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THE POPULAR SCIENCE MONTHLY. 



And now for the story of how this wonderful mineral was formed. 

 It is one of the well-established facts of geology that it is of vegetable 

 origin. This is not simply a theory, for in Nature coal can be seen in 

 various stages of formation where vegetable tissue is heaped up and ac- 

 cumulated in bogs. As we dig down into these bogs where the woody 

 matter is surrounded by moisture, and in a favorable position for slow 

 decomposition, it is seen that it is transformed into a dark, combusti- 

 ble compound which is first called peat ; then, as it becomes harder 

 and more changed, lignite ; while the oldest peat-bogs in Europe have, 

 at or near their bottom, thin layers of hard, black matter that neither 

 examination by the eye nor analysis by the chemist can distinguish 

 from true coal, and which, therefore, must be true coal. " In Holland, 

 Denmark, and Sweden, the thick deposits of peat are separated into 

 distinct beds by strata of sand and mud, giving the best possible elucida- 

 tion of the process of stratification of the coal-measures" (Lesquereux). 

 For their formation these bogs require a basin rendered impermeable 

 by a substratum of clay and an active growth of aquatic or semi-aiirial 

 plants, having their roots in water, while their branches and leaves ex- 

 pand on the surface thereof, or rise in the air above it, constantly grow- 

 ing in the same place, whose debris, falling year after year, is heaped 

 up and preserved against atmospheric decomposition by stagnant water 

 or great humidity in the air. It was during the Carboniferous epoch, 

 when our principal and most valuable seams of coal were deposited, 

 that all these favorable circumstances were in their highest develop- 

 ment. For a dense vegetation we also want a warm, moist, and equable 

 climate, and air more or less charged with carbonic-acid gas, as that is 

 the food of plants (just as the oxygen gives life to man), though it is 

 poisonous to warm-blooded animals, it being impossible for such to live 

 in an atmosphere containing more than about one per cent, of it. 



During the Carboniferous age of the earth's history the water cov- 

 ered very much more of area than it now does, and portions of the con- 

 tinents were so little raised above its surface that a slight elevation or 

 depression would change them from marshes and lagoons into dry land, 

 or sink them below the surface of the sea. When air passes over, or 

 rests on, oceans, or lakes, or rivers, etc., it becomes laden with vapor, 



