THE ORIGIN AND DESCENT OF ROCKS. 477 



organic action in the sea-water/ this organic action being more effective 

 in some areas than in others because of the unequal distribution of Hfe 

 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 ; 

 but the further processes appear to have been effective only where 

 the preliminary enrichment had taken place. At any rate, the work- 

 able deposits are singularly localized, while the concentrative processes 

 are very general. 



Metallic material is sometimes partially concentrated in sandstones 

 and shales in the process of sedimentation, though more rarely. The 

 copper-bearing shale (Kupferschiefer) of the Zechstein group in Ger- 

 many, so extensively worked along the flanks of the Harz Mountains, 

 is a striking example. 



It is in every way reasonable to suppose that land-waters, on reach- 

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

 favorable for the precipitation of their metallic contents, and that the 

 ratio of these precipitates to other material might be relatively high 

 in the more favorable situations, and that this enrichment of the country 

 rock may be a condition precedent to a sufficient subsequent concen- 

 tration to yield workable accumulations. 



It is, therefore, inferred that while the processes of sedimentation 

 tended on the whole to leanness, they gave rise to (1) some very impor- 

 tant ore-deposits, notably the chief iron ores, the greatest of all ores in 

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

 certain other areas which made them productive under subsequent 

 concentrative processes, 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 sedimentary enrichment, so far as the rock is 

 secondary. The determining conditions in both cases are obscure and 

 unpredictable, but the recognition of such regions, and of the function of 

 preliminary diffuse regional enrichment, contributes to a comprehen- 

 sive \dew of the complex processes of ore concentration. The subse- 

 quent processes consist in the further concentration of the ore material 



1 Chamberlin. Geol. of Wis., Vol. IV, p. 599 et seq., 1882. 



