But how are the great ledges of rock above the coal to be explained? Let us see. The rocks in question are very often sandstone, and if we examine them carefully we find that they are spread out in layers, that they contain the remains of many marine animals, and that the surface of a very large number of the layers shows signs of having been washed by waves. With the exception that they are somewhat harder and the organic remains belong to more old-fashioned types, these rocks are exact duplicates of the widely spread layers of sand that the ocean is piling along all our shores to-day, and contain the clearest evidence that they have been swept into place by the waves of some old sea. But how came the coal beneath the sea? That is an important question, and the full answer to it would show that no truth is more plainly taught by all the records of the past and present than that the earth's surface is a very uncertain and unstable affair. You will find your answer to the question on the shores of Greenland,Fig. 7.—Stigmaria ficoides. where the coast, for hundreds of miles, is slowly sinking into the sea; the result in historical times being sufficient to convert old marshes into shallow bays over which sands are swept by each returning tide. You will find your answer along the coast of New Jersey, in the buried forests with their prostrate trees and upright stumps, all carried down in very recent times by the subsiding land just within reach of the sea. You will find your answer in the buried forests of the delta of the Mississippi. You will find the same answer in a hundred other places. Large areas of land in different parts of the world are gradually subsiding, and large areas in the Carboniferous age sank down in the same way. Each coal-seam is the record of peat-bog and forest; the overlying rocks record a period of submergence.

But the movements in the earth's surface at present are not all downward; there are, perhaps, as many cases in which the land is rising. During the Carboniferous age, the same thing was true, and it often happened that the area that was carried beneath the sea was, in time, reëlevated, and became the platform on which other forests and peat-bogs renewed the work of coal-making. Thus it is that in many coal-fields we find a number of seams, one above the other, and thus it is that we have registered in the coal-beds of Nova Scotia not less than seventy-six distinct upward and downward movements of the surface. The sinking of the old marsh beneath the sea might seem to involve the loss of all the materials that had been accumulating during periods, perhaps, for which years would furnish no adequate unit of measure, and yet this very movement was essential to the preservation of the great magazines of energy on which human progress so much depends.