SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 85 



ferruginous cherts, and ferrous silicate rocks interbedded with 

 elastics and extrusives. Their average iron content ranges from 

 about 25 to 35 per cent iron. The ores were developed by the 

 solution and removal of silica, and the oxidation, solution, trans- 

 portation, and redeposition of the iron, in the exposed portions of 

 the formations, having structural peculiarities favorable to the 

 circulation of water, such as jointing, pitching basements, faults, etc. 



The ores mined are mainly hematite and Hmonite, although some 

 magnetite has been developed through deep-seated conditions of 

 high pressure and temperature. Their iron content, while steadily 

 falHng, has generally been above 50 per cent, but 40 per cent ores 

 are now mined as mixers for higher-grade materials. The estimated 

 tonnage of the available ores, including all ores above 55 per cent 

 iron, and 25 per cent of all formations containing from 45 to 55 per 

 cent iron is 3,500,000,000 tons. Iron formations containing from 

 35 to 45 per cent iron are classed as available in the future. Their 

 tonnage is enormous, 72,000,000,000 tons being taken as an arbi- 

 trary estimate. 



The pre-Cambrian Adirondack area of New York contains 

 commercial deposits of non-titaniferous magnetites, titaniferous 

 magnetites, and red hematites. 



The non-titaniferous magnetites are intimately associated with 

 sedimentary rocks, marble, schists, and gneisses. Separation is 

 generally made by magnetic methods and the ore shipped to the 

 iron centers, excepting a small tonnage which is smelted in local coke 

 ovens. The ores are both Bessemer and non-Bessemer. The 

 lowest grades contain about 35 per cent iron when mined, the 

 richest from 60 to 65 per cent. The concentrates run from 60 to 

 65 per cent. No fair estimate of the tonnage can be made. The 

 known deposits of high grade are beheved to contain 35,000,000 tons, 

 and the leaner deposits carrying over 35 per cent iron may yield 

 75,000,000 tons of concentrates. 



The ores are beheved to have been developed in connection with 

 the intrusion of igneous rocks. 



The titaniferous magnetites carry at least 8 per cent of titanic 

 acid and on an average about 15 per cent. They are found at the 

 margins of a gabbro-anorthosite area, and are spoken of as magmatic 



