76 LAKES OF NORTH AMEIUCA. 



Knowing the succession in which various salts are eliminated when 

 waters are concentrated by evaporation, it becomes possible to determine 

 in some instances, from the succession of salts discovered in a desiccated 

 lake basin, what changes occurred in the life of the lake from which they 

 were precipitated. In the case of Lake Lahontan, described in advance, 

 this method has led to interesting conclusions. In a similar way, the 

 chemical composition of a lake enables one to draw important inferences 

 in reference to its past history. A lake in which the rarer elements found 

 in tributary streams are almnclant, must have undergone a long period of 

 concentration, and formed cTfeposits of the more common and less readily 

 soluble salts. If a lake occupying an inclosed basin which has never 

 overflowed, contains but a small percentage of the salts most common in 

 the inflowing streams, it is evident that there must be some ^Drocess by 

 which such salts may be eliminated without being flooded out. Search 

 should then be made for this new principle. 



When lake waters are concentrated the first precipitates formed, as 

 already seen, are ferric oxide and calcium carljonate. These substances are 

 retained in solution mainly by reason of the presence of carbon dioxide, 

 carbonic acid, in the water. As evaporation progresses and also when the 

 waters are agitated, as in the breaking of waves on shore, the carbon 

 dioxide escapes and the iron and lime previously held in solution are 

 precipitated. The iron while in solution is in chemical combination with. 

 the carbon dioxide, forming ferric carbonate, when it loses its carbonic 

 acid it is precipitated as ferric oxide. The lime in solution is believed 

 to be in the form of the bicarbonate, and on losing carbon dioxide is pre- 

 cipitated as the carbonate. 



It is a fact of geological interest that iron and lime held in solution 

 may also be precipitated on account of the withdrawal of carbon dioxide 

 through the agency of plant life. Low forms of vegetation, known as 

 algae, thrive in the waters of both fresh and saline lakes and even in hot 

 springs where the temperature approximates to that of boiling water. 

 Through the vital action of this vegetation carbon dioxide is removed 

 from water in much ' the same manner that higher forms of plant life 

 eliminate it from the atmosphere. Carbon is assimilated and oxygen 

 liberated. Iron on parting with its carbon dioxide unites with the liber- 

 ated oxygen and is precipitated as ferric oxide. 



It has recently been shown that large deposits of both calcareous tufa 

 and silicious sinter are deposited thi-ough the agency of fresh water algae 

 from the waters of hot springs in the Yellowstone Park. The silicia 



