Over the site of the Appalachians and most of the eastern half of the Mississippi Valley area the land either stood near sea level and was often swampy or marshy, or at other times it was a little below sea level, allowing tidewater to overspread the area. Such conditions alternated repeatedly, usually more or less locally, over different parts of the districts in which the great coal mines of the east are located. Under such conditions strata from 1,000 to 8,000 feet thick accumulated. Remarkable physical geography of this kind resulted in the growth and accumulation of vast quantities of vegetable matter which has changed into the world’s greatest coal beds. Similar conditions prevailed over parts of Nova Scotia, New Brunswick, and Rhode Island, where strata fully 13,000 feet thick accumulated.
“Perhaps the most perfect resemblance to coal-forming condition is that now found on such coastal plains as that of southern Florida and the Dismal Swamps of Virginia and North Carolina. Both of these areas are very level, though with slight depressions in which there is either standing water or swamp condition. In both regions there is such general interference with free drainage that there are extensive areas of swamp, and in both there are beds of vegetable accumulations. In each of these areas there is a general absence of sediment and therefore a marked variety of vegetable deposit. If either of these areas were submerged beneath the sea, the vegetable remains would be buried and a further step made toward the formation of a coal bed. Reelevation, making a coastal plain, would permit the accumulation of another coal bed above the first, and this process might be continued again and again.” (H. Ries.) But it is not necessary to assume repeated oscillations of a swamp area up and down as the only way of accounting for a succession of coal beds one above another in a given region, because a general, but intermittent, subsidence, with possibly some upward movements, would occasionally cause the prolific plant life of a swamp to be killed, after which sediment would deposit over the site. Shoaling of water by accumulation of sediment would permit the development of more swamp plant life.
Fig. 39.—Map showing the general relations of land and water, including the great coal-plant swamp areas (vertical lines), in North America during the Pennsylvanian period at least 10,000,000 years ago. Lined areas represent land. (After Willis, courtesy of the Journal of Geology.)
In most coal-mining districts there are at least several coal beds, one above another. In Illinois there are nine; in Pennsylvania at least twenty; in Alabama, thirty-five; and in Nova Scotia seventy-six, but not all are important commercially. Each coal bed in such a region represents a swamp which existed in Pennsylvanian time at least ten or twelve million years ago, and in which there grew a luxuriant vegetation. Many individual swamps of that time were of wide extent. The famous Pittsburgh bituminous coal bed represents probably the largest one of all. It extends from western Pennsylvania into parts of Ohio and West Virginia over an area of fully 15,000 square miles. More than 6,000 square miles of it are being worked and the coal bed averages seven feet in thickness over an area of 2,000 square miles. Among the various anthracite coal beds of the same age in eastern Pennsylvania the Mammoth bed is exceptionally thick, reaching a maximum of fifty feet or more.
In order that the reader may not gain the impression that coal beds make up a very considerable bulk of the strata in coal-mining regions, we should state that, on the average, coal actually constitutes less than 2 per cent of the containing strata.
Some idea of the tremendous length of the geologic ages may be gained by a consideration of the time which must reasonably be allowed for the accumulation of so many coal beds and their containing strata. It has been estimated that a luxuriant growth of vegetation would produce 100 tons of dried organic matter per hundred years. Compressed to the specific gravity of coal (1.4) this would form a layer less than two-thirds of an inch deep on an acre. During the chemical alteration of vegetable matter to coal about four-fifths of the organic matter disappears in the form of gases. On this basis, then, it would take about 10,000 years to accumulate the vegetable matter represented in a coal bed one foot thick. When we realize that the total thickness of the coal beds of the Pennsylvanian system of strata in the great mining regions is commonly from 100 to 250 feet, we conclude that the time they represent is from 1,000,000 to 2,500,000 years. It seems most reasonable that the time necessary for the deposition of the containing strata must have been at least as long. It is, therefore, a fair conclusion that the Pennsylvanian period lasted from 2,000,000 to 5,000,000 years.