C. H. Smyth, Jr. — Clinton Iron Ore. 489 



chiefly of bryozoan fragments more or less completely replaced 

 by iron, as described by Dr. Foerste, there is often a coating 

 of the fragments with layers of iron associated with silica, 

 precisely as in the oolites. 



The treatment of these samples with hydrochloric acid 

 further shows that the lime of the bryozoa has often been 

 replaced not by iron alone, but by iron and silica, for there is 

 left as a residue from the treatment a siliceous cast of each 

 fragment. In most cases, the iron is removed from this by 

 the acid, but in some samples the silica prevents the complete 

 solution of the iron, even with prolonged digestion. Ap- 

 parently there is an intimate association in the deposition of 

 iron and silica. 



The above facts, together with others gathered in the field, 

 seem to have some bearing upon the question of the origin of 

 the Clinton ore. Most of the recent writers on this subject 

 seem to agree that the ore is formed by the replacement of 

 limestone strata by iron derived from the overlying rocks. 

 The iron is taken into solution by circulating meteoric waters 

 containing carbon dioxide and other products of organic decay. 

 This solution coming into contact with limestone, chemical 

 action ensues, the iron is precipitated, and the lime passes 

 away in solution. 



While there can be no doubt that many ore bodies have been 

 formed in this way, and among them portions of the Clinton 

 ore, still, several facts lead to the conclusion that other* portions 

 of this deposit require a different explanation. Compared with 

 the great extent of Clinton ore, the writer's observations have 

 been so limited as to make it, perhaps, unwise to base upon 

 them any broad generalizations. The following discussion 

 will, therefore, be confined almost entirely to the type locality 

 and such other localities as have been examined in person. 



As above stated, the ore at Clinton is associated with un- 

 doubted shore deposits, and though all of the rocks are calca- 

 reous, there are none that could be called limestones. 



During that portion of Silurian time represented by these 

 strata, the region was one of shoal waters or extensive mud-flats. 

 These conditions are favorable for the formation of an iron ore 

 by sedimentation, and it seems unnecessary to bring in the 

 idea of replacement. If the ore represents an oolitic limestone, 

 each spherule has been altered from the outside toward the 

 center. This alteration has been by the replacement of the 

 calcite by silica and iron carbonate. It would seem as though, 

 after the exterior layers were thus altered, they must, to a 

 greater or less extent, protect the interior layers from change, 

 and that there would often be some trace of the original calcite. 

 In no case has this been seen, even in the leanest ores, although 



