THE AGE OF STEEL 



comes from the Lake Superior mines, and the systems 

 of mining pursued there may be considered as repre- 

 sentative of the most advanced modern methods. 



Where the iron ore of these mines is found near the 

 surface of the earth, the great system of " open -pit" 

 mining is practised; but as only a relatively small 

 portion of the ore is so situated, modifications of older 

 mining methods are still employed. Of these the 

 three most important are known as " overhead scoop- 

 ing," "caving," and " milling." 



In the overhead method a shaft is sunk into the 

 earth to a depth of several hundred feet, according to 

 the depth of the ore, this shaft being lined with timbers 

 for support. From this shaft horizontal tunnels are 

 made in all directions in the ore deposits, and through 

 these tunnels the ore is conveyed to the shaft and thence 

 to the surface. As the ore is removed and the earth 

 thus honeycombed in all directions, supports of various 

 kinds must be made to prevent caving. For this pur- 

 pose columns of the ore itself may be left, or supports 

 of masonry or wood or steel may be introduced. 

 Under certain circumstances, however, these supports 

 are not employed, the earth being allowed gradually 

 to cave in at the surface as the ore is removed, this 

 being the method of mining known as " caving." 



Where the ore deposit occurs in a favorable hillside 

 the "milling" system is frequently employed. In 

 working this system a large horizontal tunnel, twenty 

 or more feet in diameter, is dug into the hillside. Per- 

 pendicular shafts are then sunk from the top of the 

 hill, connected with openings leading directly into 



VOL. VI. 18 [ 2 73] 



