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



These secondary blocks are generally discontinuous longitudinally — 

 that is, the} 7 may run for a half mile, a mile, or a number of miles, and 

 there cease. This is most strikingly shown in the subordinate valley 

 ridges, several of which have been described, for they form islands in 

 the midst of the lakebeds and alluvium. The longitudinal irregularity 

 of the valley block in another district is also well shown at the east base 

 of the Sierra Nevada, on the Carson topographic sheet,* where the main 

 intermontane valley is broken by subordinate blocks into a series of 

 subordinate valleys. 



It is an interesting fact that the greater number of these secondary 

 fault planes are roughly parallel to the primary fault plane, and the 

 throws are in the same sense. If in the main fault the west side was 

 the downthrow, so is it generally in the subordinate faults. This may 

 be seen in the detailed section and in the section of the Sierra Nevada 

 fault scarp (plate 16). In the Humboldt Lake range, where the primary 

 fault is on the west side, secondary faults on the east side show this same 

 approximate parallelism. The most eastern of these shown on the sec- 

 tion is 19,000 feet from the west base basalt, only 3,000 and 4,000 feet from 

 the east base of the range, and has a throw of 300 feet (perpendicular to 

 the layers). 



RELATION OF MOUNTAIN TO VALLEY BLOCKS 



The most striking feature of the detailed section is the breadth of the 

 valleys compared with the ranges. Quantitatively they are the chief 

 features. The structure of the Humboldt mountains, and others sup- 

 posed to be like them, has sometimes been represented by a great tilted 

 block that passes down under the valley until it strikes the fault plane 

 along the side of the next range. A study of the detailed section shows 

 that no such conception can be held for the Humboldt Lake range and 

 the valley of the Carson sink where crossed by the section. The basalt 

 and bedrock reach the surface twice on the way over. If the basalt slope 

 of the range were continued only to vertically below the west edge of 

 the divide near the center of the valley, it would sink 7,000 feet below 

 the valley floor — a depth in the upper part of the valley that we can 

 hardly imagine filled by deposition since the faulting took place. We 

 know there are at least three secondary valley blocks from the divide to 

 the East range. Why not several between the divide and the Humboldt 

 Lake range, which do not come to the surface ? Whether there is flexure 

 at the east side of the Humboldt mountains, or faulting, is not determin- 

 able by observation, but the general nature of the readjustments through- 

 out the region would favor the latter. 



* U. S. Geol. Survey Topographic Atlas, Carson, Nevada, sheet. 



