210 GEOLOGICAL HrSTORY OF LAKE LAHONTAN. 



of the solution from which the calcium carbonate was crystallized. The 

 lithoid and dendritic varieties having a much closer resemblance to each 

 other than either has to the middle or thinolite member; it is apparent that 

 the chemical conditions which favored their deposition, although not ident- 

 ical, were not widely divergent. As the first and third members of the 

 series were deposited when the lake was deep and of broad extent, we may 

 conclude that they were precipitated from comparatively dilute solutions ; 

 the thinolite, on the other hand, is only found low in the basin, and must 

 consequently have crystallized from waters that were more concentrated 



The structure of the isolated domes, towers, castles, etc., corresponds, 

 even in minute detail, with the structure of the tufa layers sheathing the 

 sides of the basin ; thus adding strength, if additional evidence were needed, 

 to the conclusion that they were deposited dui'ing three well-defined stages 

 in the history of the former lake. 



CONDITIONS FAVORING THE DEPOSITION OF TUFA. 



From the facts already gathered concerning the history of Lake La- 

 hontan, it is evident that the principal condition which favored the precipi- 

 tation of the calcium carbonate dissolved in its waters, was concentration 

 by evaporation ; the tributary streams at the same time continuing to sup- 

 ply fresh quantities of calcium carbonate. We do not forget, however, that 

 chemical reactions must have taken place among the various salts as the 

 lake became concentrated, which would aflfect the nature of the precipitates. 

 The conditions under wliich a mixture of saline substances exists in solu- 

 ~tion are too little known, however, to enable one to determine what 

 changes may have taken place 



The conditions favoring the formation of lithoid tufa seem simple in 

 their nature and not difficult to determine. This variety is apparently iden- 

 tical with that precipitated in neighboring Quaternary lakes, and is very 

 similar to the deposits now forming about many springs, or being- precipitated 

 from the spray of water-falls, and from the waters of lakes in which evap- 

 (natidu equals or exceeds supply. The de])osIts now forming owe their de- 

 position to the loss of carbon dioxide, and to evaporation ; and are so sim- 

 ilar to tile first-iormed Lahoiitan tufa, that there seems no doubt but that 



