IRON ORES 79 



ore is removed from the portion of one level the superincum- 

 bent rock or earth is allowed to cave upon the ore below, and 

 frequently the settling of the material is facilitated b}^ the use 

 of explosives. By this method but little ore need be left in a 

 deposit, and if care is exercised the risks are not great and but 

 little waste becomes mixed with the ore. This method is 

 applied more to soft or moderately soft ores than to those 

 which are harder, but it is used in some hard ore mines where 

 the roof or hanging wall is insecure. 



Drifting is emplo^^ed in all underground mining, but 

 where a series of parallel drifts, one advanced more than an- 

 other, are employed to slice off the deposit the method is 

 specifically known as the drifting or slicing system. 



Room mining may be considered either as digging out 

 cavities which alternate with pillars, or as opening cavities of 

 considerable length, width, and height, usually from foot wall 

 to hanging wall, and supporting the last named and the roof 

 by an elaborate arrangement of timbers known as square sets, 

 in which the timbers are so placed as to form the outlines of a 

 series of cubes resting upon one another by carefully fitted 

 joints. This method is largety employed in removing soft 

 hematite, and some of the cavities thus made and protected 

 are of enormous size. 



Filling is not so much a method of mining as a means of 

 protecting the workings by depositing in the cavities waste 

 rock, sand, and other refuse. This method is often more 

 economical than timber support, and is adapted to hard ore 

 mines. 



The magnetites, as a rule, are found in fairly well defined 

 veins, inclosed between walls and dipping at steep angles from 

 the horizon. Some of these veins are of enormous size, as in 

 the Lake Champlain district of New York, where the texture 

 and hardness of the ore permit of its use as pillars in the mines. 

 Other veins are narrow and tortuous, although persistent, and 

 each of these characteristics demands different methods of 

 exploitation. 



