PRE-VOLCANIC TOPOGRAPHY 307 



If, in order to get an idea of the topography at the time the basalts 

 were poured out, we imagine the dip of the inclined volcanic series to 

 decrease until the angle becomes very low, the associated bedrock being 

 necessarily depressed to the same extent, the elevation of the range as a 

 whole would diminish with the decreasing angle of dip, and as this latter 

 approached the horizontal the range as a topographic feature would ap- 

 proach extinction. 



An examination of the base on which the volcanics were deposited 

 shows a remarkable absence of relief. Where the basalt lies directly on 

 the bedrock its upper and lower surfaces are at many observed localities 

 practically parallel planes, and the same may be said of the tuffs, 

 although the base of these has been observed at but three or four places. 

 The volcanic series may be traced for many miles on the east side with- 

 out encountering any bedrock ridges or other projecting forms protrud- 

 ing through them to the surface. However, gentle broad rises and de- 

 pressions can be made out at some places, which have influenced the 

 thickness of the volcanics or the distribution of the flows. Judging from 

 this latter evidence alone, we must conclude that the relief of the pre- 

 volcanic topography of the range was very low and at many places 

 closely approximated a plain. No pre-volcanic canyons filled with tuff 

 or lava have yet been observed. In other words, the erosion period rep- 

 resented by the pronounced unconformity between the bedrock and the 

 superjacent volcanic series had produced a country of very low relief, 

 approaching, to say the least, a peneplain condition. 



What is the nature of the process by which the mountains were brought 

 to their present condition and attitude? Two possible explanations of 

 the uplifting of the range suggest themselves — anticlinal folding and 

 faulting with tilting. 



Let us first examine the fault hypothesis. As just shown, we m&y 

 infer, in the vicinity of the detailed section before the deformation of the 

 basalt flows, the existence of a country of very low relief, covered widely 

 by a volcanic series, the highest member of which was an approximately 

 flat-lying basalt flow. This may be represented roughly by the dia- 

 grammatic east-west section (figure 1). 



If, now, faulting be supposed to occur along a plane whose trace is 

 marked A B (figure 1), the eastern block rising with tilting, the western 

 block subsiding, there results a condition represented in section by figure 

 2. This section, however, possesses the essential characteristics of a 

 section of the present Humboldt Lake range, the lava-free western slope 

 showing exposures of folded bedrock and the simply tilted volcanic cov- 

 ering of the eastern slope. Other peculiarities of the Humboldt range 

 are readily explainable as incidental to such a process. For example, 



