Page:Popular Science Monthly Volume 52.djvu/632

612 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 depression 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 adoiningadjoining [sic] 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 displacement 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