334 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



Vista valley by a low alluvium covered roll. Water gathers in winter in 

 the playa called the Mud hole, where animals find drinking water until 

 summer. 



CHOCOLA TE B UTTE 



This is the most northern of a range of hills running along in front of 

 the Table Mountain range for some miles and separated from it by a 

 flat valley where crossed by the section. The interesting features are : 

 the occurrence of the volcanic series, basalt, tuff, rhyolite, resting on 

 bedrock, which is exposed a little south of where the section crosses; 

 the eastward tilt of the volcanics, which pass underneath the valley, 

 with a structure closely comparable to the structure of the Humboldt 

 Lake mountains, and the normal faulting of the volcanics on the east 

 side, three distinct planes being determined in a space of 1,000 feet. 

 The strict analogy of the whole to the Humboldt Mountains uplift is 

 very instructive. 



TABLE MOUNTAIN 



After leaving the shorelines on the east side of Chocolate butte, and 

 passing over If miles of valley bottom, the section mounts a long allu- 

 vial cone.* This is unbroken by shore features, and gives a very regular 

 slope. 



The irregularity of the bedrock complex on the mountain slope is 

 very evident. Just south of the section line diorite intrusions are en- 

 countered. A layer of gypsum, whose white outcrop is visible across 

 the broad valley, occurs about two-thirds of the way up the slope. A 

 similar deposit occurs a mile or so north of the section, on the west flank 

 of the Humboldt mountains. This emphasizes the similarity of con- 

 ditions of deposition of the Triassic group in both ranges. 



The faulting of the basalt on the west edge of the table produces a 

 striking appearance. The mountain rises by a succession of steps, each 

 rise being a fault scarp, each tread a flat basalt covered shelf. 



Outline of the geological History of the District 



triassic TlME 



The oldest events of which definite knowledge has been obtained are 

 those connected with the deposition of the Triassic strata. The nature 

 of the floor on which these rocks were laid down has not been deter- 

 mined and is not evident from a study of the area examined. The abun- 

 dance of Ceratites, Ammonites, Pseudomonotis, and other mollusks, and 

 of crinoids, and the occurrence of Ichthyopterygian vertebrates f give 



* For certain dimensions, see p. 302. 



fMerriam: Bull. Dept. Geol., University of Caiifornia, vol. iii, no. 4. 



