AGE OF MOUNTAIN BUILDING. 9 



of the Jurassic period took place the folding, flexing and faulting of the 

 beds which outlined the structural features of nearly all the meridional 

 ranges between the abrupt walls of the Wasatch and those of the Sierra 

 Nevada. At Eureka no direct evidence is offered as to the time when this 

 mountain building took place other than that the region was finally lifted 

 above the ocean after the deposition of the Upper Coal-measures. So far 

 as the mountains themselves are concerned, there is a total lack of evidence 

 that the blocking out of the ridges did not begin at the close of the Paleozoic 

 period, but, on the other hand, all observations tend to show that whenever 

 and by whatever causes the other Great Basin ranges were uplifted, the 

 same orographic conditions which prevailed elsewhere held true for the 

 Eureka Mountains. In other words, the Eureka Mountains were a part of 

 a more extended geological province. 



According to the conclusions of Mr. Clarence King, 1 based upon the 

 observations of the geologists of the Fortieth Parallel Exploration, the 

 mountains west of the Havallah Range and the meridian of 117 30' belong 

 to a post-Jurassic upheaval, and to the west of this line there existed during 

 Paleozoic time an elevated continental area which fumished the material 

 accumulated in an ocean basin to the east. At the close of the Paleozoic 

 this oceanic area, stretching as far eastward as the Wasatch, was lifted up 

 into a broad laud-mass, and the former continental region sank below the 

 water and in turn became an ocean basin. From the Wasatch westward 

 to this ancient shore line the mountain ridges exhibit much in common in 

 their structural and physical features, being made up in great measure of 

 Paleozoic strata, whereas from this boundary westward the ranges show 

 a marked contrast in the nature of their sedimentation and bear ample 

 paleontological evidence of their Mesozoic age. Over this latter area, not- 

 ably in the West Humboldt, Piute, and Augusta Mountains, limestones 

 characteristic of the Triassic and Jurassic have been described in detail by 

 the geologists of the Fortieth Parallel Exploration, 2 while to the east of this 

 shore line no Mesozoic rocks occur. Mr. King assigns excellent reasons for 



1 Geological Exploration of the Fortieth Parallel, vol. i, Systematic Geology, p. 733. Washing- 

 ton: 1878. 



"Geological Exploration of the Fortieth Parallel, vol. n. Descriptive Geology, pp. 657, 711, 

 ami 724. Washington. 1877. 



