| The “coal series” | Carbon | Oxygen | Hydrogen | Nitrogen |
| Wood (cellulose) | 50 | 43 | 6 | 1 |
| Peat | 59 | 33 | 6 | 2 |
| Lignite | 69 | 25 | 5.5 | 0.8 |
| Bituminous coal | 82 | 13 | 5 | 0.8 |
| Anthracite coal | 95 | 2.5 | 2.5 | trace |
| Graphite | 100 | .. | .. | .. |
From this table it is seen that the oxygen relatively diminishes while the carbon relatively increases, though, of course, all three elements actually decrease during the chemical change from cellulose to coal. These three elements disappear mainly in the form of gases, such as water vapor, marsh gas, and carbonic acid gas. The final or graphite stage is almost reached by the graphitic anthracite of Rhode Island, which is so nearly pure carbon as to be really useless as coal.
The conditions under which successive layers of vegetable matter (later turned into coal) become embedded in the earth’s crust have been outlined in the chapter on the “Evolution of Plants.” The most perfect conditions for prolific plant growth, and accumulation as great beds in the earth’s crust, were during the Pennsylvanian period of the late Paleozoic era in many parts of the world, but especially in the United States, China, Great Britain, and Germany. Most of the world’s great supply of coal comes from rocks of Pennsylvanian Age, while next in importance are Cretaceous rocks, and some comes from strata of other ages later than the Pennsylvanian, even as late as the Tertiary.
The United States not only has the greatest known coal fields, but it also produces far more coal than any other country. In 1918 the production was 678,000,000 tons, the greatest in our history, or enough, if loaded into cars of forty tons capacity, to fill a train which would reach around the earth at the equator about six times! Equally amazing is the fact that this coal was nearly all consumed by this one nation! In 1919 the production fell to 544,000,000 tons. Is there real danger that our supply of coal will soon run out? Hardly so when we consider, first, the fact that probably not more than 1 per cent of the readily available coal has thus far been removed, and, second, the high probability that rate of increase in coal production for the last twenty years will not continue. In fact, during the last two or three years the production has fallen off considerably. But even so, coal, which is our greatest natural resource, and which can never be replaced, should be scientifically conserved. In the case of the very restricted anthracite coal fields what might be called a crisis has already been reached, because a very considerable part of the available supply has been taken out.
Something like 350,000 square miles of the United States are underlain with one or more beds of workable coal (not including lignite)—in some areas five to twenty or more beds one above the other. There are also about 150,000 square miles of country underlain with the more or less imperfect coal called lignite. It has been estimated that there are more than a trillion tons of easily accessible coal, and another trillion tons accessible with some difficulty in the principal coal fields of the United States.
The greatest production of coal by far is from the Appalachian Mountain and Allegheny Plateau districts, from the western half of Pennsylvania to Alabama, where all the coal is bituminous of Pennsylvanian Age. Here as well as elsewhere the coal beds are interstratified with various kinds of sedimentary rocks, most commonly with shales and sandstones. In the Appalachian field the strata including coal beds are more or less folded toward the east, while they are nearly horizontal toward the west. The famous Pittsburgh coal bed is probably the most extensive important single coal bed known. It covers an area of over 12,000 square miles and is workable, with a thickness of five to fifteen feet, over an area of 6,000 square miles of parts of western Pennsylvania, Ohio, and West Virginia.
Fig. 76.—Map of the United States, showing the principal coal fields. Cross-lined areas represent lignitic coals. (After U. S. Geological Survey.)
The greatest production of anthracite coal by far is from central-eastern Pennsylvania, where strata of Pennsylvania Age, including a number of anthracite beds, are mostly highly folded. Most remarkable of all in this district is the so-called “mammoth bed” of anthracite, nearly everywhere present, with a thickness up to as much as fifty or sixty feet. Less than 500 square miles are there underlain by workable anthracite coal.