SUMMARY 499 



subsequently oxidized in the residual material. Some are formed in 

 seams and cavities on and in the beds of slate, limestone, and sandstone. 

 The deposits increase in size largely by the segregation of the oxide into 

 the scattered nodular and flake-like masses in the underlying limestones? 

 which are subsequently leached out, leaving the ores in residual mate- 

 rial similar to that in the overlying clay-ore mass which settles down 

 upon it. The ore nodules and flakes may form at any point in the 

 limestone, but the most favorable horizon for their concentration is at or 

 near the contact of the limestone with an underlying bed of slate or 

 sandstone, which forms a collecting place for the ores. The intercalated 

 slates weather to a white clay, which thus forms the repository for many 

 of the ore masses. 



Literature on the Limonite Ores 



The following bibliography gives a brief summary of what has been 

 published about the origin of the limonite ores in question. In general, 

 reference is made only to the literature bearing directly on the origin of 

 the limestone valley limonites, but a few specific references on other 

 points are mentioned :* 



Benton, E. R. : Tenth Census, volume xv, 1880. 



The ores are pseudomorphs after broken limestone b}' filling the cracks and 

 thickening the films, changing from solid limestone above to irregular shell ore 

 at the bottom. 



Bischop, Gustav : Elements of Chemical and Physical Geology, volume 2. 



Hydrated oxide of iron decomposes silicate of alumina. From a solution 

 of bicarbonate of iron and bicarbonate of lime a current of atmospheric air 

 causes all the iron to be precipitated as hydrated peroxide before the lime 

 begins to separate. 



Dana, James D. : American Journal of Science, third series, 1884, volume 28, 

 page 398. 



The limonite ore beds of the eastern United States result from the oxida- 

 tion in situ chiefly of ferriferous limestone. The ferriferous limestones were 

 formed in interior basins or marshes by iron bicarbonate or salt of an organic 

 acid washed down from the land over areas of calcareous deposits. These ore 

 beds, although superficial, can not be said to be modern. They have prob- 

 ably been in progress ever since the land emerged from the ocean. 



Davis, O. W., Jr. : The iron ores of Maine, Journal United States Association of 

 Charcoal Iron Workers, 1886, volume 1, page 66. 



The ores in the vicinity of Katahdin, Maine, which occur in Silurian clay 

 slates, have been formed by the decomposition and oxidation of pyrites. 



*A full bibliography on iron ores is published in Bulletin number x of the Minnesota Geological 

 Survey. 



LXXI— Bur,T„ Geot,. Soc. Am., Vol. 11, 1899 



