In the lowest and wettest parts of those forests there occurred, just as there may occur to-day, a large accumulation of fallen trees and other vegetable refuse. In the ordinary way these would simply rot, due to the work of insects and the fungi of decay, and in a few score years there would be nothing to show. “Dust to dust” would be, and is, the history of so many living things that it is only some machinery for arresting this process which will give us very different results. In the case of coal formation the original impetus appears to be certain microscopic organisms, probably saprophytic fungi, fossil remains of which have been identified, which work upon the fallen mass of vegetation and start its decay, but which can only do so while their prey is still within the influence of the air. The initial stages of decay must, therefore, have been going on while the water was low enough for these organisms to work. But in many parts of that ancient landscape the water level was a fluctuating quantity, due to local conditions or to changes in the earth’s crust. So that many times partially decomposed vegetation masses would become submerged, stopping the work of these organisms of decay that demand air, but providing the only conditions under which certain others could complete the transformation. These bacterial organisms that will work only when deprived of air continue the process, but in a different way. For one thing, the lack of air delays decomposition or almost stops it, as witness the resistance of logs under water, some of which are known to be hundreds of years old. And forest stumps off the coast of Cape May, in New Jersey, are in nearly as perfect a state as when first submerged, over 40,000 years ago. In the production of coal these anærobic (living without air) bacteria release oxygen and hydrogen from the partly decayed mass, leaving as a residue a substance known as peat, which is largely carbon. The transformation of peat into coal depends upon requisite pressure of the strata that may be laid down on top of the peat bed, and probably upon chemical changes that go on after such covering strata have been laid down.
The fact that coal is sometimes found only in thin veins, with layers of shale and other material between, tells us that its origin must often have been a precarious affair, where alternate emergence and submergence would permit first the vegetation to develop and then its transformation to peat, followed by the deposition of fine sands or silt covering the bed. Several such cycles occurred, sometimes separated by untold ages of time, or again by much briefer periods. Certain mines, however, contain over 200 feet of solid coal. The length of time necessary for such a vast accumulation, or how many generations of these ancient plants went into their making, is beyond calculation. With the mining of coal running into the hundreds of millions of tons yearly, we get some idea of how great were those Carboniferous forests, and how extensive they were is proved from the widely separated localities in which coal mines are found.
The Carboniferous age of fern, cycad, and conifer ancestors was by no means a quiet, orderly period, as from geological evidence it appears to have been much subject to alternate emergence and submergence of great tracts of land. Compared with what followed, it actually was a period of comparative quietness, however, and it must, in at least most parts of the world, have permitted the slow development of certain of its plant groups to a state of perfection never reached since. This is particularly true of the ancient relatives of our club mosses and horsetails.
Perhaps one of the most obvious questions to ask about these plants is how long ago they lived, and upon the answer to such a question depend many others. What, for instance, is the position of the Carboniferous as compared to what preceded it and came after? How old is the earth and when did life first appear on it? The evidence upon which such questions are answered comes from the estimates of physicists as to the age of the earth; from students of fossil animals and plants; from astronomers, from geologists and other students. A compromise of these different estimates, and one that has consequently been widely accepted, gives the age of the earth, dating from the time of its having a definite crust with land and water masses, as somewhere near a hundred million years. Such figures are beyond our comprehension and consequently mean almost nothing, but the proportion in time of the different stages of the development of plants may be stated with greater certainty. Taking the total age of the earth as 100 per cent, the period when there is no record of life of any sort may be set down as about 45 per cent of the total, the reign of algæ and development of land plants about 8 per cent, the carboniferous or coal-forming plants about 28 per cent, which leaves only 19 per cent from that distant time to the present. And many things happened in this comparatively brief fifth of the plant world’s history, among them the origin of some plants that have come straight down to us, without discoverable change.
Found in most fossil strata and in a practically unchanged condition from the upper part of the Carboniferous to the most recent fossil records. Now unknown as a wild tree and preserved for us through its cultivation in ancient temple gardens in eastern Asia.