Steel and Iron 437 



conditions alternated, producing stratified deposits of the above 

 sort. At last the sea floor was upraised to form part of the land. 



Some ore bodies are found which show evidences of having 

 been formed in underground cavities. In some brown ore de- 

 posits there have been discovered iron stalactites (page 411), 

 which plainly indicate this manner of formation. 



One other kind of ore formation may be mentioned. Where 

 limestone strata are steepty inclined, and are accessible to water 

 that has become a carrier of iron and carbon dioxid, one or 

 more layers of the limestone may be dissolved and replaced with 

 iron oxids. Such formations may be extensive and may be 

 profitably worked. In the southern iron ore regions sandstone 

 has been similarly replaced by iron oxid. Ores thus formed are 

 generally brown ores. 



III. REDUCTION OF IRON ORES 



212. Manufacturing centers. Mention is made in the 

 section on Inland Waterways, page 386, of current methods of 

 transportation and handling of iron ore. There are many 

 interesting problems connected with the transportation and 

 manufacture of iron, which may be briefly suggested here as 

 subjects for individual study. Why are iron ores not reduced, 

 and pig iron converted into finished iron and steel products, 

 where the ores are mined ? What conditions determine chiefly 

 the location of iron manufacturing centers? Why are Pitts- 

 burgh, Chicago, and Birmingham the leaders in iron manu- 

 facturing ? 



Exercise. Birmingham produces pig iron at less cost per ton than 

 Pittsburgh or Chicago, although it pays more for coke and labor than 

 either of these. A ton of iron is estimated to cost at Birmingham, 

 $9.88 ; at Pittsburgh, $13.03 ; at Chicago, $13.19. What reasons do 

 you find to account for the difference? 



213. The blast furnace. The manufacture of pig iron or 

 the reduction of ores consists in removing the oxygen and other 



