Page:The moon (1917).djvu/11

130 The question of the origin of the various lunar surface features is one on which astronomers are still in doubt. It is not difficult to conceive of the formation of the mountain ranges, lofty as some of them are, and of the valleys or canyons and of the smaller craters, at least, by forces similar to those which have produced corresponding features upon our Earth, especially when we consider the fact that, because of the Moon's smaller mass, a given force acting against gravity there would be about six times as effective as here. But the bright lines or rays running out from some of the craters are unlike anything familiar to us on the Earth's surface, and there are great difficulties in the way of accounting for the craters themselves.

By many astronomers they have been ascribed to volcanic origin as their name indicates. The chief objections to this theory are, (1) that there are so many craters and that they are often so very large—60 or even 100 or more miles in diameter; (2) that the material in the surrounding walls and peaks is not sufficient to fill the crater bowls; and (3) that there are few or no evidences of lava flows. Other astronomers believe that the craters originated in impacts from failing meteors. Here again, the objections are of great force: (1) the bombardment must have been a terrific one to form so many and such large craters and since the Earth and Moon revolve in the same general path about the Sun, and the Earth is so much the larger of the two, craters should be correspondingly more numerous and as large upon the Earth. As a matter of fact only one crater on the Earth—the celebrated "crater mound" in Arizona—is known that was probably formed by a falling meteorite. Advocates of this theory, however, point out that the lunar craters were undoubtedly formed many millions of years ago and that erosion on the Earth has probably erased all traces of the craters formed here at that time. Granting this, it would seem that there should be evidences of their former existence in the rock strata examined by geologists and so far as I am aware these have not been found. (2) An even greater objection to the theory is that, to form the lunar craters we see, the meteors must all have fallen vertically upon the Moon's surface, and it can readily be shown that this would be possible, if at all, in only a very small percentage of cases. Nearly all collisions must have been with meteors moving at acute angles with the surface at the time of contact and evidences of this fact should be visible for there is no "weathering" on the Moon.