C. H. Smyth, Jr. — Clinton Iron Ore. 493 



It seems to be generally assumed that if an ore becomes 

 hard and calcareous below drainage level, it must be a result 

 of replacement of a calcareous layer by iron. While this is, 

 doubtless, often true, there are many cases where the evidence 

 is against it. That there has been a replacement of the calcite 

 of various organic forms by iron there is no doubt, for the ore 

 is often largely made up of such casts. But there seems to be 

 good reason for believing that this replacement, in many cases, 

 occurred before the fossils were incorporated into the rocks of 

 which they now form part, or, at least, before the rock was 

 lithifiecl by the calcareous cement. The replacement of bryo- 

 zoa, etc., is just as complete in the hard, calcareous ores, as in 

 the softer varieties. The completely replaced fragments are 

 often found scattered through, and surrounded by, pure calcite. 

 As in the case of the oolites, how is it possible that the cal- 

 cite of the fossils could have been replaced by iron, after the 

 fossils had been cemented together in this way? Even assum- 

 ing the fragments to consist of aragonite, and therefore more 

 readily attacked than the calcite cement, it would be impos- 

 sible for the ferruginous solution to reach them, completely 

 enclosed as they are in the cement. Many specimens collected 

 not only in New York, but also in Tennessee, Georgia and 

 Alabama illustrate these facts on both a macroscopic and 

 microscopic scale. There seems to be no relation between the 

 richness of the ores and the completeness of replacement of 

 organic forms. 



These points are well illustrated by the mode of occurrence 

 of the ore at Ontario, Wayne Co., K. Y. Here there is a bed 

 of ore some two feet thick, capj)ed by six feet of fairly pure 

 limestone with shale partings. All above this has been re- 

 moved b} r erosion. The replacement theory supposes the iron 

 to have passed through this limestone, replacing the lower two 

 feet. It is difficult to understand why the upper portion 

 should not cause precipitation as readily as the lower two 

 feet, but its light gray color shows that it has not. This 

 might possibly be explained by assuming that an impervious 

 layer at the bottom checked the water, and thus allowed more 

 time for the reaction. Granting this to be the case, we should 

 expect a gradual transition from the ore to the limestone. 

 But, as a matter of fact, the only sense in which this is true, 

 is that some of the fragments of which the ore is composed 

 are scattered through the lower portion of the limestone ; they 

 are just as ferruginous as in the ore below and are inclosed in 

 pure limestone. Thus, there is a mechanical, but in no sense, 

 a chemical transition such as would result from replacement. 

 The contact is exactly similar to that of a limestone and fine 

 conglomerate, the lower portion of the former including some 



