10 GEOLOGY OF THE EUEEKA DISTRICT. 



the opinion that all the Great Basin ranges across Utah and Nevada were 

 uplifted at the same time under identical dynamic influences, and conse- 

 quently owe their origin mainly to a post-Jurassic movement. 



This indicates a marked unconformity between the Carboniferous and 

 Triassic, but it neither necessitates nor precludes the beginning of mountain 

 building over the Paleozoic area at the time of the uplifting of the conti- 

 nental laud-mass from beneath the ocean. Nowhere throughout this region, 

 any more than at Eureka, have the Great Basin ranges as yet offered any 

 direct evidence of folding accompanying this elevation, yet it would seem 

 highly probable that some crumpling of strata might have taken place before 

 the main blocking out of the mountain ridges at the close of Jurassic time. 



Most of the Great Basin ranges are narrow, longitudinal ridges, and 

 while they present much in common as to their origin and primary struc- 

 ture, each possesses its own special physical features due to local dynamic 

 conditions. Most of them are formed by direct lateral compression result- 

 ing in anticlinal folds, occasionally accompanied by synclines. Some of 

 them are simple mouoclinal ridges, representing one side of an anticlinal 

 axis. Still others exhibit great complexity of structure with both folding 

 and faulting along the meridional axes of the ranges, with which are asso- 

 ciated transverse faults and folds striking obliquely across the topograph- 

 ical trend of the uplifted mass. 



Orographic Blocks. The Eureka Mountains lie near the western edge of 

 what was at one time the Paleozoic ocean. The nearness of these uplifted 

 beds to an older pre-Paleozoic continent is in some measure indicated by the 

 relatively great amount of disturbance of strata and plication of mountain 

 masses as compared with the more gently inclined strata, and simplicity of 

 structure found farther to the eastward. Unlike the ordinary type of nar- 

 row ridges, the Eureka Mountains exhibit a solid mountain mass over 20 

 miles in width, including several uplifted blocks whose length does not 

 greatly exceed their width. Taken together they present a compact mass 

 of mountains thrown up by intense lateral compression accompanied by 

 longitudinal strain. The forces which brought about the elevation of the 

 mountains produced an intricate structure with powerful flexures and folds 

 and broke up this immense thickness of sediments into individual blocks 



