USEFUL J//.T l/.> 215 



hematites. They occur as replacement depo-it> in limestones of 

 Tria-sic age. Sometimes all traces of tin- original linn-stones 

 have Keen removed and so complete has been the interchange of 

 material that a fairly compact bed of ore rests directly upon the 

 associated underlying Paleozoic rocks. It appears to be generally 

 accepted that the replacement of the limestone was brought about 

 by ferruginous solutions which found their way into the calca- 

 eous rocks in post-Eocene times. 



Geological Horizon. Iron ores are comfined to no particular 

 geological age. They are especially abundant in the pre-Cam- 

 brian, Cambrian, Ordovician, Silurian, Carboniferous and Terti- 

 ary ages. Bog iron ores are in the process of formation even at 

 the present time. 



Methods of Extraction. At the Illinois Steel Works of South 

 Chicago the ore used in the manufacture of pig iron comes from 

 the Lake Superior district. The ore is removed from boats by 

 means of electric cranes and transported either to the storage 

 yards or to the furnaces for immediate treatment. The type of 

 furnace used is known as the blast furnace. Each furnace 

 shaped like an inverted bottle is about 80 ft. high and tapers 

 more rapidly toward the base than toward the top. The upper 

 part of the furnace into which the ore, coke and limestone are 

 fed is called the throat. From the throat down to the widest 

 part of the furnace it is called the stack. The lowest part is 

 called the bosh. The part of the furnace on which the ore lies 

 is called the hearth. Here is where the greatest heat is produced 

 and where the ore is smelted. About 4 ft. above the hearth are 

 the tuyeres through which a blast of air is introduced into the 

 furnace. The blast is heated by the hot gases which are generated 

 in the furnace. The zone of fusion is just above the place where 

 the bosh joins the hearth. The fluid iron sinks to the bottom 

 of the furnace while the resulting slag with its lighter specific 

 gravity floats on the surface of the molten metal. The slag is 

 drawn off into V-shaped troughs through which a stream of cold 

 water is flowing. The slag is therefore shorted and transported 

 to large cylindrical tanks from which it is subsequently removed 

 and utilized in the manufacture of cement. 



The molten metal is drawn off into ladles containing about 

 20 tons each or into sand troughs each containing about 300 Ib. 

 of the metal. These bars, not unlike cord wood in shape, are 

 pig iron. 



