THE ORIGIN AND DESCENT OF ROCKS. 479 



enally pure ferric oxides along certain lines of ground-water circula- 

 tion, and in certain areas of free leaching. Van Hise has shown the 

 definite relation between the water circulation and the production of 

 the high-grade ores.* Vast quantities of unconcentrated lean ores lie 

 in the tracts not thus purified and enriched by circulating waters. This 

 does not appear to be simply residual concentration. The waters seem 

 to have added ferric oxide brought from above, while they carried away 

 the '^impurities/'' siHca, carbon dioxide, etc. Perhaps this is an instance 

 of mass action in which the ore present aided in causing additions to itself. 



Concentration by solution and reprecipitation. — By a process almost 

 the opposite of residual concentration, ore material is often leached out 

 of the surface-rock by water circulating slowly through its pores, cleav- 

 age planes, and minute crevices, and is carried on with the circulation 

 until it reaches some substance w^hich causes a reaction that precipitates 

 the ore material. This substance may be a constituent of some rock 

 which the circulating water encounters, such as organic matter. More 

 commonly, the precipitation seems to be due to the minghng of waters 

 charged with different mineral substances, the minghng inducing reac- 

 tion and the precipitation of the ore. Precipitation, however, does not 

 necessarily follow such commingling. The junctions of underground 

 waterways are sometimes characterized by barrenness instead of rich- 

 ness. In the expressive phraseology of the miners, a tributary cur- 

 rent sometimes ''makes" and sometimes ''cuts out.'' In chemical 

 phrase, when the mingling waters reduce the solubihty of the appropri- 

 ate substance sufficiently, an ore-deposit is formed; when they increase 

 its solubility, they promote barrenness. Changes of pressure and tem- 

 perature may enter into the process, and mass action may lend its aid 

 when once a deposit is started. 



More concretely stated, the general process of underground ore 

 formation appears to be this: the permeating waters dissolve the ore 

 material disseminated through the rock and carry it thence into the 

 main channels of circulation, usually the fissures, broken tracts, porous 

 belts, or cavernous spaces. If precipitating conditions are found there, 

 deposition takes place. The precipitating conditions may be merely 

 changes of physical state, such as cooling or rehef of pressure, but prob- 

 ably much more generally they consist in the comminghng and mutual 

 reaction of waters that have pursued different courses and become differ- 

 »Vau Hise, Mono. XIX. U. S. Geol. Smtv., pp. 268-295, 1892. 



