1136 A TREATISE ON METAMORPHISM. 



descending- waters is to produce a differentiation of the products, in a verti- 

 cal sense, one mineral or one combination of minerals being at one horizon 

 and another mineral or combination of minerals being at another horizon. 

 Commonly these horizons have a depth varying from a few meters to many 

 or even hundreds of meters. This is very different from the phenomena 

 of crustification, in which the change of minerals is lateral within very 

 short distances at right angles to the walls. 



It is certain that for various reasons, as fully explained under "Ore- 

 shoots," there are great variations vertically in the richness of deposits pro- 

 duced by deep waters. A case of such extreme variation is that of the 

 Comstock lode of Nevada, where there were numerous great bonanzas 

 separated from one another by comparatively lean material; and yet in this 

 vein the evidence of water ascending under pressure is most conclusive, 

 with no evidence of descending water; but the variations in tenor in 

 deposits of the deep circulation are in both directions. The bonanzas may 

 be found lower than the poorer material, and vice versa. In contrast with 

 this, descending- waters produce an upper rich belt which is not again dupli- 

 cated below. While the deposits of the deep circulation may vary greatly 

 in values, in many cases there is remarkably even tenor for great vertical 

 depths. The best illustration of this known to me is the copper deposits 

 of the Lake Superior region which are known to extend from the surface 

 to a vertical depth of 1,600 meters, equivalent to 2,066 meters along the 

 dip, 40°, with values sufficient to warrant working throughout the distance. 



As already explained, the copper was in all probability reduced and 

 precipitated directly as metallic copper from upward-moving cupriferous 

 solutions. The reducing agents were ferrous solutions derived from the 

 iron-bearing silicates, and ferrous compounds in the solid form, magnetite 

 and silicate. It is well- known that metallic copper once formed is but 

 slowly affected by the oxidizing action. Oxidation has in fact occurred in 

 the Lake Superior region, but not to an important extent. An oxidized 

 belt may have formed in pre-Glacial time, but if so it was swept away 

 by glacial erosion, and a sufficient interval has not elapsed to form another. 

 The ore deposits now worked have apparently remained practically un- 

 changed since their first concentration. In this fact we have the explana- 

 tion of the extraordinary depths to which these deposits extend. 



