180 GEOLOGY OF THE EUREKA DISTRICT. 



base of the Silurian system. The transition beds show a constant change 

 in the deposit, all of them pi-esenting more or less mixed material, develop- 

 ing into argillites, grits, and arenaceous schists, finally passing into distinctly 

 bedded uniform limestone. Overhang the Pogonip rests the Eureka 

 quartzite, about 500 feet in thickness, but singularly uniform in its material 

 and wholly uiilike all the other siliceous formations, being formed of pm-e 

 white siliceous grains completely metamorphosed to quartzite. Through 

 this formation, except at its base, there is a marked absence of mixed sedi- 

 ments and off-shore material. All the conditions of deposition suggest deeper 

 water and a quieter sea bottom. Immediately above the Eureka quartzite 

 comes an immense development of limestone, with occasional iuterbedded 

 sandstones at varying intervals, but comparatively free of earthy matter. 

 Taken together, the Lone Mountain and Nevada formations which make up 

 these limestones measure nearly 8,000 feet in thickness, apparently laid 

 down without any decided break in the conditions of deposition, although 

 the accumulation of such a vast amount of calcareous sediment must have 

 occupied a long period of time. That it was sufficient, notwithstanding its 

 uniformity, to bring about marked chang'es in the life of the sea is shown 

 by its faunal development. There exists no greater break in the character 

 of the sediments than that found between the Nevada limestone and the 

 overlying White Pine shale. The change from the calcareous deposit of a 

 quiet ocean with a marine fauna to an argillaceous deposit full of carbon- 

 aceous material with evidence of cross-bedding, rapid currents, and shallow 

 water recalls a retreating sea and the proximity of a land surface. 



Remains of vegetable life are of rare occurrence in the Paleozoic 

 rocks of the Great Basin and eastward of the East Humboldt Range are 

 quite unknown. The White Pine shale, both at Eureka and White Pine, 

 carries innumerable fragmentary bits of twigs and stems throughout the 

 entu'e formation, although for the inost part too poorly preserved for 

 specific determination, yet indicating land areas throughout a long period 

 of time. Over the White Pine shale was laid down the Diamond Peak 

 quartzite, a uniform deposit of fine grained siliceous material without any 

 special evidence of the proximity of land, either in the life or mode of 

 deposition. An interstratified bed of limestone carrying Productus semire- 



