gives a circular movement to all fluids that are drawn towards a common centre. The centrifugal force thus generated tends to throw off matter from the equatorial regions of the great orb, but is restrained by the attraction of gravitation, which would prevent any separation of the parts, if the sun itself did not now begin to cool down, and consequently to shrink in size. Under this cooling process, a crust is formed upon the surface, too rigid to yield to the force of gravity, and the parts within, continuing to shrink, separate from this envelope; so that there is now a central orb, revolving more rapidly from its greater density and smaller diameter, and surrounded by an exterior shell, or band, like Saturn's ring, rotating at its original speed. As we cannot suppose that the ring would usually be of uniform thickness and strength, it eventually breaks up into fragments, the larger of which attracts the smaller into itself, and the whole is formed by its revolving motion into an oblate spheroid circling round the contracted sun in the centre. In this manner, the planet Uranus was shelled off from our sun, which originally filled the whole of the vast sphere, of which the distance from Uranus to the centre of the present sun is but the radius. The planet itself, by the same process of cooling, shrinking, and thus forming exterior rings, threw off successively all its six satellites; and the sun, also, continuing to contract from the loss of heat, formed another ring, and thus constituted the planet Saturn. In this way were formed successively all the planets and satellites of the present solar system. The original diameter of our earth was equal, of course, to the present diameter of the moon's orbit. In the case of Saturn, the two rings formed around it happened to be of unusual homogeneity and equal thickness, so that they were not broken up, but have preserved their primitive shape. A ring was formed from the sun in the space between the present orbits of Mars and Jupiter; but when it was broken up, the fragments did not congregate into one, but spherified separately, so as to form the four smaller planets which now revolve in that opening.

Having thus explained the genesis of the solar system, we come down to the history of our own earth, since it shelled off the ring which formed our moon. Continuing to cool down and shrink, a thin but rigid crust of primary rocks, still bearing marks of the intense heat to which they have been subjected, was formed upon its surface; and then the vapors, with which the atmosphere had been charged, were condensed, and formed seas, which covered the whole, or the greater part, of the earth's rind. The continual agitation of these waters, and their high temperature, as they were still nearly at the boiling point, disintegrated and wore down many of these rocks, and, in the lapse of ages, deposited their remains, in thick layers of sand and mud, at the bottom of the seas. Baked by the heat from beneath, and pressed by the weight of superincumbent waters, these layers slowly hardened into stratified rocks. Forms of vegetable and animal life, though only of the lowest type, the origin of which is to be explained hereafter, now began to appear. Some sea-plants, zoöphytes, infusory animalcules, and a few of the molluscous tribe, all low down in the order of being, but important from their immense numbers and joint action, commenced their work of absorbing the carbonic acid with which the air was overcharged, and building up vast piers and mounds of stone from their own remains. Meanwhile, the internal fires of the earth occasionally broke through the rocky crust that imprisoned them, threw up liquid primitive rock through the rents, and distorted and tilted up the strata that had been formed above.

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