QUATEENARY DEAI1>1AGE. 29 



a surface 52,000 square; miles in cxto-iit ; Lake Lahoutaii's hydrograpliic 

 basin embraced 40,775 sqiuu'c miles. 



The Bonneville basin has its lowest depression along its eastern border, 

 now occupied by Great S;dt Lake; and its form was largely determined bv 

 the Wasatch fault, hi the Lahontan area the lowest depression is situated 

 near tlie base of the Sierra Nevada, and the topography of the basin. is de- 

 termined, to a considerable extent, by the fault which follows the eastern 

 base of that range. 



The Bonneville and Lahontan drainage areas had a common divide for 

 about 2") miles, between the 41st and 42d parallels, and a- little east of the 

 115th meridian. Southward of the 41st parallel the boundaries of the two 

 great hydrograpliic areas diverge, the included space being divided by short 

 mountain ranges into a number of independent basins, some of which held 

 Quaternary lakes of considerable size. 



The direction of the streams in the northern part of the Great Basin 

 shows that the area is divided by a central axis, irregular in its trend, from 

 which the surface has a general slope, ])oth eastward and westward, to the 

 bases of the inclosing mountains. 



From the Bonneville-Lahontan divide, north of Toano, the Humboldt 

 River flows westward through a narrow and rugged valle}' which crosses 

 the structui-al features of the country nearly at right angles. The course 

 of the river seems to have been determined in Tertiary times, or perhaps 

 earlier. During the Quaternar}' the Upper Humboldt Valley was occupied 

 by a stream larger than the present, which emptied into Lake Lahontan a 

 few miles east of the present site of Golconda. Before reaching the lake, 

 the Quaternary rivei' received considerable additions from the north thrt>ugh 

 the channels of the North Fork, Maggi, Rock, and Rabbit creeks, and the 

 Little Ilundjoldt River. Its most important tributary, however, in ancient 

 as in modern times, came from tiie southAvard, and flowed through the nar- 

 row Reese River Valley. 



On the north the Lahontan drainage area Avas bordered by the rim of 

 the Great Basin, and by a nundjer of small and inde{)endent areas of inte- 

 rior drainage, situated mostly in Uregon and in the nortliwestern corner of 

 Nevada. On the west the divide coincided for not less than 260 miles with 



