[hard rock] Underground hard rock mining refers to various underground mining techniques used to excavate hard minerals, mainly those containing metals such as ore containing gold, silver, iron, copper, zinc, nickel, tin and lead, but also involves using the same techniques for excavating ores of gems such as diamonds. In contrast soft rock...
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http://en.wikipedia.org/wiki/Underground_mining_(hard_rock)
[soft rock] Underground mining (soft rock) refers to a group of underground mining techniques used to extract coal, oil shale and other minerals or geological materials from sedimentary (`soft`) rocks. Because deposits in sedimentary rocks are commonly layered and relatively less hard, the mining methods used differ from those used to mine ...
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http://en.wikipedia.org/wiki/Underground_mining_(soft_rock)
(from the article `mining`) When any ore body lies a considerable distance below the surface, the amount of waste that has to be removed in order to uncover the ore through ... ...all the early coal seams were worked from the surface, in fully exposed outcroppings. In the later Middle Ages, however, exhaustion of outcrop ... In under...
Found on
http://www.britannica.com/eb/a-z/u/7
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