SALINE DEPOSITS OF GREAT SALT LAKE. 185 



deposits, in some instances, gives evidence as to whether the beds in which 

 they occur were j)i'ecipitated from ocean or inland waters. 



The precipitation of salts in inclosed lakes is still farther illustrated by 

 Great Salt Lake, Utah, the composition of which in the years 1850, 1869, 

 and 1873, is given in Table 0. At the present time the lake is lower than 

 in 1873, when the last analysis was made, but there is no reason to suppose 

 that there has been any change in the salts with which the waters are 

 charged. As in all inclosed lakes, the percentage of total salts in a given 

 quantity of the brine changes as the waters rise and fall. To illustrate, 

 the amount of saline matter contributed to Great Salt Lake during a single 

 year, for example, is so small in comparison with the quantity which the lake 

 holds in solution, and varies so little from year to year, thatthe composition of 

 the residue obtained by evaporating a sample of the lake brine would 

 remain practically constant for a long period, provided precipitation did not 

 take place. The amount of water reaching the lake varies with the seasons, 

 and also undergoes secular fluctuations, dependent on climatic changes, 

 extending over a term of yeai-s. The brine is thus diluted when the lake 

 is unusually full, and greatly concentrated when the lake is reduced abnor- 

 mally by evaporation. 



Owing to the large amount of sodium sulphate dissolved in the waters 

 of Great Salt Lake, and since it is much more soluble in warm than in 

 cold water, its precipitation takes place during cold weather. When the 

 temperature rises it is redissolved. If the waters of the lake are cooled 

 artificially to about 20° Fahrenheit, an abundant precipitation of floc- 

 culent sodium sulphate takes jjlace. Each year, on the approach of cold 

 weather, the waters of the lake lose their transparency and become cloudy 

 and opalescent, owing to the precipitation of sodium sulphate in a state of 

 minute subdivision. In the depth of winter the temperature of the atmos- 

 phere about the lake sometimes falls as low as — 20° F. On these occasions 

 sodium sulphate is precipitated in immense quantities and collects along the 

 shores in thousands of tons. Nature has here anticipated Balard's process 

 for obtaining sodium sulphate, and is carrying on the operation on a grand 

 scale. 



