OUTLINK OK THIS VOLUME. xvii 



fooling niolteu muss. The widtli of tlio dike has iinicli to do in detoniiiniiig tin- physical loiiditiniis 

 {foverniug crystallizatiuii. As regards the age of the dikes little is known other than that they jieue- 

 trate Silurian strata. 



CiIAPTEU VIII. The Enreka District offers no direct proof of the age or duration of voleanic 

 energy, although evidence hased upon observations elsewhere in the Great Basin points to the 

 conclusion that the lavas helong to the Tertiary period, and probably the greater part of them 

 to the Pliocene epoch. They broke out in four ways: First, through profound fissures along 

 meridional lines of displacement; second, following lines of orographic fracture, they border 

 and encircle large uplifted masses of sedimentary strata; third, they occur as dikes penetrating the 

 sedimentary rocks; fourth, they occur in one or two relatively large bodies, notably Richmond Moun- 

 tain and Pinto Peak, along Hues of displacement. The sequence of lavas was hornblende-audesite, 

 hornblende-mica-andesite, dacite, rhyolite, pyroxene-andesite, and basalt. The lavas display a great 

 variety of volcanic products in both chemical and mineral composition. They are all derived from a 

 common source, a homogeneous molten mass. They are due to a proc<!ss of diU'erentiation by molec- 

 ular change within the molten mass under varying conditions of pressure and temperature. Starting 

 with a magma of intermediate composition, the extreme products of such a dift'erentiation are rhyolite 

 and basalt. 



Chaptkk IX. In the Eureka District the ores oiciir in sedimentary rocks belonging to the 

 Cambrian, Silurian, and Devonian periods, and may be found in all horizons, except the Secret 

 Canyon and Hamburg shale, from the base of the Prospect Mountain limestone to the summit of the 

 Nevada limestone. Through 17.000 feet of strata ores have been dej>osited in sufficiently large 

 bodies to encourage mining exploration. The most productive dejiosits have been found in Cambrian 

 rocks, but this is owing to orograjjhic- and structural conditions rather than the geological age of 

 strata or chemical nature of sediments. Nearly all the more ])roductive mines are included within 

 the beds which form the Prospect Mountain uplift between the Hoosac and Spring Valley faults. The 

 ore followed the rhyolite and is consefiuently Pliocene or post-Pliocene age. All the ores came from 

 below and were originally deposited as suli)hides. They were subsequently oxidized by atmospheric 

 agencies, mainly surface waters percolating through the rocks. 



In Appendix A. Mr. C. D. Walcott gives a systematic list of fossils lioni each formation found 

 at Eureka. 



In Appendix B, Mr. ,Tose])li 1'. Iddiugs discusses the microscopi(-al petrography of the crystal- 

 line rocks. It is a thorough study of the mineral and .structural character of the rocks and is illu.s- 

 trated by several plates. 



MON XX II 



