324 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



Occurrence of secondary blocks. — But, first, it would be well to show 

 that Table mountain is broken by minor faults into secondary blocks. 

 Standing on the top of the " table " and following the basalt flats with 

 the eye, one is immediately struck by the fact that there is not one single 

 uniform table, but several. One table will spread out for a quarter or 

 a half or even a whole mile, and then suddenly end in a drop or rise 

 leading to another table. In other words, the general tableland is broken 

 up into a number of secondary plane surfaced tables, which are sepa- 

 rated by sharp vertical breaks or by erosion channels. They do not flow 

 into one another by lava cascades ; the result is due to normal faulting. 



Basalt in the valley of Carson sink. — About two-sevenths the distance 

 from the western base of Table mountain to the Humboldt Lake range 

 a line of hills rises out of the alluvium of the intermontane valley and 

 runs for some miles at a small angle to the front of the East range. A 

 general east-west profile across the summit of the most northern of these 

 hills is given in the detailed section. It rises about 850 feet above the 

 surrounding valley floor. In structure it is a miniature of the Humboldt 

 Lake range. It is capped by basalt, which passes, though somewhat 

 broken by normal faulting, from the summit to the east base and is 

 absent from the west slope. The basalt is underlain by tuff, and this 

 by rhyolite lava, all dipping east. Where crossed by the detailed sec- 

 tion the bedrock does not appear at the surface, but half a mile south 

 the volcanic series is seen to be underlain by quartzite, diorite, etcetera, 

 striking north and south and dipping high to the west. 



The divide. — About 3£ miles west of the base of these hills a low 

 divide occurs. This low, flat ridge, though only 150 feet above the valley 

 bottom where crossed by the detailed section and probably only 100 feet 

 where crossed by the wagon road, separates completely Buena Vista 

 valley from the valley of Carson sink. It runs from the south end of 

 the Star Peak range southward, diagonally across the valley, toward the 

 East range. It is in part capped by basalt, but where crossed by the 

 detailed section the bedrock reaches the surface. The basalt is flat-lying 

 over the greatly eroded and planed surface of the bedrock complex. 



It may be here pointed out that the report of the Fortieth Parallel 

 Survey gives a wrong idea of this basalt-covered divide and one quite 

 out of consonance with the general conceptions presented in this paper. 

 Referring to this ridge, it says : * 



" It is of considerable interest, as it forms a divide across the valley, completely 

 shutting in the Carson and Humboldt desert to the south, and is one of the few 

 prominent cross-ridges of Tertiary eruptive masses connecting the longitudinal 

 ranges of the basin." 



* Fortieth Parallel Survey Report, vol. ii, p. 704. 



