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



Basic dikes. — At several points on the lower part of the western slope 

 basic dikes were found, running roughly parallel to the range trend. 

 They are black and granular on fresh fracture, and weather with a thick 

 limonite film. In the field it was suspected that the} 7 were the feeders 

 of the basalt flows and had been truncated b} 7 the fault plane. Under 

 the microscope they are seen to be quite fresh basaltic rocks, made up of 

 a noncrystalline aggregate of basic plagioclase, augite, and magnetite. 

 The last two are more abundant than in the basalts above described, but 

 olivine was not recognized in the sections examined. 



SUMMARY OF STRATIGRAPHIC EVIDENCE OF FAULTING 



It has been shown that the structure of the folded, faulted, and partly 

 altered bedrock series is distinctly out of consonance with the trend and 

 form of the range, and that its mass was degraded to low relief before 

 the eruption of the volcanics. These volcanics were laid down at a low 

 angle, and their deformation is approximately a measure of the deforma- 

 tion of the rocks of the range in post-basaltic times. The occupation of 

 the whole east slope from summit to base for many miles by the volcanics, 

 together with their entire absence from the west slope and the occurrence 

 of rocks of almost identical mineralogical and structural features at the 

 west base of the range, all of them being affected by simple tilting, com- 

 bined with normal faulting, is all easily explained by faulting along the 

 west base and a lifting and tilting of the range toward the east, the west 

 slope being the eroded remains of the fault scarp. None of the dis- 

 tinctive features can be explained by anticlinal uplift. 



PHYSIOGRAPHIC EVIDENCE OF FA ULTING 



Relationship of structure to range lines and fronts. — Lack of depend- 

 ence between ridge lines and associated tectonic features, and continuity 

 of range front irrespective of the parallelism or obliquity of rock struc- 

 tures, form an important criterion of dominant lateral faulting in moun- 

 tain upheavals. This was in part first stated by Gilbert in giving his 

 generalizations on the Basin Range structure, as follows : * 



" Furthermore, ridge lines are more persistent than structures. In the same 

 continuous ridge are monoclinals with opposed dip, as in the Timpahute, Pahran- 

 agat, and House ranges — or monoclinal and anticlinal, as in the Spring Mountain 

 and Snake ranges." 



Recently W. M. Davis, in critically discussing the argument and con- 

 clusions in Spurr's paper on "The Origin and Structure of the Basin 



* U. S. Geog. Survey West of the 100th Meridian, vol. iii, p. 41. 





