Appearing sometimes on the shores of the "seas," sometimes in the midst of broad plains, sometimes along the course of mountain chains, and sometimes in magnificent rows, following for hundreds of miles the meridians of the lunar globe, are tremendous, mountain-walled, circular chasms, called craters. Frequently they have in the middle of their depressed interior floors a peak, or a cluster of peaks. Their inner and outer walls are seamed with ridges, and what look like gigantic streams of frozen lava surround them. The resemblance that they bear to the craters of volcanoes is, at first sight, so striking that probably nobody would ever have thought of questioning the truth of the statement that they are such craters but for their incredible magnitude. Many of them exceed fifty miles in diameter, and some of them sink two, three, four, and more miles below the loftiest points upon their walls! There is a chasm, 140 miles long and 70 broad, named Newton, situated about 200 miles from the south pole of the moon, whose floor lies 24,000 feet below the summit of a peak that towers just above it on the east! This abyss is so profound that the shadows of its enclosing precipices never entirely quit it, and the larger part of its bottom is buried in endless night. One can not but shudder at the thought of standing on the broken walls of Newton, and gazing down into a cavity of such stupendous depth that if Chimborazo were thrown into it, the head of the mighty Andean peak would be thousands of feet beneath the observer. A different example of the crater mountains of the moon is the celebrated Tycho, situated in latitude about 43 deg. south, corresponding with the latitude of southern New Zealand on the earth. Tycho is nearly circular and a little more than 54 miles across. The highest point on its wall is about 17,000 feet above the interior. In the middle of its floor is a mountain 5,000 or 6,000 feet high. Tycho is especially remarkable for the vast system of whitish streaks, or rays, which starting from its outer walls, spread in all directions over the face of the moon, many of them, running, without deviation, hundreds of miles across mountains, craters, and plains. These rays are among the greatest of lunar mysteries, and we shall have more to say of them. [Illustration: THE LUNAR ALPS, APENNINES, AND CAUCASUS. Photographed with the Lick Telescope.]