12 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



return of humid conditions the water that reoccupies the old basui may be 

 comparatively, if not absolutely, fresh. 



To the artist the scenery of the arid lands of the Far West contrasts 

 with that of more humid regions by tlte russet-brown desolation of the 

 valleys, the brilliant colors of the naked rocks, and the sharp, angular out- 

 lines of the mountains. A country without water is necessarily a desert, 

 while with abundant moisture, at least in tropical and temperate latitudes, 

 it becomes a garden of luxuriant vegetation. In the most desert portions 

 of the Great Basin the annual precipitation does not exceed four inches, 

 while in the valleys on the borders of the basin it probably reaches 20 or 

 30 inches. Throughout this region the only fruitful areas are along the 

 margins of streams, or where springs come to the surface. In such places, 

 where water can be had for irrigation, one finds oases of delicious shade, 

 with green fields and orchards yielding an unusually abundant harvest. 

 Thus in nearly all its physical features the Great Basin stands in marked 

 contrast with those favored lands where rain is more abundant and more 

 evenly distributed. 



The rainfall that a region receives is a jiotent though silent factor, which 

 controls an almost infinite series of results in its physical history and topog- 

 raphy. In a humid region vegetation is usually luxuriant; the rock forms 

 are masked by forests, erosion is rapid, and the rocks are commonly buried 

 beneath the accumulations of their own debris or concealed by layers of 

 vegetable and animal mould that in turn are clothed with vegetation. The 

 hills have flowing outlines and are dark with foliage. The valleys have 

 gently sloping sides that conduct the drainage into streams meandering 

 through broad plains, and the whole scene has the softness and beauty of a 

 garden. In an arid land like the Great Basin all this is changed. The 

 mountains are rugged and angular, usually unclothed by vegetation, and 

 receive their color from the rocks of which they are composed. From the 

 gorges and canons sculjDtured in the mountain sides alluvial cones descend 

 to the plain. These sometimes have an extent of several miles, and they 

 are steep or gentle in slope according to the grade of the streams that formed 

 them. The valleys, even more dreary than the mountains, are without 

 arboreal vegetation and without streams, and form a picture of desolation 



