308 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



the occurrence of irregular areas of basalt along the west base could be 

 accounted for by some irregularity in the settling of the valley block, 

 whereby it is traversed by smaller faults breaking it here and there into 

 secondary blocks. The visible basalt lies on small secondary blocks 

 that have lagged behind, while that associated with the main valley 

 block has been covered and concealed by lake beds and alluvium. Sub- 

 sidiary faulting might be expected in the mountain block also, where it 

 would break the basalt flow into parts that would be displaced with re- 

 spect to each other. Such changes are represented in figure 3, with 

 which may be compared the actually observed condition in the Hum- 

 boldt Lake range, as shown in the detailed section. 



W 



B 



/ 



/ 



^\\\V\VV\\\\\\^\^S^: 



g ^^^s^isssSsggsssgl 



Rhyolite and Tuffs ^^ 



Figure 1. — Structural Features. 



Diagrammatic east-west section to illustrate the chief structural features in the vicinity of the 

 detailed section at the time of the outpouring of the basalt. Tertiary lake beds and slight irregu- 

 larity of lower surface of basalt not represented. The bedrock structure is typical, not represent- 

 ing any particular section. 



Figure 2.— Simple Faulting. 



Diagram illustrating a possible result of simple faulting with tilting along the line A B of the 



country represented by figure 1. 



Western Intermontane 

 Valley 



w 



West Base 

 Basalt 



Secondary 



Mountain 



Blocks 



Eastern 



Intermontane 



Valley 



E 



l°o°o I Lahontan Beds and Alluvium 



Figure 3. — Formation of Minor Structural Features. 



Diagram illustrating the inferred mode of formation of the more important minor structural 

 features of the Humboldt Lake range as a simple modification of figure 2. The valleys are repre- 

 sented filled with alluvium and Quaternary, which hide any basalt that may exist on the sunken 

 blocks. The effect of erosion on the fault-scarps and the existence of alluvial cones is not repre- 

 sented, but the transition is easy from this diagram to the detailed section (plate 21) of the 

 Humboldt range. 



