176 GEOLOGY OF THE EUREKA DISTEICT. 



ban-iers. In the neighborhood of the East Humboldt Range a pre-Cam- 

 brian barrier may have existed, but the evidence seems rather in favor of 

 islands rising out of an ocean, which stretched across the entire area between 

 broad continental regions. 



The evidence of this ancient shore-line rests, as has been pointed out 

 elsewhere, upon the complete and unmistakable differences in the charac- 

 ter of the beds which now lie uncomformaldy upon the older rocks of 

 the continental area and the deposits upon the ocean bottom to the 

 east. Upon this pre-Cambrian continent no Paleozoic rocks have as yet 

 been recoo-nized in western Nevada, while the enormous thickness of 

 uncomformable strata laid down since the uidifting of the Paleozoic area 

 bears ample evidence of a Jura-Trias fauna, as shown in the Piute and 

 West Humboldt ranges. Again, the Mesozoic rocks of the Wasatch and 

 those of western Nevada bear but slight resemblance to each other, either 

 in the nature of the material deposited or in the character of the life repre- 

 sented. Across this broad intervening area all evidences of Mesozoic sedi- 

 ments are wanting and the opposite sides are sharply contrasted by their 

 physical and faunal distinctions. In all probabilit}- the Paleozoic ocean in 

 Nevada presented an indented shore-line with a general northeast and 

 southwest trend, accompanied by outlying islands stretching far eastward 

 and rising high above the water level. This ancient coast line has never 

 been traced, consequently its outlines are most indefinitely determined. It 

 is obscured by enormous quantities of erupted material, in places literally 

 mountain high, burying for long distances all traces of preexisting rocks. 



The course of this eruptive action was in great part determined by the 

 profound displacements which accompanied the elevation and transformed 

 an ocean bottom into an area of dry land. Measured at right angles to the 

 supposed trend of the sliore are broad areas 40 miles in width across which 

 no other rocks are exposed than Tertiary lavas or the recent Pleistocene 

 deposits of the valleys. That these lavas followed the trend of the old 

 shore is indicated by their parallelism with the mountain uplifts, and that 

 they conformed to the course of the jjreexisting continental ranges is shown 

 by the frequent outcrops of Jura-Trias rocks j)rojecting above the vast accii- 



