As has already been said, the plants of the Carboniferous age were exceedingly abundant and luxuriant. They grew up richly from the clayey soil, and formed dense jungles in the vast marshes which covered so large an area of the earth’s surface. Ferns, mosses, and tufts of surface vegetation, and the leaves, branches, and trunks of trees fell and decayed on the place where they grew, only to make the soil more fertile and the next growth richer and more luxuriant. Year after year, century after century, this process of growth and decay went on, until the beds of vegetable matter thus deposited had reached a great thickness. But condensation was still in progress in the earth’s body, and in consequence of it her crust, of necessity, at times contracted and fell. When it did so the land sank throughout vast areas, these beds of incipient coal went down, and over the great marshes the waters swept again, bringing drift of vegetation from higher levels to add to that already buried. Then over these deposits of vegetable matter the sand and mud and gravel were laid up anew, and the clayey soil from which the next rich growth should spring was spread out upon the surface. This process was repeated again and again, as often, indeed, as we find seams of coal in any coal bed. Thus the final condition for the formation of coal was met, the exclusion of atmospheric air from this mass of decaying vegetation was complete, and under the water of the ocean, under the sand and silt of the shore, under the new deposits of succeeding ages, the transformation went on, the wood of the Carboniferous era became the coal of to-day, while above and below it the sand and clay were hardened into rock and shale. The remarkable features of the vegetation of the coal era were the size and abundance of its plants. Trees of that time whose trunks were from one to three feet in diameter, and which grew to a height of from forty to one hundred feet, are represented in our day by mere stems a fraction of an inch in diameter and but one or two feet high. A comparison of quantity would show differences as great as does the comparison of size. But at that time all the conditions were favorable for the rapid and enormous growth of vegetation. The air was laden with carbon, which is the principal food for plants; so laden, indeed, that man, who is eminently an oxygen-breathing animal, could not have lived in it. The great humidity of the atmosphere was another element favorable to growth. Vegetation never lacked for an abundance of moisture either at root or leaf. Then, too, the climate was universally warm. Over the entire surface of the earth the heat was greater than it is to-day at the torrid zone. It must be remembered that the internal fires of the globe have been constantly cooling and receding, and that the earth, in the Carboniferous age, was subjected to the greater power of a larger sun than shines upon us to-day.