CHAPTER II. 



GEOLOGICAL SKETCH OF THE EUREKA DISTRICT. 



Sedimentary rocks, belonging either to the Paleozoic or Quaternary 

 period, form by far the greater part of the mountains and valleys of the 

 Eureka District. The beds of the Quaternary present but little of geological 

 interest, and although they extend over wide areas they are, in most 

 instances, superficial accumulations composed of detrital material brought 

 down from the mountains and deposited along their flanks, concealing the 

 underlying rocks of the foothills. Igneous rocks play a most important 

 part in the geological history of the region, but nevertheless do not form an 

 imposing feature of the individual mountain uplifts, appearing either as ex- 

 travasated masses along lines of faulting, or as larger bodies encircling and 

 lying outside the main blocks of sedimentary formations. The older crys- 

 talline rocks offer a still less marked topographical feature of the country, 

 occupying very limited areas in the older Paleozoic limestones, where they 

 appear as intruded masses exposed by erosion. 



It is doubtful if within the province of the Great Basin there can be 

 found any region of equally restricted area surpassing the Eureka District 

 in its grand exposures of Paleozoic formations, especially of the lower and 

 middle portions. 



The great thickness of limestone aiid sandstone of which the Paleozoic 

 is composed was laid down under varying conditions of depth of water and 

 rapidity of deposition, with only one well recognized unconformity from its 

 base to summit. In this region the Paleozoic age was a time of compara- 

 tive freedom from dynamic movements. Most geologists who have given 

 any attention to the history of the Great Basin ranges substantially agree 

 that the movements that finally built up the mountains began after the 

 close of Paleozoic time, and that between the Carboniferous and the close 



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