domain, whether of land or water, was so different from ours, and now all passed away. A higher temperature, from central heat, will not explain these facts, for that should have prevailed more in the devonian, and still more in the silurian periods,—and of this we have no evidence. Appearances would rather support an opposite conclusion. The sweep of the comet again, resorted to upon occasions, may have destroyed, but could not maintain, such a state of things. A change of the polar axis, of the most inconsiderable extent, is demonstrated to be highly improbable, or almost impossible. And now, in the unwearied march of science, often baffled but never cast down, it has been announced as the probable solution of all the changes of the past, THE PROGRESSION OF THE WHOLE SOLAR SYSTEM, whereby the earth, and all the sister planets, are dragged through infinite space, and brought successively within the sphere of new constellations—now in a hotter and now in a milder efflux of ether—combining its own with a more general movement in a universal whirl—and thus constantly subjected, in all its parts, to ever-varying external influences! This, at least, is the ingenious theory of M. Poisson, which, he thinks, will account for the central heat of the globe, dipped for a time into a burning atmosphere, and cooling off more rapidly on the surface, and will give a no less plausible explanation as to the extent and frequency of change effected on the surface. Geology and astronomy become, when viewed in this light, correlative sciences, and impart an illustrative interest to the researches of each other. The lofty flights of the one are brought down, as it were, to more earthly things; while the geologist, on the other hand, is lifted from his miry pit and downward studies, to meditate on the “sweet influences and bands” that harmonize and link all the planets in their orbits, and rejoices to see his own earth taking part in the eternal music of the spheres. He is pleased to believe, according to the view of the astronomer, that this ball of stone and clay enjoys at times a vitality all over, which warms and cherishes into life natant forms, and creeping things, and flying dragons, whose development of powers could not have been sustained, on so great a scale, in the lower and less favored regions. But while the cause may be adequate to the effect—and in the approximation to the truth there is a feeling of satisfaction, an elevation of vision and elasticity of thought, as “Rays divine dart round the globe,” —still the speculation referred to belongs rather to the poetry than to the philosophy of science, influencing the imagination more than the judgment, and trenching on relations that lie beyond the field of legitimate research. CHAPTER V. THE CRETACEOUS SYSTEM.