118 



KEMP'S ORE DEPOSITS. 



edge of the chemical composition of the porphyries is, however, as 

 yet very imperfect. An eruptive origin was originally assigned to 

 these ores by J. D. Whitney (Metallic Wealth of the United States, 

 p. 479, 1854), just as to the Lake Superior hematites. The later 

 investigations of Adolph Schmidt for the Missouri Survey in 1871 

 arrived at a different conclusion. Dr. Schmidt considered them, 

 whether occurring in an apparent bed, as at Pilot Knob, or in 

 various more or less irregular veins, as at Iron Mountain, to have 

 been formed either by a replacement of the porphyries with iron 

 oxide deposited from solution, or by a filling in the same way of 

 fissures, probably formed by the contraction of the porphyry in 

 cooling. In the valuable report on iron ores by F. L. Nason in 

 the Missouri Geological Survey a sedimentary origin is advo- 

 cated for the Pilot Knob beds. They are conceived to have been 

 deposited in a body of water in a hollow, between formerly exist- 

 ing porphyry hills, which rose above. In the course of weathering, 

 the hills became the valleys and the early sedimentary beds the hill- 

 top. It is, however, somewhat difficult to understand how the more 

 or less incoherent sediments withstood degradation better than the 

 hard, firm porphyry hills. Some such origin as sedimentation or re- 

 placement is, however, the only reasonable one. It is not improbable 

 that the Pilot Knob ores originated in the saturation and more or less 

 complete replacement of tuf aceous layers with infiltrating iron oxide. 

 An extended table of analyses of Iron Mountain ores will be 

 found in Mineral Resources of the United States, 1889-90, p. 47. 1 



ANALYSES OP HEMATITES, BED AND SPECULAR. 



(The same discrimination must be employed in looking over these 

 analyses that was emphasized under limonite.) 



1 G. C. Broadhead, "The Geological History of the Ozark Uplift," 



