SECTION OF HUMBOLDT LAKE RANGE 331 



slope from the Humboldt mountains. At present the central part of 

 this valley is covered with the horizontally laid and exceedingly fine 

 grained alluvial deposits of Humboldt river, which, after its journey of 

 perhaps 340 miles, is dissipated and disappears entirely a few miles 

 south of the line where the section crosses the valley. Under this allu- 

 vium lie the Lahontan beds, of unknown depth, but probably a few 

 hundred feet. They are not exposed by a cut where crossed by the sec- 

 tion, but their presence is inferred from the relation of the valley floor 

 to bordering shore features. Some miles up the river they are beauti- 

 fully displayed in the river trench, which is a hundred feet or more deep 

 in places. They are chiefly well stratified, but unconsolidated light col- 

 ored sands and clays. 



The alluvial cone on the east side of the valley has already been 

 described. 



THE HUMBOLDT LAKE MOUNTAINS 



The depth of the basalt on the hill at the base of the mountains and 

 what rock, if any, separates it from the bedrock are not determinable. 

 It has a dip to the west, but only one-half that of the general range 

 slope. An erosion trench from one to over two hundred feet below the 

 summit of the basalt hill separates it from the range slope. The moun- 

 tain side of this depression is everywhere bedrock. The area of basalt 

 a mile or two north of this is broader and practically horizontal except 

 at its southeast end, where it dips south, as shown in plate 17, figure 1. 

 This northern basalt is furrowed by Lahontan shorelines carrying rolled 

 basalt pebbles, and it also carries deposits of lake tufa. 



The section starts up the mountains on the side of a canyon, and the 

 swinging of this canyon and the occurrence of diagonally feeding streams 

 are the chief causes of the irregularity of the slope. 



At about a mile from the west base the main drainage channel turns 

 almost at right angle and heads north. If we follow the line of the 

 trunk stream eastward a divide of vertical slates 250 feet wide at its base 

 and 60 or 70 feet high must be crossed, and then a canyon continues the 

 line up to the summit. The drainage down this upper canyon turns at 

 the low divide, runs south for a short distance, and then turns west 

 down the next canyon of the lower slope. These peculiar north-south 

 stretches trace closely the locus of a fault along which the east side 

 has risen and the west side dropped. This is described because it is a 

 type of a number of cases observed in these mountains where stream 

 lines follow fault planes and where fault planes explain what would 

 otherwise appear anomalous drainage configurations. 



We may call those streams which follow a fault plane consequent, 

 when the stream was forced into that path by the actual uplifting of one 



