Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams.The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure.
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Two dominant methods account for the vast majority of underground coal mining: "room and pillar" and "longwall" mining.. In room and pillar mining, seams of coal are partially mined, leaving large pillars of coal intact to support the overlying layers of rock.
US California: Slumburbia. February 10, 2010 New York Times* By TIMOTHY EGAN. In Lathrop, Manteca and Tracy, California, among some of the world's most productive farmland, you can find streets of foreclosed home, looking like a 21st century ghost town, with rock-bottom discounts on empty starter mansions.
Similarly, you may sometimes find on a mountain-side a large rock poised so delicately that a touch will set it crashing down into the valley, while the rocks all round are so firm that only a considerable force can dislodge them What is analogous in these two cases is the existence of a great store of energy in unstable equilibrium ready to burst into violent motion by the addition of a very ...