336 G. D. LOTTDERBACR — STRUCTURE OF THE HUMBOLDT REGION 



found anywhere in the vicinity. It is probable that the great amount 

 of deformation by folding produced at that time resulted in the forma- 

 tion of mountains of considerable magnitude. We may conclude this 

 both from the extent of the folding, and especially from the magnitude 

 of the resulting denudation, which has stripped large areas of granite of 

 their thick covering of Mesozoic rocks. If we divide the circle of azi- 

 muths into four parts by northeast-southwest and northwest-southeast 

 diameters, we may sa) 7 that the axes of uplift — the anticlinal axes — as 

 far as observed, lie almost entirely in the north and south quadrants, 

 and particularly cluster near the north-south line. Axes lying in the 

 east-west quadrants are very rare and are neither long nor important. 

 As a general rule, it may be stated that the axes of folding commonly 

 lie more east of north and west of south than the ridge lines of the pres- 

 ent ranges. 



It was probably at the time of the post-Jurassic upheaval that the 

 greater part of the granite was intruded. As far as the direct evidence 

 of intrusion at the localities studied goes, this might have taken place 

 at the close of the Triassic, as the only rock of definitely determinable 

 age with which the granite has been found in intrusive contact is Trias- 

 sic ; but the comparatively small disturbances at that time and the great 

 disturbances at the end of the Jurassic, combined with the fact that 

 farther west the intrusion of great masses of granite in post-Jurassic and 

 pre-Chico (Upper Cretaceous) time* is well established, makes a strong 

 case for the close of the Jurassic as the time of the intrusion of the gran- 

 ites of the Humboldt and the East ranges. 



Nothing has been said about volcanic activity during the early Meso- 

 zoic, or about the intrusion of the diorites. The existence of both has 

 been determined, but the character and extent of the Triassic eruptives 

 and the age of the diorites were not investigated. 



THE GREAT EROSION INTERVAL 



Following the post-Jurassic disturbances, a long period of subaerial 

 erosion was inaugurated. There were apparently no indications of a 

 " great basin " at that time, and the drainage probably passed freely to 

 the sea, for no deposits corresponding to Cretaceous time are found any- 

 where in the Basin region today, while exceedingly thick deposits occur 

 along the coast and along that region east of the Great basin that was 

 covered by the epicontinental Cretaceous sea. These deposits show by 

 their character, thickness, and remarkable changes in their included 

 organic forms that the Cretaceous was a very long period. 



*See citations, p. 319. 



