186 GEOLOGICAL HISTOEY OP LAKE LAHONTAN. 



From the analyses of the tributary streams we know that large quan- 

 tities of calcium carbonate are contributed to Great Salt Lake in solution ; 

 but the chemist fails to find this salt in the brine itself. The extreme 

 scai'city of animal and plant life in the waters shows that it could not be 

 removed by organic agencies ; it must, therefore, either be precipitated or 

 perhaps in part decomposed and changed to the chloride. The very small per- 

 centage of calcium in the lake, however, is sufficient proof that this element 

 must have been precipitated, probably as the carbonate, when the river and 

 lake waters were mingled The presence of large quantities of oolitic sand 

 along the shores of the lake, which is apparently still in process of forma- 

 tion, is strong evidence in support of this hypothesis This lake furnishes 

 a typical instance of the concentration of ordinary meteoric waters by evap- 

 oration. We may conclude, therefore, from this and other instances which 

 might be enumerated, that in inclosed lakes, as in the ocean, calcium car- 

 bonate will be the first salt precipitated as evaporation progresses. 



Great Salt Lake is now so concentrated that the crystallization of com- 

 mon salt is taking place in certain portions where the water is shallow. 

 Over hundreds of acres along its southwest border the bottom is covered 

 with a continuous crust of salt crystals, forming a pavement sufficiently 

 strong to support a horse and rider. This condition was observed during 

 the arid season ; whether the salt is redissolved during the rainy season or 

 not, has not been determined. From the observations recorded above we 

 learn that precipitation is now taking place in three separate ways in the 

 waters of a single lake ; (a) calcium carbonate is thrown down, probably 

 throughout the year, as the tributary streams mingle with the brine of the 

 lake ; (h) sodium sulphate is precipitated in all portions of the lake when 

 its temperature falls below 20° F.; (c) sodium chloride crystallizes in the 

 very shallow portions during the arid season. An example of this nature 

 teaches that a stratified saline deposit might form in an inclosed basin as a 

 result of fractional crystallization dependent upon changes of temperature. 

 The deposits found in the smaller of the Soda Lakes at Ragtown, Nevada 

 (see page 79), apparently illustrate such an occurrence. 



