Some geologists have believed that when the old swamps now represented by coal seams were accumulating thick beds of peat, the climate was torrid, while the earth's atmos phere contained a higher percentage of carbon dioxide than now. It may be said, however, that a torrid climate is not necessary for the existence of great swamps; and as to a higher percentage of carbon dioxide in the air, were the present land surface of the globe coverod with a layer of coal one foot thick, the carbon contained therein, if restored to the atmosphere, would mean an increase in the amount of carbon dioxide of only about .5 of 1 per cent. This about represents the difference between air in the city and in the country, and the effect of such an increase in stimulating plant growth is at best doubtful.
Following the submerging of the fallen vegetable substance biochemical changes take place through the action of certain bacteria and the material soon loses its vegetable structure and becomes metamorphic. Being later subject to stresses of earth movements, either through foldings of the strata, with more or less heat, or, through subsidence and the deposit of sedi mentary rocics, to heavy pressures this meta morphic bacterial residue becomes a true rock. The great variation in its constitution is due to the endless variation of conditions during its transmutation.
As to the age of the coal formations it may be said that true peat deposits are all of later age than the Tertiary, and in North America only peat and inboherent lignite occur in later formations than the middle Tertiary (Miocene) though the early Tertiary (Eocene) forma tions contain great areas of lignite. In Europe there are lignitic deposits in the Triassic and in the Carboniferous. The great coal-forming age in North America and in Europe was the Carboniferous, though true coals occur in the Permian and in the Triassic, and in North America there are very important coal fields of Cretaceous Age. Isolated pockets of coal, but no workable coal fields, occur in pre-Carbon iferous formations. See CARBONIFEROUS SYS TEM.
An idea of the relative compositions of peat, lignite, and true coals may be had from the following table: Varieties of Coal.— Various elaborate schemes of classifying coal have been advo .cated, but have not gained popular acceptance. In fact in the coal trade in this country the word coal is often applied to a true ligiute. However, since the chief heat-producing ele ment in coal is carbon, a classification in wide use is based on the value of the coal as fuel; that is, on the percentage of carbon present and the condition of the carbon.
Part of the carbon is fixed, that is, cannot be driven off by heating in a retort; part is combined with the hydrogen and nitrogen as volatile hydrocarbon compounds which can be driven off. The percentage of fixed carbon is highest in anthracites, but even the semi graphitic anthracite of Rhode Island contains a considerable percentage of hydro-carbons The proportion of the volatile hydro-carbons to the fixed carbon in a coal is called its fuel ratio. On this basis the distinctions usually made are lignite, bituminous, semi-bituminous, semi-anthracite and anthracite. Cannel coal, in which the percentage of volatile hydro carbons is very high, is believed to be of dif ferent origin from other coals. It may repre sent accumulations of seeds, spores, resins or gums, and possibly of fish remains, in pools in the ancient swamps. Generally spealcing, in anthracite the volatile matter is below 6 per cent in semi-anthracite below 10 per cent, in . .
semi-bituminous between 12 and 18 per cent and in bituminous above 18 per cent. In cannel coal the volatile matter may be as high as 50 or even 65 per cent.
Lignite, or brown coal, is brown to black in color, though the powder is always brown. It often shows plainly its vegetable origin, containing stems that look like undecomposed wood. The lustre may be resinous or dull; the specific gravity is .5 to 1.5, some kinds floating on water. Lignites burn easily with a smoky flame, generally contain a high percentage of watcr, crumble easily and slack to mud on long exposure.
Bituminous or soft coal is black; the pow der is black; the lustre may be resinous or dull; the specific gravity is 1.25 to 1.4. It con tains less water titan lignite and bears trans portation better. Bituminous coals are sub divided according to their properties or uses, into coking, free-burning, smokeless, gas coals, etc. Coking coals partly fuse or cake in burning. If low in ash and very low in sulphur they are highly valued for forging and for making coke and gas. The famous coals of the Cumberland region in Mary land are classified as semi-bituminous. In coking coals the less volatile hydrocarbons present, the higher the yield of coke, but in gas coals the more hydrocarbons the better. A good gas coal will give 10,000 cubic feet of gas per ton. Smokeless coals burn with little smoke and are used for domestic purposes and for steam production.