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| Coal |
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| Coal varies in quality according to the amount of pressure and heat to which it is subjected to during its formation. |
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| Coal consists largely of carbon, hydrogen and oxygen and a small amount of sulphur. It comes in 3 forms. |
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| A Swampy Forest |
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| Coal is formed in layers called seams. Layers of other rocks are there amidst coal seams. A mine shaft is dug through these rocks and the coal seam is reached. Then tunnels are dug into the coal. |
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| Twisting of the rocks may sometimes bring the coal nearer to the earth's surface, so that an open-cast mine can be made use of. The figure shows an open cast mine. |
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| Sometimes a valley is formed naturally due to erosion of rivers exposing a coal seam, making man's work easy. Hence drift mining can be done by digging into the valley. |
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