SPRINGS OF THE LAHONTAN BASIN. 47 



which serves to equahze its volume, as well as to clear its waters of im- 

 purities in suspension. A knowledge of the composition of the rivers 

 entering the Lahontan basin and tlieir average volume enables one to esti- 

 mate roughly the amount of mineral matter in solution that these streams 

 are now contributing to the lakes that they supply. Thi.s subject will be 

 reverted to in discussing the chemical history of Lake Lahontan. 



SPRINGS. 



Springs have been grouped with reference to their mode of occurrence, 

 in two convenient classes: (a) Hillside springs and (b) Fissure springs. 



Hillside springs are usually formed by the gathering of percolating 

 meteoric waters in inclined porous strata, which alternate with less pervious 

 beds, and outcrop on a hillside or among mountains in such a manner as to 

 afford an escape for the subterranean waters. The source of the water 

 forming hillside springs is in the rainfall of the immediate neighborhood. 

 They are commonly small, and their temperature is approximately the same 

 as the mean temperature of the locality at which they are found. Springs 

 of this class are usually agreeable to the taste and useful for domestic pur- 

 poses, for the reason that they are seldom liighl}' charged with mineral 

 matter. 



In western Nevada the conditions favoring the formation of hillside 

 springs are almost entirely absent. The rocks throughout the region are 

 very largely volcanic without definite stratification, and the rainfall is lim- 

 ited to a few inches annually. Owing to these unfavorable conditions, there 

 are no springs of this class in the Lahontan basin to claim our attention. 



Fissure springs occur where the earth's crust has been broken, usually 

 with some displacement, to a great depth. Their water supply, as in the 

 first instance, is from meteoric souices, but is derived from regions remote 

 from the point of discharge. Owing to the depth to which the water of 

 fissure springs frequently descends during its s-jbterranean passage, it is 

 commonly highly heated and not unfrequently reaches the surface with the 

 temperature of boiling water. The heat and tlie great pressure to which tlie 



