CONTENTS 291 



Page 



Discussion of special features of detailed section 328 



Method of preparing the section 328 



The west end 329 



Lone mountain 329 



The Humboldt valley 330 



The Humboldt Lake mountains 331 



The Carson desert 333 



The low divide 333 



Mud Hole flat 333 



Chocolate butte 334 



Table mountain , 334 



Outline of the geological history of the district 334 



Triassic time 334 



Jurassic time 335 



Post-Jurassic upheaval „ 335 



The great erosion interval 336 



Volcanic activity 337 



Oogenic disturbances 338 



Recent 339 



Comparison with the Sierra Nevada 340 



Discussion of certain features connected with the faulting 341 



Magnitude , 341 



The secondary faulting 341 



Relation of mountain to valley blocks 342 



Range slopes and fault planes 343 



Use of volcanic rocks in deciphering orogenic movements 344 



Directions to visiting geologists = 345 



Introduction 



The existence of mountain ranges in western America formed by the 

 tilting or relative uplifting of great blocks of the earth's crust, acting as 

 comparatively rigid masses, with subordinate or no flexing or folding of 

 the strata, was first pointed out by G. K. Gilbert in 1873.* He presented 

 and discussed a number of sections and descriptions of ranges in Utah 

 and eastern Nevada, showing the futility of any folding and denudation 

 hypothesis, and the applicability of the notion of origin by faulting. 

 As the ranges of the Great basin form a rather distinct geographical 

 system, and as the faulted block structure was supposed to be the pre- 

 vailing type, Gilbert discussed it as the structure of the Basin Range 

 system. Not long afterward, Powell,f in outlining the different types 



* U. S. Geographical Survey West of the Hundredth Meridian (Wheeler Survey), vol. iii, pp. 21-42. 

 t U. S. Geol. and Geog. Survey of the Territories ; Geology of the Eastern Portion of the Uintah 

 Mountains, p. 16. 



