THE CARSON DESERT 333 



be flatter if the mountains were so depressed that the volcanics became 

 almost horizontal again, is perhaps an indication of the original lacco- 

 litic character of the intrusive. This point was, however, not further 

 investigated. 



The alluvial slope on the east side of the mountains does not represent 

 a complete cone, for the section strikes the main cone quite a distance 

 from its apex, which is south of the section line. 



THE CARSON DESERT 



There are 5 miles of almost perfectly flat valley between the bottom 

 of the cone and the low divide between the Carson desert and Buena 

 Vista valley. There is no stream in this valley, and during most of the 

 year it is perfectly dry. Good shorelines are cut into the mountain 

 flanks. Most remarkable of all, a series of great semicircular lines may 

 be seen crossing the valley bottom, each with its concavity toward the 

 distant Carson sink. These are the traces of a receding shallow lake. 

 It would seem as if but a few months had elapsed since the waters were 

 on the valley bottom. One would think that a single dust storm such 

 as spreads over the Humboldt valley would obliterate all these lines, 

 and that a single year of winds would cover up the bunches of tufa ; but 

 there is practically no sand or dust drifting in this valley. Its bottom 

 is like a well kept floor, except where the occasional wagon wheel cuts 

 up the dust on the road. The valley is underlain by Lahontan sedi- 

 ments to an unknown thickness. 



The lowest part of this, valley where crossed by the section is at the 

 same altitude as the lowest part of the Humboldt valley. 



THE LOW DIVIDE 



The nature of the peculiar low divide between the Carson desert and 

 Buena Vista valley has already been discussed.* Where crossed by the 

 section it is of diorite, but a horizontal sheet of basalt caps it a short 

 distance to the north. The higher lake waters covered it many feet, but 

 when lower the waves swept over it, removing some of the basalt, and 

 when still lower notched the sides. 



MUD HOLE FLAT 



Between the divide and Chocolate butte is a broad stretch of lake bot- 

 tom in the middle of which is a depression that was an old strait of the 

 lake when the latter was at a very low level. This whole area forms a 

 basin separated from the Carson desert by the divide, and from Buena 



* See page 324. 



