ABSENCE OF AN OUTLET. 33 



Pass, in Idaho, and then cut down its outlet to the depth of 370 feet. The 

 level of the first point of discharge determined the horizon of the highest 

 of the Bonneville terraces. The bottom of the gorge cut by the overflowing 

 stream determined the level of the Provo Beach, tlie most strongly accented 

 of all the Bonneville water lines. 



As previous writers on Lake Lahontan have conjectured that there was 

 an outlet at its southern end" this was the first portion to be explored when 

 the present study was undertaken. On examining Gabb's Valley, it was 

 found that a mountain barrier intervened between it and the Lahontan 

 basin to the northwest, thus proving that it was not occupied by that lake, 

 and, moreover, was not included in its hydrographic basin. From the 

 southern end of the Carson Desert a long narrow arm of the former lake 

 extended into the desert valley which opens southward from Allen's Springs. 

 The southern end of this valley is low and filled with alluvium which has 

 been formed into gravel bars by the waters of Lake Lahontan; these sweep 

 about the end of the basin in graceful curves, the highest in the series 

 coinciding with the horizon of the highest water level of Lake Lahontan, 

 thus proving conclusively that the lake did not here find an outlet. 



The highest of the Lahontan beaches at the eastern end of Alkali Val- 

 ley is far below the lowest part of Sand Spring Pass, which proves that La- 

 hontan did not enter Fairview Valley through this gap. 



The Lahontan beach may be traced with ease along the steej) vol- 

 canic bluifs bordering the Carson Desert on the south, from Allen's Spring, 

 to where the Carson River breaks through the range. The lake extended 

 through the Carson River canon and occupied Churchill Valle}' and the 

 valley of the Carson River as far as Dayton. Opposite Old Camp Churchill 

 there is a narrow gap in the hills bordering the valley on the south, wliich 

 at first gives promise of having been an outlet of the ancient lake. On fol- 

 lowing up this valley we find it ascending with a low grade and opening 

 through a narrow gap into Mason Valley, about which there are beach 

 lines, showing that it too was once filled by a lake. The highest beacli in 

 Mason Valley is on a level with the top of a narrow divide which has been 



>3King: U. S. Geological Exploration of the Fortieth Parallel, Vol. I, p. 507. Whitney: Cli- 

 ui.atic Changes in Later Geological Time, p. 110. Memoirs of the Museun: of Comparative Zoology of 

 liege. Vol. VII, : 



M<iK. s.i—3 



