We may remark, in passing, that the chronology of the events of which we now speak is not very accurately determined; the only thing certain about it is, that a series of ages, so protracted that the imagination cannot conceive their number, elapsed between the successive epochs in the history of the earth's crust. Some of the convulsions caused by the fiery mass within threw up rock above the surface of the waters, and thus the dry land began to appear. Islands were formed, and immediately land-plants made their appearance, of excessive luxuriance, under the tropical temperature that still prevailed all over the globe, and began their office of absorbing carbon, and storing it up for future use. Land-animals as yet were not, for the excess of carbonic acid in the atmosphere rendered it incapable of supporting animal life. But the richness of this island vegetation gradually purified the air; while the decaying plants themselves, being accumulated into vast beds and strata, and subjected, through the changes of the earth's surface, to the pressure of mighty waters, gradually formed immense deposits of coal, for the subsequent service of man. Animals of a higher grade were now formed; fishes became abundant, and amphibious monsters, huge lizards and other reptiles, with an imperfect apparatus of respiration, began to breathe an atmosphere not yet fitted for birds and mammifers. It is not necessary to trace out the comparatively well known facts and theories of geological science, that are incorporated into this history. It is enough, for the present purpose, to point out a few of the general conclusions of the geologist respecting the several great changes that the earth's crust has undergone, and the distinct races of vegetables and animals which have successively tenanted the earth's surface. These changes and these races have borne a constant relation to each other; as the scenes shifted, the inhabitants also changed, the latter being always adapted to the circumstances in which they were placed. There has been a constant progress, the soil and the atmosphere becoming more and more fitted for the support of the higher forms of life; and when all things were thus made ready for them, these higher forms have appeared, and the lower orders of being, which formerly occupied the scene, have entirely died out, so that their remains, entombed in the solid rock, are now the only indications of their past existence. In the era of the primary rocks, as we have seen, there was no organization or life, as there was nothing to support it. In the succeeding period, zooephytes and mollusca appeared; these were followed by fishes, and then land rose above the surface of the waters. Land-plants and animals came next, though of a low type; continually advancing orders of beings, reptiles, birds, and mammifers, suited to the improved condition of things, successively appeared, until, at the latest epoch, man entered upon the