LAKES BONNEVILLE AND LAHONTAN. 193 



Mono, described by RiisselV bave risen nearly or quite to overflowino-. 

 Tbe climate of the Great Basin of interior tb-ainag-e, wbicb comprises the 

 areas of these lakes, was marked at these times by considerably greater 

 humidity than now, though to less deg'ree than the present climate of the 

 eastern' half of the United States. The humid epochs were divided by 

 a long interval of aridlt\', in which, as Gilbert and Russell have shown, 

 the lakes were perhaps wholly evaporated, their soluble salt and alkaline 

 mineral matter becoming intermingled and covered with playa silts, so that 

 it could not be redissolved by the water of the lakes dui-ing their second 

 rise, which therefore may haye been nearly fresh. 



Lake Bomieville, the largest one of the many lakes which were formed 

 during the Pleistocene period in the Great Basin, covered at its maximum 

 stage an area of 19,750 square miles, lying mostly in northwestern Utah, 

 but extending also into the borders of Nevada and Idaho. It was about 

 ten times as large as its present representative. Great Salt Lake, which, 

 having a mean height of 4,208 feet above the sea, lies 1,000 feet below the 

 highest of the ancient shore-lines. The maximum depth of the Pleistocene 

 lake was about 1,0.50 feet, while that of Great Salt Lake, in its range from 

 the lowest to the highest stage within the past forty years, is from 36 to 49 

 feet. The hydrographic basin of Lake Bonneville comprised a fourth part 

 of the Great Basin, whose total area is estimated to be 210,000 square miles; 

 almost another quarter was tributary to the companion Lake Lahontan, 

 which attained an extent of 8,422 square miles, occupying a very irregular 

 tract of interlocking valleys among mountain ranges in western Nevada; 

 and the remaining half of this arid region held some twenty-five smaller 

 lakes, much exceeding, however, the saline lakes and playas to which they 

 are now reduced. 



The first great rise of Lake Bonneville, lifting its level to within 90 

 feet of the lowest point of the inclosing watershed, is recorded by numer- 

 ous beaches, marking the oscillations of the lake level under the varjnng 

 influence of secular climatic changes, and by a thick deposit of yellow 



' Geological History of Lake LiiUontau. By I. C. Russell. Monographs of the U. S. Geol. Survey, 

 Vol. XI. Quaternary History of Mono Valley, California. By i. C Kussell. Eighth Annual Report 

 of the U. S. Geol. Survey. 



MON XXV 13 



