may perhaps be looked upon as the remnants of the great meteor-swarm which completed the Martian development, and which are perhaps themselves destined at some distant period to fall into the planet. Should future astronomers witness the phenomenon the effect produced upon its surface would be full of instruction.
As the result of such an origin as that suggested, Mars would possess a structure which, in the essential feature of heat-distribution, would be the very opposite of that which is believed to characterise the earth, yet it might have been produced by a very slight modification of the same process. This peculiar heat-distribution, together with a much smaller mass and gravitative force, would lead to a very different development of the surface and an altogether diverse geological history from ours, which has throughout been profoundly influenced by its heated interior, its vast supply of water, and the continuous physical and chemical reactions between the interior and the crust.
These reactions have, in our case, been of substantially the same nature, and very nearly of the same degree of intensity throughout the whole vast eons of geological time, and they have resulted in a wonderfully complex succession of rock-formations—volcanic, plutonic, and sedimentary—more or less intermingled throughout the whole series, here remaining horizontal as when first deposited, there
