Coal is the litter of primeval swamps and forests. Year after year the débris of the humid jungles accumulated in shallow water or in the boggy soil, where it underwent partial decay, and was thus converted, first of all, into the slimy or spongy material known as “peat.” Similar deposits are in process of formation in the swamps of the present day, and the peat obtained from them is dried and used as fuel on an extensive scale in some parts of the world; especially Ireland, Holland, Germany and Scandinavia. [Illustration: CONCRETE PORTAL OF A “DRIFT” MINE] Gradual changes in the elevation of the land led to the submergence of the prehistoric peat bogs, during successive intervals of time, by lakes or shallow seas. Thus their vegetation was killed, and they were overspread with layers of mud or sand, above which, during a subsequent period of elevation, a new peat bog would form; and this process was repeated several times. The conversion of the peat into coal appears to have resulted from the pressure of the overlying strata, probably aided by the internal heat of the earth. Much of its moisture was squeezed and evaporated out; the proportions of its component gases were reduced; and the result was a hard mineral, which has earned the popular name of “black diamond” because consisting chiefly of carbon--the same chemical element which, in a pure and crystalline form, constitutes the true diamond. Chemically, coal consists of carbon; the gases hydrogen, nitrogen and oxygen; sulphur; and ash (the mineral matter that remains after combustion). [Illustration: Courtesy of United States National Museum Comparative coal supplies of the world. The nick in the smallest cube shows how much hard coal has been used up. Soft coal cube has hardly been scratched]