Vol. 4] 



Lawson. — The Robinson Mining District. 



295 



The Ely Limestone. — Beneath this formation is the Ely lime- 

 stone, consisting of regularly stratified, thick-bedded, more or 

 less cherty limestone to a thickness of about 1,500 feet. Its 

 regolith is scant, the weathered surfaces are often of a light 

 lilac gray color, and it forms the boldest bluffs, the most precip- 

 itous slopes and the highest summits of the district. It also car- 

 ries Carboniferous fossils, but the only forms identified by Mr. 

 Girty to whom they were submitted are crinoidal fragments and 

 Product us aff. P. porreetus Kut. 



THE DEVONIAN. 



The White Pine Shale. — Below the Ely limestone are forma- 

 tions of supposed Devonian age. The first of these in descending 

 order is a body of shale of somewhat varied character. A good 

 deal of it is ordinary, blue-black, argillaceous shale. Most of the 

 formation is, however, rather siliceous and yields a regolith filled 

 with flat, angular rock chips, which have a yellowish color. Cer- 

 tain portions of the formation, as for example, to the north and 

 northwest of Copper Flat and in the vicinity of Pilot Knob, are 

 carbonaceous and have a dead black color. In other cases, as 

 in the vicinity of the Chainman Mine and Pilot Knob, the rock 

 while having an evenly and thinly laminated shaly structure is 

 almost wholly quartzose in composition. In several portions of 

 the shale belt, the shale has been bleached to a snow white or 

 a cream white color, due, it is presumed, to the action of emana- 

 tions from neighboring irruptive rocks, or more probably of 

 sulphuric acid arising from oxidation of the overlying pyri^if- 

 erous porphyry. Some of these have a chalky consistency. A 

 chemical examination of two samples of this bleached shale from 

 the gulch to the east of Jupiter Ridge was made for the writer by 

 Mr. Herbert Ross of the Ruth Mine with the following results : 





I. 



II. 



SiO, 



65.0 



C7.1 



A1A 



28.5 



28.1 



CaO 



1.8 



1.0 



H 2 



5.1 



3.7 





100.4 



99.9 



