Let the reader call to mind what we have already explained about the origin and formation of Coal; and then let him examine the structure of the Carboniferous Rocks. In the great Coal-fields of Wales, for instance, he will find, in a depth of 12,000 feet, from fifty to a hundred distinct beds of coals, spread out one above another, with intervening strata of clay several feet thick. Now each one of these beds represents an ancient forest which must have grown up and flourished and decayed; or else an immense and varied mass of Drift-wood, transported from a distance by the action of moving water, and deposited near the mouth of some great river. In either case a considerable lapse of time would have been necessary for such an accumulation of vegetable matter as would furnish the elements even of a single seam of Coal. And, when that period came to an end, only one little stage in the long series had been accomplished: one stratum of a few feet had been laid down in that great Formation which was to reach at length a height of more than two miles. A new condition of things then ensued. This layer of vegetable matter, sunk below the waters, was gradually covered over with a thick deposit of clay, which, in course of time, was to emerge, and become dry land, and give birth to a second forest, destined in its turn to wither and decay. Or, at least, when the stratum of clay had been deposited, it was to be overlaid, in some way or another, with a second layer of vegetable matter sufficient for the production of a second bed of coal. And so this process must have gone on, doubtless with many and long interruptions, for a hundred times in succession. Then it must be remembered that the Coal-bearing strata represent but one of many periods, and that not the longest in the Geological Calendar. Before the age of the Coal, England was for centuries at the bottom of the sea, while the Old Red Sandstone was slowly spread out over its existing surface. And after the age of the Coal, England was again submerged, and gigantic Ichthyosaurs with their companions of the deep, sported in the waters that rolled over her plains and covered the tops of her mountains; and, when they had run their course, left their remains buried in the clays of Oxfordshire and Warwickshire and Dorsetshire.