We have not, in this enumeration of the mineral strata of the earth’s crust, as yet spoken of any of the calcareous deposits. They are very numerous, some of them of prodigious thickness, and belong to the formations of every epoch. There is not one, but many, alternations of limestone connected with every such formation. Whence the source of all this material? The primary beds are not in sufficient mass to have furnished supplies for every succeeding age. The mountain limestone alone, of the middle secondary epoch, contains more calcareous matter than is to be found in the three antecedent periods. The lias, oolites, and chalks are likewise of vast thickness. The beds of the tertiary group are less considerable; but in the gypseous marls, and numerous alternating bands throughout the clays and sands of the formation, there is the clearest evidence that the stores of nature were still abundant. Nor are they yet exhausted. What supplies in every river, sea, and ocean of the world! What countless myriads of living animals are now employed in elaborating the material! And when we again inquire, whence is it all? the answer is, that throughout all time, a wise and bountiful Providence has thereby provided the pabulum for its successive creations of organized bodies—the law of their nature is to pile up rocks—and in all the monuments of the past, we discern the style and architecture of the builders of the present. Look, then, to your still active, living, working chronometer. With what incredible swiftness do these minute creatures ply their labors! how many fathoms of coral reef will they rear in a season! When the hapless mariner returns, after a brief short interval, what hazards to run from structures which now for the first time appall him with their formidable barriers! Millions of years! Not even thousands are needed to construct islands, and to pillar the floor of the ocean, over vast expanded areas, with broad, massive, indurated rock.[13]