OUTLINE OF THIS VOLUME. 



CHAPTER I. The area covered by the geological survey of the Eureka Mountains embraces a 

 region of country 20 miles square. The mountains are situated on the Nevada plateau and form a 

 somewhat isolated mass, surrounded on all sides by the broad detrital valleys so characteristic of 

 the Great Basiu. These valleys which encircle the mountains have an average elevation above sea 

 level of 6,000 feet. Rising above them the highest peaks attain altitudes varying from 9,000 to 10,500 

 feet. In strong contrast with most of the Great Basin ranges, the Eureka Mountains present a rough 

 and rugged appearance, with varied topographical features. 



CHAPTER II. Sedimentary rocks belonging either to the Paleozoic or Quaternary age form the 

 greater part of the mountains and valleys. Quaternary beds present little of geological interest, 

 although they extend over wide areas, being mainly superficial accumulations composed of detrital 

 material brought down from the mountains and deposited along their flanks and out over the broad 

 plains. A great thickness of limestone, sandstone, and shale, which make up the Paleozoic series of 

 rocks, was laid down under varying conditions of depth of water and rapidity of deposition with 

 only one well recognized unconformity from base to summit. In this region the Paleozoic age was a 

 time of comparative freedom from dynamic movements. Eureka presents no direct evidence as to 

 the time mountain building took place other than that the region was elevated into a broad conti- 

 nental land mass after the deposition of the Upper Coal-measure limestone. Reasons are assigned for 

 supposing that all the Great Basin ranges owe their origin to a post-Jurassic movement. The folding, 

 flexing, and faulting which outlined the mountains broke up this mass of sediments into six sharply 

 denned orographic blocks, each with well marked structural peculiarities. These mountain blocks 

 have been designated as follows: Prospect Ridge, Fish Creek Mountains, Silverado and County Peak 

 group, Mahogany Hills, Diamond Mountains, and Carbon Ridge and Spring Hill group. Taken 

 together these six blocks present a compact mass of mountains, the result of intense lateral com- 

 pression and longitudinal strain. Profound longitudinal faults extend the entire length of the moun- 

 tains, showing a displacement of beds of over 13,000 feet. The Paleozoic sediments measure 30,000 

 feet in thickness, with Cambrian, Silurian, Devonian, and Carboniferous, all well represented by 

 characteristic fauna. In these four periods fourteen epochs have been recognized. 



CHAPTER III. Cambrian rocks measure 7,700 feet, divided into five epochs, as follows: Pros- 

 pect Mountain quartzite, Prospect Mountain limestone, Secret Canyon shale, Hamburg limestone, and 

 Hamburg shale. The Middle, Lower, and Upper Cambrian are all exposed. On the crest of Prospect 

 Ridge, at the base of the Cambrian limestone, occurs the Olenellus shale, the oldest fossiliferous strata 

 recognized in the Great Basin. Hamburg Ridge carries a Potsdam fauna both at its base and summit. 

 Conformably overlying the Cambrian come the Silurian rocks, 5,000 feet in thickness. They 

 fall readily into three epochs, two limestones and an intervening body of quartzite. They have 

 been designated Pogonip limestone, Eureka quartzite, and Lone Mountain limestone. The qnartzite 



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