CONDITIONS UNDER WHICH TUFA IS DEPOSITED. 187 



DEPOSITION OF CAIiCIUM CARBONATE. 



In considering the natnral methods by which calcium carbonate is pre- 

 cipitated from solution, we have, as preliminai'y data, that this salt is more 

 abundant than any otiier in ordinary surface waters, constituting, usually, 

 nearly one-half of the total of solids in solution in average rivers ; that it 

 is the first to be precipitated when such waters are evaporated ; and that 

 calcium ordinarily exists in solution as the bicarbonate, CaO COj, the pres- 

 ence of free carbon dioxide (carbonic acid) being essential for its solution. 



Under the conditions prevalent in nature it is evident, from the elemen- 

 tary laws of chemistry, that the precipitation of calcium carbonate may 

 take place in at least three ways : (a) By evaporation, concentration being 

 carried beyond the point of saturation, (b) By the loss of the carbon diox- 

 ide necessary to hold the salt in solution. This gas escapes when pressure 

 is removed or temperature increased ; it passes off gradually when carbon- 

 ated waters are exposed to the air, especially if agitated, as in the spray of 

 water-falls and in breaking waves, (c) By chemical reaction, as when an 

 alkaline carbonate is added to water holding calcium chloride in solution. 

 Of these three methods we need to consider at this time only the results of 

 concentration and loss of carbon dioxide ; as dissolved gases are driven off 

 during evaporation, these two methods act together. So far as the chem- 

 istry of inclosed lakes is concerned it is evident that the precipitation of 

 calcium carbonate must depend almost entirely on concentration by evapo- 

 ration ; chemical reaction may in certain cases play an important part, as 

 when a spring holding CaO CO2 rises in an alkaline lake; but, in general, 

 the conditions under which a number of salts may exist in a solution is too 

 little known to warrant one in ascribing a reaction to this cause when it may 

 be more simply explained as a result of evaporation. 



With this elementary sketch of the chemistry of natural waters we will 

 proceed with our study of the history of Lake Lahontan. 



