Coal does not occur like chalk or granite, in great hills. Even an enormously thick peat-bog when dried and hardened into coal will not make a very thick layer and so coal is found in 'seams'. These are layers of irregular thickness rarely as much as 10 ft thick, and often as little as two feet - or less. The rock above and below coal is usually shale or slate (hardened mud), and so coal seams are, as far as we know, not found at very great depths. The deepest coal mine is no deeper than a mile and a quarter.
If there is good reason to hope that a coal seam lies under a piece of land, the first thing to do is to find out whether the coal exists, and if so, how deep it is, how thick the seams are, and whether it is of good quality.
The best thing to do is to sink a trial shaft several feet in diameter, but as this is very expensive, it is more usual to drill boreholes. A hollow drill, usually a diamond drill, is used. The 'cores' cut out by it are brought up and examined. The holes may be a mile deep, but in England we rarely go so deep.
If the coal is of good quality, and the seams thick enough to work, the next step is to sink a shaft down to the coal. This will ultimately be the passage by which men, coal and air are brought up and down and through which the subterranean water, which almost always gives trouble, is pumped to the surface.
This is a summary of part of an article by F. Sherwood Taylor, who made science intelligible to the public a couple of generations ago - N.D.
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