THE TRINITY MOUNTAINS 327 



subordinate ridge, and the basalt capping on the low divide all indicate 

 that since the period of basalt eruption erosion has been comparatively 

 slight, both on the mountain tops and in the intermontane valley. The 

 first decipherable event after the basalt extrusion and its dislocation is 

 the formation of the alluvial cones, followed by lake Lahontan. Some 

 of the lower shorelines have been cut in the valley basalt. 



Conclusions. — From the above discussion we must conclude that the 

 history of the Table Mountain range and that of the valley range of hills 

 have been very similar to the history of the Humboldt mountains. In 

 fact, the same succession of events has been made out for each of these 

 now elevated areas. The only noteworthy new feature is that presented 

 by Table mountain of a crust block elevated along fault planes on both 

 sides to form a mountain range with but very slight tilting. 



THE TRINITY MOUNTAINS 



Extent of investigation. — The Trinity mountains are the next west of 

 the Humboldt. No general study was made of them, but the detailed 

 section was run across into their lower slopes, and their east flanks were 

 studied for some miles north and south. A number of similarities to 

 the conditions in the other ranges were found. 



The bedrock complex. — Where examined along the east flank the bed- 

 rock complex consists of granite with thick layers of hornfels, into which 

 it has, of course, been intruded. The metamorphism has been so com- 

 plete that the relationship of this hornfels to the rocks of the other ranges 

 can not be determined. 



The superjacent volcanics. — The volcanic rocks are displayed in a com- 

 plete series identical with that on the east side of the Humboldt Lake 

 range : 



Basalt (summit of series, always at surface). 

 Tuff. 



Rhyolite lava. 

 Tuff (base of series). 



The tuffs are sometimes very pumiceous and are generally distinctly 

 stratified. Layers of sand up to 6 inches in thickness were found at one 

 locality. The thickness of the basal tuff series varies from a few feet to 

 several hundred. The upper tuffs are generally comparatively thin and, 

 where not capped by basalt, are removed from the rhyolite surface. 

 Where carefully observed at two different localities, the basal tuffs ap- 

 peared to have been tilted before the outpouring of the rhyolite. 



There are indications that the rhyolite was furrowed by corrasion be- 

 fore the outpouring of the basalt, although no distinct angular uncon- 

 formity between them has been observed. 



