ACADEMY OF SCIENCES. 245 



distance, as sccu from San Francisco, is not, as mii^lit be expected, of detrital 

 material, but of rock as hard as the pliocene. It appears to be a terrace of 

 marine abrasion rather than of subaqueous deposit, and to belong to the older 

 day of the San TJriino mountains — the cretaceous. * * * « 



The remaining peculiar surface and scenic features of the vicinity of San 

 Francisco, are found in the channels of subaqueous erosion, like that continuing 

 from the Mission Dolores through the New Park to the ocean. These were 

 minor (jlolden Gates when our city hills and Lone Mountain were Alcatrazes, 

 long after the close of the pliocene. It is an open question whether the elong. 

 ated and level sandy beaches, similar to that found on the north side of Lone 

 Mountain, halfway to its top, are ascribable to Neptune or Ventosus. 



A discussion followed, in the course of which Prof. Davidson, Mr. 

 Stretch and Prof. Bolander participated. 



Mr. Stretch remarked that the terraces of Mono Lake, and also 

 of the lakes in Nevada, indicated a gradual and uniform rise. 



Notes on the Silver Mines of Pioche. 



BY F. E. DUKAND. 



The silver mines of Pioche are situated on veins running in an easterly and 

 westerly direction. These veins are of different characters. The Meadow Val- 

 ley, Pioche, Raymond & Ely, Washington and Creole, appear to be on a vein 

 with a bifurcation, or on two veins crossing each other. Higher on the hill are 

 two other veins, distant about 18 feet ; their general direction is also easterly 

 and westerly, and parallel with the first vein. On these second veins are situ- 

 ated the American Flag, Arkansas, Chapman, Huhn & Hunt, Dcsdemona, 

 Ivanhoe, etc. Farther east is the vein of the Peavine, running north and south. 

 All these veins are enclosed between walls of very hard, stratified quartzite ; this 

 quartzite is generally colored by peroxide of iron. 



The veins, which are nearly vertical, cut the stratification at right angles ; 

 but the convulsions which have produced this fissure have lifted one side, as the 

 planes of stratification do not correspond across the vein. In some places the 

 quartzite is covered by micaceous slate, which lies in unconformable stratifica- 

 tion. Above this slate is a very thick formation of stratified crystalline mag- 

 nesian limestone. The line of contact of the slate and limestone contains a great 

 amount of metallic oxides (iron or manganese), hematite, braunite, manganite 

 and pyrolusite. 



The silver veins of Pioche are generally very regular; they are encased in 

 hard rock, requiring little timbering, and there is no water in the mines. The 

 •deepest worked mine is about 500 feet below the surface. In the superior 

 parts, the ores are chloride of silver, mixed with crystallized quartz ; but gen- 

 erally the ores are carbonate of lead containing silver, very likely in the state 



