

ORE-DEPOSITS 43 



circuits, carried to the seas various metallic substances in solution. 

 In the main these substances appear to have been widely diffused, 

 and to have been distributed very sparsely through the sediments, 

 for sediments seem to contain less ore material than igneous rocks. 

 There are however important exceptions to this general rule Of 

 sedimentary leanness. 



The iron-ore beds of Clinton age ranging from New York to 

 Alabama, and appearing also in Wisconsin and Nova Scotia, form 

 a stratum in the midst of ordinary sediments, and contain marine 

 fossils. The great iron-ore beds of Lake Superior also were sedi- 

 mentary in origin, and so, probably, were most other important 

 iron deposits. Not all sedimentary iron-ore deposits are of marine 

 origin, and most of them are not clastic. Many of the sedimentary 

 iron ores have been changed greatly from the condition in which 

 the ferruginous matter was first deposited. In this change, ground- 

 water has been the chief agent. Beds of clastic iron ore are known 

 in Europe. The ore matter was in older rocks, and was segregated, 

 mechanically, during sedimentation, because it was much heavier 

 than other contemporaneous sediments. Its superior weight had 

 .much the same effect as greater coarseness. 



Some limestones appear to have been enriched locally, in a lean 

 way, in lead and zinc, and rarely in copper, in the course of their 

 formation. This lean enrichment at the time of deposition probably 

 determined the development of ore regions later. The lead and zinc 

 ore regions of the Mississippi basin have been regarded as areas of 

 this sort, the subsequent concentration of the metal into ores being 

 the work of ground-water. The lean enrichment accompanying 

 sedimentation has been attributed to solutions of the metals brought 

 to the sea from neighboring lands, the metals being then precipi- 

 tated 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. 



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

 the margins of the water-basins, must here and there find con- 

 ditions favorable for the precipitation of their metallic contents, 

 it is inferred that while the processes of sedimentation tended on 

 the whole to leanness, they gave rise to (i) some very important 

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



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



