CHAPTER VII. 



PRE-TERTIARY IGNEOUS ROCKS. 



igneous rocks have played a most important part in the development 

 of the geological history of the Eureka District. They may be separated 

 into two distinct groups : first, those which reached their present position in 

 pre-Tertiary times; second, a younger and much more extended series of 

 eruptions, those of Tertiary and post-Tertiary age. Not only do they be- 

 long to distinct geological periods, but their mode of occurrence is quite 

 unlike and their petrographical characters in every way different. 



Granite, granite-porphyry, and quartz-porphyry are the types of the 

 pre-Tertiary rocks. Their sm-face exposures are very restricted, being 

 quite insignificant as compared with the more recent volcanic lavas, and 

 only to a very limited degree have their extrusions influenced the present 

 physical features of the country. 



Granite.— Between the Sierra and the Wasatch there are probably few of 

 the many longitudinal ranges which rib the Great Basin, other than those 

 made up entirely of volcanic lavas, that do not show one or more bodies of 

 granite or crystalline schists of greater or less extent. Along the lines of 

 upheaval of one or two of these ranges, the accumulations of recent lavas 

 have been on so vast a scale that all direct evidences of an older preexist- 

 ing range are to-day wholly wanting. In some instances granite and gneisses 

 cover large tracts of country and occasionally culminate in peaks rising 

 high above the surrounding i-egions, but so abrupt are the changes in the 

 Archean topography that they occur for the most part only as subordinate 

 exposures over limited areas. The granite is found cropping out along the 

 base of the foot-hills beneath the Paleozoic sediments, occasionally occupy- 

 ing low passes tln-ough breaks in the ranges, or, as is frequently the case, 

 they are associated with extrusions of volcanic lavas and accidentally left 

 bare or else uncovered by recent erosion. Westward of the Salt Lake Basin, 



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