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226 



THE AUBIFEROUS GRAVELS OF THE SIERRA NEVADA. 



150 feet above it. It is known as the u silicon lead." # The thickness of the 

 bed of infusorial matter could not be ascertained, as the workings did not 

 expose its full extent. It appeared, however, to be underlain by "white 

 lava/' and immediately above it was a stratum of quartzose and metamor- 

 phic gravel some four or five feet in thickness, and over this the dark vol- 

 canic breccia which forms the crest of the hill. 



i 



i 



Still another locality of infusorial silica is that in Chalk Ravine, at the 

 head of Shirt Tail Canon. This place was not visited by any person con- 

 nected with the Geological Survey. As described by Dr. J. P. Smith, 

 " Chalk Ravine " is a little branch of Shirt Tail Canon, situated about three 

 miles southwest of Damascus. The bed is said to have a thickness of fifty 

 feet or more, and the material to be an excellent polishing powder. Fossil 

 leaves have also been found here, it is said. 



It must have been apparent, from the detailed description of the gravels 

 and the associated volcanic deposits, that the epoch of their deposition was 

 on the whole a turbulent one. Hence it will not be surprising to the reader 

 to learn that the infusorial strata are considerably more extensively de- 

 veloped in some other portions of the Cordilleras than they are in that region 

 with which this volume is specially occupied. In the northeastern counties 

 of California, on the high volcanic plateau which occupies that portion of the 

 State, and where — so far as we know — no gravels occur, the infusorial rocks 

 occasionally acquire a great thickness. The same is the case all along on 

 the eastern slope of the Sierra Nevada, in the State of Nevada, and also in 

 Oregon, wdiere volcanic rocks are abundant and gravels of rare occurrence. 

 Hence it is that the most prolific localities — those furnishing the most abun- 



4 



dant material for description to the Berlin microscopist — have been those 

 outside of the gravel region. Some of the most interesting localities are in 

 Nevada, in quite close proximity to the boundary of California. Our parties 

 at various times obtained abundant specimens from the Silver Peak mining 

 district, from the vicinity of Aurora, and from points north of Virginia City 

 and along the Truckee River. Mr. King indicates the localities of infuso- 

 rial silica observed by the Fortieth Parallel Survey in this region, as fol- 

 lows : a The infusorial silica overlying the palagonite has its most important 

 outcrop at Fossil Hill, and along the whole northeastern edge of the Kaw r soh 



* Whether it was to the material obtained at this locality that the term a electro silicon " was first 

 applied the writer does not know. Tie has seen infusorial silica, labelled with this name, from various 

 places in California and Nevada, offered for sale in San Francisco. 





