24 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



been broken by a, vast nehvork of fractures accompanied by a tilting of the 

 included blocks, wliich have given origin to orogi-aphic basins on a grand 

 scale. On the borders of the region, in the glaciated valleys of the Sierra 

 Nevada and Wasatch mountains, rock basins due directly to the erosion of 

 glaciers may be counted by hundreds if not by thousands. From almost 

 any of the peaks of the High Sierra more than a score of lakes of this char- 

 acter may be observed. Lakes occupying barrier basins are also numerous 

 in the cafions of the Cordilleras where ancient moraines obsti'uct the drain- 

 age. A number of the Sierra Nevada lakes which owe their origin to ero- 

 sion and decomposition, resulting mainly from glacial action, will be de- 

 scribed in connection with the Quaternar}^ history of the Mono basin in 

 the Sixth Annual Report of the United States Geological Survey. At pres- 

 ent we are constrained to confine our attention to the more central portion 

 of the Great Basin The ar6a formerly occupied by glaciers in this region 

 is very limited, and as flowing ice has been the principal agent in the for- 

 mation of basins of erosion, this type of lake-basin is wanting, except about 

 the summits of some of the highest of the basin ranges. Barrier basins, 

 produced by the deposition of the current-borne dehris of ancient lakes in 

 such a manner as to obstruct the drainage of valleys, are not Tincommon in 

 the interior portion of the Great Basin, l)ut the depression,s characteristic of 

 the region are due to other causes. 



ORIGIN OF THE LAHONTAN BASIN. 



The more pronounced topograi)hic features of the Great Basin have 

 been found to be the result of orographic displacement. The typical 

 mountain structure of the region is monoclinal ; the elements being oro- 

 graphic blocks bounded by faults, and so tilted that their upturned edges 

 form mountain crests with a steep descent on one side and a more gentle 

 slope in the opposite direction. The upheaved edges of faulted blocks 

 usually appear as long and narrow ranges. Their depressed borders under- 



