136 qricKsiLVER deposits of the pacific slope. 



conditions magnesia passed into solution is not known, nor even what salt 

 of magnesia was dissolved. The structural evidence, however, strongly 

 favors the supposition that the silica of the sandstones reacted directly 

 upon magnesian solutions. Were it otherwise, the sandstones might have 

 been impregnated with hydrous and anhydrous, ferromagnesian silicates, 

 l)ut the quartz grains could not have been attacked, as they certainly are 

 in the altered sandstones, while in the recrystallized rocks they have alto- 

 gether disappeared. 



The experimental researches of Dr. Hunt and of Mr. Daubree indi- 

 cate the kind of reaction which must be supposed to have gone on in the 

 rocks of the Coast Ranges. As has already been stated. Dr. Hunt found 

 tliat when alkaline carbonates in solution are heated with .silica and mag- 

 nesium carbonate an alkaline silicate is formed which reacts upon the 

 magnesium carbonate, yielding an insoluble magnesium silicate and regen- 

 erating the alkaline silicate. So, also, Mr. Daubree discovered that water 

 heated at a high pressui-e in glass with kaolin results in the formation of 

 zeolites, feldspar, pyroxene, and quartz. In such experiments the tem- 

 perature and pressure must of course be reduced below the boiling point 

 before any satisfactory examination of the results can be made. Conse- 

 quently, it is not at present possible to say under what special conditions 

 of temperature and pressure each mineral was formed. Tlie heated waters 

 in the granite and the overlying strata of the Coast Ranges must have 

 contained carbonic acid in solution. It is not inconsistent with any known 

 facts to suppose that these waters attacked the granite at great depths, dis- 

 solving alkalis, magnesium, and iron, with perhaps a certain amount of 

 silica; nor is there anything to forbid the supposition that at lower press- 

 ures, and perhaps also at lower temperatures, such solutions brought in 

 contact with calcareous sandstone would form feldspars, pyroxene, amphi- 

 bole, and serpentine. In the experiments solution and precipitation were 

 confined to the same locality, while in the Coast Ranges solution went on 

 at great depths and precipitation at more moderate ones; but nothing is as 

 vet known which makes it necessary to suppose that the conditions of tem- 

 perature and pressure or the general character of the reactions in the 

 Coast Ranges differed essentially from those in tlie experimental investiga- 



