112 THE DESICCATION OF LATER GEOLOGICAL TIMES. 



and entire desiccation such as is now presented in parts of the Great Basin. 

 Let the desiccation continue and the drainage to the sea be cut oft', then only 

 the deeper portions of the lalce-Ulve expansions of the rivers would remain ; 

 and, if the climatic conditions did not change, but still favored diminution of 

 the water, we should finally have the alkaline flats and salt lakes of the Great 

 Basin only remaining. The disajjpearance of the glaciers and the formation 

 of the terraces are the preliminary steps to such a condition. 



The whole region of the Cordilleras has been from early times one highly 

 favorable, in its orographic conditions, to the development of lakes. A great 

 plateau-like area traversed by numerous ranges of mountains of course neces- 

 sitates a system of valleys of corresponding complexity. If we imagine the 

 corrugating forces to act always in one direction, and with uniform force, 

 these viilleys would not be likely to possess the necessary conditions for the 

 formation of extensive lake systems ; the depressions between the ranges 

 would be parallel to each other and would not vary much in depth in their 

 different portions. If the uplifting force died out equally in both directions 

 from the centre, the result would be a system of valleys draining both ways 

 from the highest point of the convex surface. If, on the other hand, the 

 whole uplifted mass sloped one way, the waters would necessarily flow toward 

 one side in preference. But if we suppose a force tending to pi'oduce a 

 corrugation of the surface, and acting transversely to that which has pre- 

 ceded it in point of time, it is evident that the result will be that the valleys 

 will be broken up into sections having difierent elevations above the sea- 

 level. We might suppose a secondary corrugation of this kind carried so far 

 as to almost obliterate the previous one, or at least to become the dominant 

 system ; but this seems to have been rarely if ever the case in nature, for a 

 certain persistence in the direction of the mountain-building forces seems to 

 have been maintained in all the great chains, even where these continued to 

 be developed during several successive geological periods. It has been stated 

 in the Auriferous Gravels* that there was proof that the Sierra Nevada has 

 undergone a certain amount of thrust, or been subjected to the pressure of 

 forces acting in a direction longitudinal to the trend of the range itself The 

 form of the valleys throughout the Great Basin indicates a similar condition 

 of things to have existed there in former geological ages, and there can be 

 little doubt that the depressions afterwards occupied by lakes owe their 

 oiigiii to this cause. Here, then, we have a good illustration of the differ- 



* See Aurifurous Gravels, p. 49. 



