Page:Popular Science Monthly Volume 44.djvu/164

154 seems like the lower Amazon, too broad for us to see its banks, coming from the high Andes and the lower plains, and going to its rest in the ocean.

According to the well-approved nebular hypothesis of Kant and Laplace, the material of our earth and moon became separated from the condensing mass of the sun after the outer planets had been similarly produced, but before the birth of Venus and Mercury. At early stages in the condensation of the revolving nebula, it had thrown off successively the portions of matter which were afterward gathered, by their independent condensation and revolution, to form Neptune, Uranus, Saturn, Jupiter, and Mars; while another portion, which was probably never united like the other planets, made the many small asteroids. The matter which has been changed into our splendidly luminous sun was at one time very attenuated and occupied the whole space inclosed by the orbit of the outermost planet, which was developed from a comparatively very small cloud or ring of this matter, centrifugally detached from the revolving exceedingly tenuous mass. In like manner the material of each of the planets, including the earth, was shed from the whirling nebula at times during its decrease in volume when its circumference was approximately coincident with their orbits.

Again, in their turn the planetary masses have undergone the same evolutionary process, taking a rotary motion and throwing off, as they condensed, the material which now circles about them in the shape of moons and rings. In the case of our own planet and its single large satellite, probably the far greater part of the original cloud or ring whence they were produced had assumed a somewhat globular or discoid form and taken a movement of revolution which still continues as the earth's daily rotation, before the moon's mass was separated from that of the earth. It seems to me, however, very improbable that the present contour of our globe should preserve, as suggested by Fisher, the scars of this loss in the depressions of the deep ocean basins.

Many relatively small portions of the ring of matter producing the earth and moon may have become early separated from the chief condensing mass, and after its division in our globe and its satellite have been drawn by gravitation into them, marring the face of the moon, as Gilbert supposes, with its multitudes of both small and very large crateriform scars. On the earth, too, if this hypothesis be true, such falling asteroid-like bodies must also have made similar small and huge blots by their violent impact; but they evidently were effaced by the slow processes of atmospheric and stream erosion, or in basin areas were deeply covered by sediments, before the formation of the oldest of our