or previously formed crystals has not been 

 determined. Questions of this character are in a great measure beyond 

 laboratory experiment for the reason that large bodies must be dealt with 

 in order to reproduce the conditions of nature. 



That the shells of mollusks occui' in thousands in both the lithoid and 

 the dendritic tufa, is also proof that the waters of Lake Lahontan were only 

 moderately concentrated at the time these deposits were formed. No traces 

 of fossils have been found in the thinolite crystals. 



When springs rise in the bottom of a lake a new element is introduced 

 into its cheniicid history. Suldacustral springs, charged with carbon dioxide 



