PRINCIPLES CONTROLLING ORE DEPOSITS 765 



horizontal extents of the deposits may be much greater than the 

 vertical extents. Reduced to a simple and broad statement, The 

 first concentration of many ore deposits is the work of a relatively deep 

 water circulation, while the reconcejitration is the result of reactions 

 upon an earlier conce?itratio?i through the agency of a relatively shallow 

 water circulation. Commonly the deep water circulation is lacking in 

 free oxygen a?id contains reduci?ig agents, a?id the shallow water con- 

 tains free oxygen. The deep water is therefore a reducing, and the 

 shallow water an oxidizing agent. 



In addition to the general factors already considered there 

 are many special factors which have a most important, indeed) 

 very often a controlling influence in the production of ore-chutes 

 and in the localization of ore in certain areas and districts. 

 Some of these factors are the complexity of openings, the pres- 

 ence of impervious strata at various depths, the presence of 

 pitching folds, the character of the topography. I see however 

 that my time is nearly gone, and I shall not take up their discus- 

 sion this evening, but must refer those especially interested in 

 this phase of the subject to my full paper already repeatedly 

 mentioned. 1 I must however note that impervious strata are 

 frequently of controlling importance in the underground circula- 

 tion. Often deep and shallow water circulations are separated 

 by such strata. Often also as the result of the removal of 

 impervious strata by denudation, the previous deep circulation 

 ceases and the action of the shallow circulation is inaugurated. 



At this point it may be well to briefly recall the most funda- 

 mental features of the water circulation which produces the ore 

 deposits. First comes the downward-moving, lateral-moving 

 waters of meteoric origin which take into solution metalliferous 

 material. These waters at depth are converged into trunk chan- 

 nels, and there, while ascending, the first concentration of ore 

 deposits may result. After this first concentration many of the 

 ore deposits which are worked by man have undergone a later 

 concentration not less important than the earlier, as a result of 



1 Some Principles Controlling the Deposition of Ores, by C. R. Van Hise : Trans. 

 Am. Inst. Min. Eng., Vol. XXX, 1900, pp. 1 12-146. 



