The illustrations furnished by the Solar System, supposing it to have had a nebular origin, are so obvious as scarcely to need indicating. That as a whole, it underwent a gradual concentration while assuming its present distribution of parts; and that there subsequently took place a like concentration of the matter forming each planet and satellite, is the leading feature of the hypothesis. The process of integration is here seen in its simplest and most decided form. Geologic evolution, if we trace it up from that molten state of the Earth’s substance which we are obliged to postulate, supplies us with more varied facts of like meaning. The advance from a thin crust, at first everywhere fissured and moveable, to a crust so solid and thick as to be but now and then very partially dislocated by disturbing forces, exemplifies the unifying process; as does likewise the advance from a surface covered with small patches of land and water, to one divided into continents and oceans—an advance also resulting from the Earth’s gradual solidification. Moreover, the collection of detritus into strata of great extent, and the union of such strata into extensive “systems,” becomes possible only as surfaces of land and water become wide, and subsidences great, in both area and depth; whence it follows that integrations of this order must have grown more pronounced as the Earth’s crust thickened. Different and simpler instances of the process through which mixed materials are separated, and the kindred units aggregated into masses, are exhibited in the detailed structure of the Earth. The phenomena of crystallization may be cited _en masse_, as showing how the unifications of similar elements take place wherever the conditions permit. Not only do we see this where there is little or no hindrance to the approach of the particles, as in the cases of crystals formed from solutions, or by sublimation; but it is also seen where there are great obstacles to their approach. The flints and the nodules of iron pyrites that are found in chalk, as well as the silicious concretions which occasionally occur in limestone, can be interpreted only as aggregations of atoms of silex or sulphuret of iron, originally diffused almost uniformly through the deposit, but gradually collected round certain centres, notwithstanding the solid or semi-solid state of the surrounding matter. Iron-stone as it ordinarily occurs, presents a similar phenomenon to be similarly explained; and what is called bog iron-ore supplies the conditions and the result in still more obvious correlation.