538 KIRK BRYAN 



Ground water is brought to the surface primarily by the change in 

 slope, but the dense silts and clays of the center of the basin tend 

 to act as a dam to prevent further movement down the slope. 

 Thus by their presence they assist the water to rise and may 

 determine the exact location of certain springs. Such border 

 springs are common in the intermontane valleys or bolsons of the 

 western United States. Those of the Big Smoky Valley, Nevada, 

 are shown in Figure 5 and have been described' as follows: 



The main west-side spring line of the upper valley extends, with a 

 sinuous course due to the different sizes of alluvial fans, from the Vigus 

 ranch to Wood's ranch, a distance of more than 30 miles, and includes 

 innumerable springs that discharge a part of the copious underground supply 

 received from the Toyabe Range. On the east side of the upper valley there 

 is no spring line comparable to that on the west side, probably because the 

 supply from the Toquima Range is smaller than that from the Toyabe Range, 

 but numerous springs similar to those on the west side are found for a 

 distance of 3 miles in the vicinity of the Charnock ranch. 



Concerning the Darrough Hot Springs, which lie along this 

 same line, Meinzer^ says: 



The water issues from bowldery fill, but probably comes originally from 

 the underlying rock, the heat being due either to igneous intrusion or to 

 faulting that opened deep fissures, or to both causes. Less than 100 feet from 

 the main hot spring and at a level a few feet higher there is a small spring 

 that issues at a temperature of only 60° "F., which is almost the normal 

 temperature for this region. 



b. springs in porous rock overlying impervious rock 

 (contact springs) 



Where porous rock overlies impervious material the water that 

 accumulates in the porous rock is forced to the surface at the 

 contact. Springs so formed may therefore be called contact 

 springs. The form and attitude of the surface of the underlying 

 impervious material divides them into three general types. In one 

 the surface is regular and horizontal, in another the surface is 

 regular but inclined, and in the third the surface is irregular. 



^O. E. Meinzer, "Geology and Water Resources of Big Smoky, Clayton, and 

 Alkali Spring Valleys, Nevada," U.S. Geol. Survey, Water-Supply Paper 42 j (191 7), 

 pp. 86, 87, Plate II. 



^ Ibid., p. 89. 



