28 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



If desirable, illustrations of Basin Range structure might be multiplied 

 almost without number, not onl}^ in the Lahontan basin, but throughout 

 Nevada, Utah, and Arizona, and in parts of Oregon and California. On the 

 accompanying map, Plate III, an attempt is made to represent the course 

 of the faults that determined the main features in the present topography 

 of the Lahontan basin. The data for completing a map of this nature, 

 however, so as to present an accurate outline of the orography of the region, 

 have not been obtained, for the reason that special attention has not been 

 directed to the subject The lines of displacement that are shown have 

 been sketched from actual observation, and serve, at least in the absence 

 of more complete data, to indicate the vastness of the system of fractures 

 tliat have given diversity to the topography of the region. Could every 

 fault be indicated the map would be covered by an irregular network of 

 intersecting lines. 



The depression formerly occupied by Lake Lahontan may be taken as 

 the type of a compound rock-basin due to displacement, many of the minor 

 valleys of which it is composed being examples of fault-basins of the simplest 

 kind. 



GEOGRAPniCAL EXTENT. 



THF. IIYDROGRAPniC BASIN. 



During the Quaternary period, as at the present time, the region of 

 interior draiuaire between the Sierra Nevada and the Wasatch mountains 

 was divided into a large number of interior drainage areas or hydrographic 

 basins, two of which were of large size, and have claimed special attention. 

 These are included between the 3Sth and 42d parallels of latitude, and 

 together occup\- the entire breadth of the Great Basin. The one to the 

 eastward embraced northern and western Utah, together with small portions 

 of Idaho and W3'oming, and delivered its drainage to Lake Bonneville. 

 The hydrographic area to tlie westward included the northwestern part of 

 Nevada, together with small portions of California and Oregon, and dis- 

 charg(-(l into Lake Lahontan. Lake Bonneville received the drainage from 



