Now, however slowly we suppose the earth's crust to be changing, it must be admitted that in the course of the next fifteen hundred millions of years the earth will have parted with far the greater part, if not with the whole, of that inherent heat on which the present movements of her surface depend. We know that these movements at once depend upon and indicate processes of contraction. We know that such processes cannot continue at their present rate for many millions of years. If we assume that the rate of contraction will steadily diminish--which is equivalent, be it noticed, to the assumption that the earth's vulcanian or subterranean energies will be diminished--the duration of the process will be greater. But even on such an assumption, controlled by consideration of the evidence we have respecting the rate at which terrestrial contraction is diminishing, it is certain that long before a period of fifteen hundred millions of years has elapsed, the process of contraction will to all intents and purposes be completed. We must assume, then, as altogether the most probable view, that the moon has reached this stage of planetary decrepitude, even if she has not become an absolutely dead world. We can hardly reject the reasoning which would show that the moon is far older than has been assumed when long stages of her history and our earth's have been neglected. Still less reasonable would it be to reject the conclusion that at the very least she has reached the hoar antiquity thus inferred. Assuming her to be no older, we yet cannot escape the conviction that her state is that of utter decrepitude. To suppose that volcanic action can now be in progress on the moon, even to as great a degree as on the earth, would be to assume that measurable sources of energy can produce practically immeasurable results. But no volcanic changes now in process on the earth could possibly be discernible at the moon's distance. How utterly unlikely does it seem, then, that any volcanic changes can be now taking place on the moon which could be recognized from the earth! It seems safe to assume that no volcanic changes at all can be in progress; but most certainly the evidence which should convince us that volcanic changes of so tremendous a character are in progress that at a distance of two hundred and sixty thousand miles terrestrial telescopists can discern them, must be of the strongest and most satisfactory character.