436 Introduction to the Study of Science 



a drift mine or a shaft. A diagrammatic sketch of a shaft, 

 with several levels or drifts which are worked at the same 

 time, is shown in Fig. 142. On the whole, the underground 

 method of mining iron ore is similar to that in mining coal, 

 lead, and other minerals. 



211. Formation of ore bodies. The position in which iron 

 ores are found suggests the question as to how they were formed. 

 Iron-bearing formations generally are, like limerock and sand- 

 rock, of sedimentary origin. Some iron deposits, as those of the 

 Lake Superior region, were probably of marine formation and 

 originally composed of iron silicate and iron carbonate. The 

 marine beds were later covered by other deposits and meta- 

 morphosed. The physical and chemical character of iron- 

 bearing formations was gradually modified. Permeating waters 

 dissolved away the silica and carbon dioxid, leaving the iron 

 oxids. In some large iron-bearing bodies of low grade, which 

 were steeply inclined or tilted, the permeating waters effected 

 a transfer of the iron from the upper parts and its concentration 

 in the lower parts of the body. This is the probable formation 

 (Fig. 142) of many ore bodies, as those in the Lake Superior 

 region. 



In some localities the ore bodies were formed directly by 

 igneous action (pages 409, 414), the iron being deposited by 

 water that was originally a part of the lava. It is supposed 

 that magnetite ore was produced in this way. 



In the Birmingham, Alabama, district as many as five layers 

 of iron ore are found separated from one another by strata of 

 shale and sandrock. The largest seam of ore is at places thirty 

 feet thick ; and a seam which is very rich in iron, and between 

 four and six feet thick, lies several feet below it, separated 

 by shale and sandrock. Where ore bodies occur as strata 

 separated by layers of shale and sandrock, there was once the 

 sandy floor of the sea. River waters in which much dissolved 

 iron was present flowed into the sea and deposited their iron 

 content. Other sands covered the first deposit of iron, and these 



