196 ECONOMIC GEOLOGY 



as in the "Great Gosson Lead" in Virginia, and Ducktown, 

 Tennessee. 



Siderite occurs in bedded deposits in the Carboniferous ter- 

 ranes of western Pennyslvania and in several other localities 

 along the Appalachian belt. It occurs in a clay bed lying beneath 

 the Tertiary formations on the west side of Chesapeake bay for a 

 distance of 50 miles. Siderite often constitutes a large part of 

 the clay-iron stones of the Coal Measures and many shaly 

 stra^fied deposits. 



The impurities present in iron-ore bodies fall into two classes : 

 (1) Those that are not seriously objectionable in small quantities 

 save as they lower the iron content of the ore and thereby 

 depreciate the ore in value. Calcium carbonate is often present 

 in hematites that occur in bedded deposits between sandstones 

 and limestones. If the lime content is high the carbon dioxide 

 must be volatilized before smelting the ore. Clayey matter is 

 often present in limonites as admixed material that requires a 

 larger percentage of flux in the treatment of the ore. 



(2) Impurities that tend to weaken the iron and therefore lessen 

 its value. Silica is undesirable in iron ores for two reasons. 

 It lowers the iron content of the ore and requires a larger amount 

 of fluxing materials in its treatment. However foundry iron 

 sometimes carries 10 per cent, or more of silicon. Sulphur is 

 objectionable because of the brittleness imparted to the result- 

 ing iron. It is found as pyrite in the magnetites but it may arise 

 from the presence of gypsum or barite in the limonites. Phos- 

 phorus is objectionable for the same reason. It is present in 

 the mineral apatite associated with some magnetites. It should 

 not be present in any iron ore in excess of one-tenth of 1 per cent. 

 It cannot be volatilized in either the blast-furnace or the acid 

 converter used in the manufacture of Bessemer steel, and there- 

 fore it appears in the finished product. Titanium is a common in- 

 j urious constituent found in many magnetites. It does not render 

 steel brittle but it renders the ore highly refractory and forces a 

 part of the iron into the slag. In the manufacture of titanium 

 steel the metal should be added directly as an alloy. 



Geographical Distribution. There are three distinct belts of 

 iron ores in the United States. (See Fig. 104.) 



(1) The Appalachian district; (2) the Lake Superior district; 

 and (3) the Cordilleran section. 



Until recently practically all of the iron ores of the United 



