i882. 15 Trans. N. Y. Ac. Set. 



thinness : a structure which would be the natural product of sedimen- 

 tation but hardly of mechanical washing. Secondly, they could never 

 have been deposited as hematite by shore waves, since this mineral 

 forms an impalpable powder which never accumulates by itself. We 

 must therefore suppose that all these great iron ore beds are pseudo- 

 morphs after magnetite, or that they are the result of sedimentation by 

 organic agencies and subsequent metamorphism. 



The iron ore of Iron JMountain is enclosed in " porphyry " — probably 

 a metamorphosed sediment really fused in place, — but some of the 

 masses of ore are thickly set with crystals of apatite, which afford an 

 equally conclusive argument against the volcanic and the mechanical 

 theories. In the ore of Simmons Mountain, masses of magnetite occur 

 which retain perfectly their radiated, limonite structure ; and there can 

 be no question but that this has been deposited by a chemico-organic 

 process. 



The deposits of magnetic iron in Southern Utah. — the most exten- 

 sive of which I have any knowledge, — are in places very conspicuously 

 and evenly stratified and thus exhibit all the signs of having been de- 

 posited as sediments. They are associated with limestone and a grani- 

 toid metamorphic rock. Along certain lines, the ore is thickly set with 

 crystals of apatite, and some of the largest masses are in great part com- 

 posed of magnetite, which shows everywhere limonite structure. From 

 these facts it is easy to see that these great deposits have not been 

 formed by mechanical agencies. 



In regard to the sparry carbonates which form vein-stones, and the 

 earthy carbonates of the Coal measures, there can be no question of 

 their origin. They have all been formed by chemical precipitation. 



The Clinton ores are interstratified with limestone, form continuous 

 sheets of great extent, and constitute on the whole the most consider- 

 able depj)sits of iron ore in this country. They were apparently formed 

 in the same way that the granular lake ores are now deposited, by the 

 precipitation of iron from solution in a water basin, surrounded by land 

 having a ferruginous drainage. The quantity of phosphorus they 

 contain, derived from the organic matter deposited with them, is a con- 

 spicuous feature in the composition of the Clinton ores, and apparently 

 explains its abundance m many of our crystalline ores. After a careful 

 study of the structure, composition and surroundings of our most im- 

 portant deposits of magnetic and specular ores, I am compelled to con- 

 clude that they have generally been derived from stratified deposits, 

 accumulated by chemico-organic agencies, and once closely resembled 

 our stratified ores of the upper Silurian and Devonian ; and that any beds 

 which accumulated as iron sand on beaches must be rare exceptions 

 to the general rule. 



