306 G. D. LOUDERBACK — STRUCTURE OF THE HUMBOLDT REGION 



sink. Except where removed by the mountain streams in forming their 

 canyons, or on the fault scarps as described above, the basalt forms the 

 surface of the eastern mountain slope. Plate 17, figure 2, shows fairly the 

 nature of the east side, all of the long eastward slopes being formed by the 

 basalt flow. This is everywhere underlain by the tuff, which is, however, 

 generally obscured by the black talus. Plate 18, figure 1, shows in the 

 distance the even slope of the basalt down to and underneath the valley 

 deposits. The underlying tuff can be made out at many places. The 

 shadow in the foreground is of the crest line directly back of the ob- 

 server and shows the characteristic slopes, the erosion cliff to the west, 

 and the dip slope to the east. The structure is shown quite well in sec- 

 tion, plate 16, figure 2. 



The slopes of basalt emerging from the valley and rising toward the 

 crest of the range can be seen for 12 or 15 miles along the east side, but 

 nowhere on the west slope has any basalt covering been found. At the 

 north end and along the southern stretches of the range, where the moun- 

 tains are low, the rhyolite passes over to the west side. In such places 

 it covers the whole range from east to west, and presents cliffs to the 

 west and long slopes to the east, corresponding to the plane of the tilted 

 flows, even on the west edge. 



Basalt has been found at the west base of the mountains just south and 

 north of Muttleberry canyon. A short distance south of the mouth of 

 the canyon a small hill (about half a mile wide) stands in front of, and 

 apparently disconnected from, the range slope. It is composed entirely 

 of basalt, very like that on the summit, which dips west at from 2 to 4 

 degrees. 



Some distance north of the canyon, on a sort of terrace in front of the 

 range, more basalt is found. It is identical with that on the summit in 

 all its details, including the differences noticed in the different layers. 

 It forms the surface of the terrace, except where obscured by alluvium, 

 etcetera, and extends a mile or more north and south and about the same 

 distance east and west. Plate 17, figure 1, shows a faulted portion of its 

 east edge looking almost due east. It dips about 20 degrees south. In 

 this plate the basalt- free slopes of the range are clearly shown. At its 

 west edge the basalt is practically horizontal. 



MOUNTAIN MOVEMENTS INDICATED BY THE VOLCANICS 



Bearing in mind the facts already presented which indicate that the 

 volcanic series was formed at a low angle of inclination to the horizontal 

 plane, the simplicity of its structure and the exceedingly slight amount 

 of alteration of its members should afford us easily decipherable evidence 

 of post- volcanic earth movements that have affected the range. 



