The form of these craters is that of pits, which generally have ring-like walls about them. These wails slope very steeply to a central cavity and more gently toward the surrounding country. In all these pits, as pointed out by Shaler, except those of the smallest size, and possibly in these, also, there is, within the ring wall and at a considerable though variable depth below its summit, a nearly flat floor, which often has a central pit of small size or, in its place, a steep cone. When this floor is more than 20 miles in diameter, and in increasing numbers as it is wider, there are generally other pits and cones irregularly scattered upon it. Thus, within the ring called Plato, which is about 60 miles in diameter, there are some scores of these lesser pits. On the interior of the ring walls of the pits over 10 miles in diameter, there are usually more or less distinct terraces, which suggest that the material now forming the solid floors they inclose was once fluid and stood at greater heights in the pit than that at which it became permanently frozen. It is, indeed, tolerably certain that the last movement of this material of the floors was one of interrupted subsidence from an originally greater elevation on the outside of the ring wall. The ring wall is commonly of irregular height, with many peaks. In some places there may be seen tongues or protrusions of the substance which forms the ring, as if it had flowed a short distance and then had cooled with steep slopes. It may also be noted: (_a_) that the pits or craters in many instances intersect each other, showing that they were not all formed at the same time, but in succession; (_b_) that the larger of them are not found on the plains (seas) but on the upland and apparently the older parts of the surface; and (_c_) that the evidence from the intersections clearly indicates that the larger of these structures are prevailingly the older and that in general the smallest were the latest formed. In other words, says Shaler, whatever was the nature of the action involved in the production of the craters, its energy diminished with time, until in the end it could no longer break the crust. These features indicate that the surface of the Moon has been subject to forces similar to those which produce volcanoes on the Earth, and it is therefore customary to refer to the crater-like mountains of the Moon as volcanoes. As the parallel cannot be drawn too closely, however, Shaler has urged that the term vulcanoids, meaning volcano-like, be applied to these mountains. 3. RILLS or CLEFTS. These are small, deep, ditch-like furrows to be found over various parts of the Moon's surface. Their course seems quite independent of the surface topography, for they traverse mountains and plains with equal facility. They are without doubt the latest formation on the Moon and some of them may have had their origin in modern times.