CHANGES IN OCEAN LEVEL. 179 



In the Prospect Mountain limestones occur narrow belts and lenticular 

 bodies of clay shales, in contradistinction to the Pogonip, which is charac- 

 terized by a series of grits and sandstones. Throughout the 6,000 feet of 

 Nevada limestone the pure sandstone beds, taken together, would scarcely 

 measure more than 300 feet, varying from 25 to 100 feet in thick- 

 ness, while argillaceous strata are exceptional occurrences. In the White 

 Pine shale, which is mainly argillaceous, occur several beds of reddish 

 sandstone near the summit, amounting in the aggregate to several hundred 

 feet. Deducting from the limestone epochs the beds that are decidedly 

 siliceous and argillaceous, but leaving the impure strata, which are mainly 

 calcareous, we find the aggregate thickness of the three classes of sedi- 

 ments as follows : Argillaceous, 3,000; siliceous, 9,000; calcareous, 18,000; 

 or as 1 : 3 : 6. 



The Prospect Mountain quartzite throughout, at least so far as it is 

 exposed at Eureka, can hardly be otherwise than an off-shore deposit. The 

 base of the formation is largely composed of coarse conglomerates, made 

 up of a great variety of unassorted siliceous pebbles, while the finer beds 

 nowhere present any considerable thickness, and show evidence of strong 

 currents. It is certain that such material could not have been transported 

 for any great distance in deep water. At the top transition beds of 

 siliceous sands pass rapidly into the limestones of the Prospect Mountain 

 series, which, across a thickness of 3,000 feet, carries but one heavy, per- 

 sistent belt of clay shale. 



The Secret Canyon shale, a remarkably uniform horizon throughout 

 its great thickness, presents a wholly different character of deposits from 

 the two underlying formations. Nowhere are there any evidences of rapid 

 deposition. Following the latter formation comes a second belt of lime- 

 stone, with occasional beds of siliceous material in place of the clayey beds 

 found in the Prospect Mountain limestone. Above this second belt of lime- 

 stone occurs the Hamburg shale, indicating a complete change in the sedi- 

 mentation and reproducing conditions like those of the Secret Canyon 

 period, although only 350 feet in thickness. In the character of its deposits 

 it changes more rapidly and shows unstable conditions in its mode of sedi- 

 mentation. Next in order is found the widespread Pogonip limestone, the 



