70 GEOLOGY 



zinc, and more rarely in copper, in the course of its formation. 

 The lead and zinc regions of the Mississippi basin have been regarded 

 as regions of special subsequent enrichment, in areas where there 

 was some enrichment at the time of sedimentation. This enrich- 

 ment accompanying sedimentation has been attributed to solutions 

 brought into the sea from neighboring lands, and precipitated by 

 organic action in the sea-water. 1 This organic action may have 

 been more effective in some areas than in others, because of the 

 unequal distribution of life and the concentration of its decaying 

 products. It is assumed that such precipitates were at first too 

 diffuse to be of value, and further concentration was required to 

 bring them together into workable deposits. 



Metallic material is sometimes concentrated to some extent 

 in sand and mud in the processes of sedimentation, though more 

 rarely. Copper-bearing shale is known, for example, in Germany, 

 Texas, and elsewhere. 



Since it is reasonable to suppose that land-waters, on reaching 

 the margins of the water-basins, must occasionally find conditions 

 favorable for the precipitation of their metallic contents, it is in- 

 ferred that while the processes of sedimentation tended on the 

 whole to leanness, they gave rise to (1) some very important ore- 

 deposits, notably the chief iron ores, the greatest of all ores in 

 quantity and in industrial value, and (2) a diffuse enrichment of 

 certain other areas which made them productive after. subsequent 

 processes of concentration, while the sedimentary formations in 

 general were left barren. 



Origin of ore regions. From these considerations it appears 

 that for the fundamental explanation of " mining regions" we 

 must look mainly (1) to magmatic differentiation, so far as the 

 country rock is igneous, and (2) to enrichment during sedimenta- 

 tion, so far as the rock is secondary. The subsequent processes 

 in ore making consist in the further concentration of the ore material 

 into sheets, lodes, veins, and similar aggregations by ground-water 

 circulation, or else in the purification of the ores by the removal 

 of useless or deleterious material. 



'Chamberliii. Geol. of Wis., Vol. IV, p. 599, et seq., 1882. 



