LAKES OF THE LAHONTAN BASIN. 75 



the numerous volcanic bombs that depress the strata on which thev rest. 

 If the cavities owed their origin to springs of very great magnitude rising in 

 t-he bottom of Lake Lahontnn, it is evident that the out-flowing waters would 

 have cut channels of overflow when the lake evaporated to a horizon below 

 the rim of unconsolidated material that surrounded them ; but the crater 

 walls ai-e now continuous and unbroken by stream channels. On the other 

 hand, had the springs become extinct before the evaporation of the lake the 

 cavities they formerly occupied would be buried beneath lake-beds. Tliis, 

 as our observations show, is not the case, but both the inner and outer 

 surfaces of the cones are free from lake sediments The last addition of 

 lapilli to the walls of the crater nuist have been of post-Lahontan date. 



The least diameter of the larger crater at the water surface is ;-i,168, 

 and its greater 4,224 feet. Its area, as stated on a previous page, is 268.5 

 acres. A sublacustral spring of these dimensions, rising with sufficient 

 force to carry blocks of basalt 1 or 2 feet in diameter to the lieight of 

 150 feet, would be a phenomenon without parallel. That the lakes occupy 

 extinct craters is recognized by Mr. Arnold Hague in his description of the 

 Carson Desert.'^" 



There are no streams either tributary to or draining these lakes ; their 

 total water supply, excepting the small amount derived from direct precipi- 

 tation, is supplied from subterranean sources. Around the immediate shores 

 of the largei- lake there are a number of fresh-water springs ; the largest of 

 these is situated on the northern border of the basin, and issues from a small 

 fault at an elevation of about ! 5 feet above the water surface. As the lake, 

 by aneroid measurements, is 50 feet below the level of the Carson River at 

 its nearest point, we may safely look to this stream as the j^robable source of 

 the water supply which reaches the craters by percolating through the inter- 

 vening marls and lapilli deposits. The bottom of the lake, as determined 

 by many soundings, is a continuation of the slope of the inner walls of the 

 crater, excepting that the conical form has been modified by shore action and 

 sedimentation, which has resulted in the formation of the terrace about the 

 present water margin. In the northern part of the lake a reef of rock pro- 

 jects above the surface, and soundings show that this is continuous from 



^ U. S. Geological Exploration Fortieth Parallel, Vol. II, p. 746. 



