The limestone is unquestionably of marine origin, as the countless myriads of testacea inclosed in it testify, and was probably constructed by the primeval families of those island-making architects by which the coral-reefs of our present seas are raised, and whose instincts have found them similar employment in all ages of the world. The limestones of the earlier systems may have been formed in the same manner; and then, as in the subsequent period, we must go to the great original storehouse of Nature for the materials on which they worked. The spoils of the primary rocks could not supply them, as the quantity of the carbonate of lime therein contained bears no proportion to the masses which constitute the mountain limestone group. But the calcareous substance was already, in some elementary form, in combination or otherwise, in existence—the animals capable of secreting and arranging it anew, as the secondary instruments of creation, were abounding in the seas—shallow bottoms over the subjacent sandstones of the devonian system, and within the required conditions of life, were prepared for their operations. The waters had now brought forth abundantly the moving creatures, which, at first more scantily distributed, produced the limestone of the silurian rocks, as the arborial remains of the land, in like stinted measure, are inclosed in the older palæozoic deposits. Their day of increase as it advanced, each after their kind, is recorded in the vast accumulations of animal and vegetable matter which compose the strata of the carboniferous system, both of an order and quality purposely so arranged, and never upon the same scale of magnitude to be repeated in the combustible mineral.