8 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



ocean, but all the snow and rain that falls inside the rim of the basin is 

 returned to the atmosphere, either by direct evaporation from the soil or 

 after finding its way into some of the lakes that occupy the depressions of 

 the irregular suiface. The climate is dry in the extreme, thie average 

 yearly rainfall probably not exceeding 1 2 or 1 5 inches. 



The ai"ea thus isolated from oceanic water systems is 800 miles in length 

 from north to south, and nearly 500 miles broad in the widest part, and 

 contains not far from 208,500 square miles — an area nearly equal to that of 

 France. The southern part of the region includes the Colorado Desert, 

 Death Valley, and much of the arid country in southern California and Ne- 

 vada. In northern Nevada the Carson and Black Rock deserts exhibit the 

 extreme of desolation. The most northerly part of the Great Basin, occup}^- 

 ing the central portion of Oregon, is less barren, its rugged surface abound- 

 ing in long and narrow mountain ranges, volcanic table lands, and isolated 

 mesas, weathering as they grow old into rounded buttes, that are covered 

 with luxuriant bunch-grass and bear a scattered growth of cedars and pines. 

 At the south the valleys of the Great Basin are low-lying. Death Valley 

 and the Colorado Desert being depressed below the level of the sea ; but at 

 the north the valleys have a general elevation of from 4,000 to 5,000 feet, 

 while the intervening mountain ranges rise from 5,000 to 7,000 feet above 

 them. 



Diversifying this region are many moimtain ranges and broad desert 

 valleys, together with rivers, lakes, and canons, topographic elements to 

 be found in all quarters of the world, but here characterized by features 

 peculiar to the Great Basin The mountains exhibit a type of structure not 

 described before this region was explored, but now recognized by geologists 

 as the " Basin Range structure." They are long narrow ridges, usually 

 bearing nearly north and south, steep upon one side, where the broken 

 edges of the composing beds are exposed, but sloping on the other, with a 

 gentle angle. conformable to the dip of the strata. They have been formed 

 by the orographic tilting of blocks that are separated by profound faults, 

 and they do not exhibit the anticlinal and synclinal structures commonly 

 observed in mountains, but are monoclinal instead. 



