ATTITUDE OF FAULT PLANES 343 



Southward from the line of the detailed section the valley of Carson 

 sink widens and the valley ridges disappear, so that, as far as observa- 

 tion goes, the structure might be, as has been elsewhere suggested, the 

 tilted Humboldt Mountain block passing down under the valley deposits 

 until it strikes against the East Range block; but even here it is pos- 

 sible that the valley block does not lie at so great a distance below the 

 alluvial surface, and the amount of post-basaltic erosion, judged from 

 the lava-covered slopes, would fall very far short of filling valleys, which 

 are broader than the ranges, several miles vertically with lake beds and 

 alluvium, as would be required by a strict adherence to a single moun- 

 tain-valley block hypothesis. 



The valley blocks are best thought of as separate and distinct from 

 the mountain blocks, even when the tilted mountain surface dips down 

 underneath the present valley floor. The former have, as a whole, rela- 

 tively sunk, the latter risen. 



RANGE SLOPES AND FA ULT PLANES 



Notwithstanding the breaking down of the fault scarp, especial^ 

 where it intersects the original surface as described above, all indications 

 point to a relatively small amount of general erosion since the produc- 

 tion of this feature. 



If in the detailed section we connect with a straight line the point 

 where the bedrock series is first exposed at the top of the alluvial cone 

 with the west edge of the table of Table mountain, the angle with the 

 horizontal is 11 degrees 21 minutes. The angle similarly obtained for 

 the west slope of Chocolate butte is 12 degrees 31 minutes ; for the west 

 slope of the Humboldt Lake range, 7 degrees 47 minutes. 



It is a very interesting fact that all of these profiles when smoothed 

 out are concave upward, the angle of slope increasing as the summit is 

 approached. This is quite different from the east side of, the Humboldt 

 Lake range, which is irregular and in part convex. The western slopes 

 are, in fact, typical erosion profiles, and have developed on the scarp 

 side better than on the basalt-covered slopes, probably because of the 

 scarp's greater initial slope and less resistant covering. 



From the youthful character of the drainage, with no appreciable lat- 

 eral cutting or undermining along the range base, the apparently slight 

 change in the basalt surface layer by general erosion, and from the 

 nature of the profile, it may be judged that the amount of post-basaltic 

 erosion has been comparatively small. It is probable, considering the 

 comparative softness of the rocks and the erosion profile, that the ero- 

 sion on the west side has been greater than on the volcanic-covered 

 slopes ; but a comparison of the drainage forms on both sides of the 



