Origin of Coal

A study of the relationships of the carboniferous to other strata, and of the fossils found in the coal measures, makes possible a fairly accurate view of the origin and mode of formation of coal. Coal-bearing strata occur first in the later primary rocks of the earth's crust, and recur in various subsequent formations. The flora and fauna existing when true coal was formed were very different from those of the present day. The vegetation of the coal period, as shown by fossil remains, was chiefly cryptogamic, consisting of giant tree-ferns, reeds, equiseta, and club mosses; but conifers and cycads also occurred, and their seeds, together with the spores of lycopods, contributed in large part to the coal. Forest trees, however, such as oak and elm, had not yet appeared.

All this vegetation flourished exceedingly in marshy land and in an atmosphere rich in carbon-dioxide gas. Dead and decaying vegetation was buried beneath succeeding layers of similar material, or of sediment which hardened into rock; thus under enormous and ever-increasing pressure, by the agency of bacteria but in the absence of air, layers of coal were gradually formed.

Various attempts have been made to imitate artificially the processes by which coal has come into existence, by submitting carbonaceous material to high pressure and temperature. Spring submitted peat to 6000 atmospheres pressure, and converted it into a hard, black, shining substance like coal. Violette heated wood to nearly 400° C. in a sealed tube; the volatile products exerted a great pressure, and the solid remaining had the appearance of a fatty coal. Cagniard de la Tour performed similar experiments, and Fremy heated carbohydrates and "ulmic acid" in sealed tubes to about 300° C., and obtained the following coal-like products, which yielded water, gas, tar, and coke by destructive distillation.

Composition od artificial coal.

	Carbon	Hydrogen.	Oxygen.
Coal from sugar	66.84	4.78	28.43
Coal from starch	68.48	4.68	26.84
Coal from gum arabic	78.78	5.00	16.22
Coal from ulmic acid (peat)	76.06	4.99	18.95
Coal from ulmic acid (vasculose)	78.78	5.31	18.26

More recently Bergius has heated peat, cellulose, and water to a temperature of 340° C. for eight hours at a pressure of 100 atmospheres, and has thus obtained a finely divided black powder, very similar to ordinary coal so far as its composition and qualitative reactions are concerned. Subsequent heating to above 300° C. under pressure of about 5000 atmospheres yielded a coherent mass resembling coal in its physical properties.