612 POPULAR SCIENCE MONTHLY. 



from the other ingredients. The lake is fed chiefly by Owen's 

 River, but at the present time it is slowly sinking. "Whether this 

 is due to growing aridity of the climate or because so much water is 

 taken from the streams for irrigation is not known. 



As we go north from the upper end of Owen's Valley a vast 

 table-land of lava and volcanic ash is encountered, extending to 

 Mono Lake. On the extreme head of Owen's River the volcanic 

 material rises to the summit of the Sierras, almost obliterating the 

 fault scarp for a number of miles. 



Mono Lake is nearly circular, with a diameter of twelve miles. 

 It has an elevation of a little more than six thousand feet, being 

 about two thousand feet higher than Owen's Lake, and like the 

 latter extends up to the very base of the fault cliffs forming the 

 eastern wall of the mountains. The water is intensely alkaline, 

 quite closely resembling in composition that of Owen's Lake. It is 

 situated in a depression in the sandy desert, being surrounded by 

 sand and volcanic rocks on all sides except the west, where rise the 

 snowy peaks of the Sierra Nevadas to an elevation of more than 

 thirteen thousand feet. Although the mountains at this point are 

 still very imposing, their grandeur does not compare with those 

 farther south in Owen's Valley. As we continue northward toward 

 Lake Tahoe it appears that there are two or more fault lines to be 

 made out, the lake itself, according to geologists, occupying a de- 

 pression caused by the sinking of one of these blocks. 



The geological history of much of the region along this series 

 of mighty fault fissures which have resulted in the formation of the 

 Sierra Nevada Mountains is as yet not thoroughly known. The 

 glacial history of Mono Lake and the adoining portion of the 

 Sierras has been studied by Russell, and is better known than that 

 farther south. It has been thought by Mr. Lindgren, of the United 

 States Geological Survey, that, following the period of volcanic 

 activity near the close of the Jurassic which finally culminated in the 

 formation of enormous fused granite magmas, the portion of the 

 crust embracing the Sierra Nevadas and the Great Basin became 

 arched upward. As the result of this, a strain was set up which 

 finally gave rise to north-and-south fault lines, and the region east 

 of that now occupied by the Sierra Nevadas began to settle. This 

 is supposed to have been inaugurated during the Cretaceous, and 

 has continued at various times and in different amounts down 

 to the present time. Toward the close of the Tertiary the dis- 

 placement of the Sierra Nevada fault block was markedly increased, 

 giving the range approximately its present outlines. There are 

 many indications that an equilibrium has not yet been reached, as 

 shown by the slip which took place at the time of the earthquake 



