ON THE FILLING OF MINERAL VEINS. 33 



1.04.10. Replacement. The conception of replacement is one 

 that has been applied of late years by some of the most reliable 

 observers. About 1873 it appears to have been first extensively 

 developed by Franz Posepny, an Austrian geologist, in relation to 

 certain lead-silver deposits at Raibl, in the Province of Kaernthen. 

 About the same time it was suggested by Pumpelly, then State 

 Geologist of Missouri, to Adolf Schmidt, who was engaged in 

 studying the iron deposits of Pilot Knob and Iron Mountain (see 

 Examples 11 and lla), and by Schmidt it was considered applicable 

 to them. ("Iron Ores and Coal Fields," Missouri Geol. Survey, 

 1873.) Some ten years later J. S. Curtis based his explanation of 

 the formation of the Eureka (Nev.) lead-silver deposits on the 

 same idea, and according to Emmons (1886) it holds good for Lead- 

 ville. R. D. Irving, who credited Pumpelly with bringing it to his 

 attention, published in 1886 an explanation of the hematite beds of 

 the Penokee-Gogebic range (Example 9c'), in which the idea is ap- 

 plied, and Van Hise has since elaborated it. In the process of re- 

 placement no great cavity is supposed to exist previously. There 

 is little, in fact, but a circulation or percolation of ore-bearing solu- 

 tions which exchange their metallic contents, molecule by molecule, 

 for the substance of the rock mass. We would not expect the ore 

 body to be as sharply defined against the walls as when it filled a 

 fissure, but rather to fade into barren material. Thus rock may 

 be impregnated but not entirely replaced, and, while apparently 

 unchanged, yet carry valuable amounts of ore. Some of the ores 

 of Aspen, Colo. (Example 30c?), are at times only to be distinguished 

 by assay from the barren limestones. Yet decomposition may 

 bring out the limits of each. 



1.04.11. The chemistry of the replacement process is none too 

 well understood, but it presents fewer difficulties when applied to a 

 soluble rock, like limestone or dolomite, than when rocks composed 

 of silicates and quartz have given away to ores. Acid solutions 

 would readily yield to calcium carbonate; but if the metals are pres- 

 ent as sulphates, some reducing agent, such as organic matter, is 

 necessary in order to change the metallic sulphate to sulphide. 1 

 Or else, if the metallic sulphides come up in solution with alkaline 

 sulphides, some third agent is needed to remove the calcium car- 

 Sulphides,'' Amer. Jour. Sci., III., xxxiii. 199; Eighth Ann. Rep. Direc- 

 tor U. S. Geol. Survey; Monograph XIII. , U. S. Geol. Survey, p. 965. 



1 Compare S. F. Emmons, " On the Replacement of Leadville Limestones 

 and Dolomites by Sulphides," Monograph XII., U. S. Geol. Survey, p. 563. 



