PRECIPITATION OF THE METALS. 1087 



Many other similar illustrations could be given. It is recognized that 

 these relations are explained in part by the ready solubility of the lime- 

 stones as compared with the acid rocks, thus giving openings for the depo- 

 sition of ores, but also largely, it is believed, by the reaction of limestone, 

 or its solutions, upon the solutions which contributed the metals. This 

 conclusion, already announced from the geological relations, without refer- 

 ence to experimental work, has been confirmed by laboratory work by 

 Mr. H. N. Stokes. Mr. Stokes has shown that when soluble zinc, lead, 

 copper, or silver salts are heated with iron bisulphide, in the presence of an 

 alkaline carbonate, the metals are thrown down as sulphides. The reactions 

 are given on p. 1117. It was suggested that calcium carbonate might be 

 substituted for the sodium carbonate and similar reactions take place, and 

 this Mr. Stokes found to be the fact. It is therefore certain that where 

 limestone is the wall rock it often has a very important influence in the 

 precipitation of the ores. 



The supposition that the limestones are a cause of precipitation does 

 not preclude the possibility that they may not also contribute the metals, 

 as in the case of the lead of the Mississippi Valley. But in such cases the 

 precipitation is probably due more to the mingling of solutions in trunk 

 channels than to the effect of the wall rock. Of course, in some cases, the 

 association of ore with limestone may be due to more than one of the factors 

 mentioned — the ready solubility of the limestone, its power to cause direct 

 precipitation, to furnish precipitating solutions, and to supply the metals or 

 some part of them. 



Another case of precipitation resulting from the influence of the wall 

 rock is the well-known occurrence of metallic copper in the openings of 

 sandstones, conglomerates, and amygdaloids of Keweenaw Point." The 

 metallic copper between the particles was doubtless precipitated by ferrous 

 solutions furnished by the wall rocks, which in many cases are basic 

 volcanics. 



A particularly clear illustration of the effect of wall rock is furnished 

 by ores in which the sulphides are confined to strata containing organic 

 matter, as in some copper deposits b and some of the gold reefs of Australia. 



ffi Irving, R. D., The copper-bearing rocks of Lake Superior: Mon. U. S. Geol. Surv., vol. 5, 1883, 

 pp. 419^30. 



&Posepny, F., The genesis of ore deposits, and Cazin, F. M. F., discussion of same: Trans. Am. 

 Inst. Min. Eng., vol. 23, 1894, pp. 316, 606-607. 



