EARLE, INTERBEDDED IRON ORE DEPOSITS 1^9 



with the lime of fossils converting it to peroxide. This secondary en- 

 richment is plausible because where the outcrop, the slope of the ground, 

 the thickness of the overlying strata and other conditions are favorable 

 to considerable infiltration of surface water, the ore carries a higher 

 amount of iron than at less favorable places. The fossils often form 

 ■one-half of the total weight of the ore; it is obvious that if part of the 

 lime thus contained is dissolved out, the remaining peroxide of iron vidll 

 form a much larger percentage of the total bulk of remaining material. 



SECONDARY ORIGIN 



Residual Ennchment Theo7-y 



The residual enrichment theory starts with the supposition that the 

 -ore beds were originally limestones rich in iron, that by a process of 

 leaching the lime carbonate was partly or wholly removed, and that the 

 iron, together with the insoluble material, was left in a much more con- 

 centrated form. 



Similar effects are known in tropical countries ; in Cuba, for instance, 

 where silica by a process of weathering known as laterization has been 

 removed from iron-bearing rocks, leaving the iron and insoluble portions 

 rich enough to be classed as an ore and mined profitably. 



I. C. Russell (292, pp. 22-23) : 



"Portions of the Silurian rocks of Alabama, readily recognized as limestones 

 when unweathered, are easily mistaken for sandstones and shales when only 

 their weathered outcrops can be seen. The Clinton ore, or fossil ore, inter- 

 bedded with strata of shale and sandstone forms one of the most character- 

 istic beds in the Upper Silurian rocks of Tennessee and Alabama. In the 

 mines of Gadsden and Attalla, Ala., where Clinton ore is worked, the strata 

 are highly inclined (a dip of 70 to 80 degrees to the southeast prevailing) and 

 well exposed for study. 



"The outcrops of the beds are soft, porous, highly fossiliferous ore, which 

 has a deep brownish red color, and is easily worked and easily smelted. The 

 ere at Attalla retains this character to the depth of about 250 feet, measured 

 down the slope, and then changes to a hard, compact, ferruginous limestone, 

 rich in fossils. The marked difference in the character of the ore in the upper 

 portions of the mines as compared with that of the lower portions is due 

 entirely to weathering. This is shown by its chemical composition. Two 

 typical samples of the ore, selected by me — one from near the surface, repre- 

 senting the ordinary character of the soft ore, and the other from a depth of 

 250 feet, representing the hard ore, but not the most calcareous variety — gave 

 on analysis the following percentages of iron, lime and carbonic acid, after 

 ■drying at 105 degrees Centigrade : ^^ ^^^^.^^^^ ,.^ ^^^^ 



Fe 57.52% 7.75% 



CaO 1.38% 47.64% 



CO, 30% 34.90%" 



