Forms of Vegetable Matter.—If vegetable matter be exposed to a certain degree of moisture and temperature, it is decomposed into the substance called peat, which is dug from swamps, and belongs to the alluvial formation.
Lignite or Brown Coal, the most perfect variety of which is jet, is found in most of the series above the coal formations; and appears to be vegetable matters like peat which have long been buried in the earth, and have undergone certain chemical changes. It generally exhibits vegetable structure.
Bituminous Coal appears to be the same substance which has been longer buried in the earth, and has undergone further changes. The proportion of bitumen is indefinite, varying from 10 to 60 per cent., and the coal is said to be dry or fat, according to the amount of bitumen present.
There are several varieties of bituminous coal.
Pitch, or caking coal, is a velvet black, highly bituminous mass, which cakes or runs together during combustion. Cherry coal is like caking coal, but it does not soften and cake. It breaks so readily that much of it is lost in the mining process. Cannel coal is nearly black, with a fine compact texture and a conchoidal fracture. It burns readily like a candle, hence its name. Splint coal is a coarse variety of cannel coal. The Albert coal of Nova Scotia is perhaps to be regarded as a species of bitumen, because the latter so much predominates. It has a bright, shining luster, and ignites instantly upon contact with flame. Coke is bituminous coal artificially deprived of its bitumen. It is light, and approaches charcoal in appearance. Coke is occasionally found in nature; especially in the neighborhood of dikes. All these varieties are found in the coal formation, and even in the Mesozoic and Tertiary series.
Anthracite is bituminous coal that has been deprived of its bitumen, usually by heat, under pressure. It thus forms a compact heavy mass, igniting with some difficulty. The anthracite of Pennsylvania, of enormous extent, is in the true coal measures, and it is a curious fact, that as we pass westward—that is, recede from the metamorphic and unstratified rocks of the Atlantic coast—the quantity of bitumen increases; so that within a few hundred miles the coal is highly charged with it. The fact makes it extremely probable that the heat, which changed the metamorphic rocks, also drove off the bitumen.
Hitchcock, Edward and Charles H. Hitchcock. Elementary Geology. New York: Ivison, Phinney & Company, 1862. 53-54.
The origin of coal, as Emily Dickinson would have studied it.