of rills occur. The appearance of this mare is as if a thin skin had formed over the liquid surface and had then been broken and crumpled and drifted to the eastern side. Toward the centre and on the western side of Humorum are some long ridges, the western ones concentric with the mare and the rills. On the eastern side are some deep rills also concentric with the mare. One of them is shown on 14A [2.2, 4.6]. The paraffine crater (Fig. 4) shows a minute ridge stretching from the nearer side toward the centre. Later a rill formed upon the farther side concentric with the crater, thus showing that the surface was subjected first to compression as the floor of the crater fell, and later to tension as it contracted and receded from the crater walls.
The direction of the rill in many cases bears no apparent relation to the surrounding formations, and seems to be the result of a general contraction of the lunar crust. When it traverses a crater it is often evident that the latter is the earlier formation, as in Hippalus, 12E [2.8, 2.9], though in the case of Ramsden [2.7, 3.9] the crater seems to have been formed later. Some extremely broad and deep concentric rills are found in the interior of Wurzelbauer [1.0, 3.9]. The appearance of a crumpled skin seen in the Mare Hiunorum is also found in other formations, as in J. F. W. Herschel, 11B [1.4, 1.6].
An extremely rough and broken surface is found just to the north of Sinus Iridum, iiB [2.0, 3.1]. Few craters are to be seen, and it looks as if it might have been a part of the original surface of the Moon, before it had been pitted with craters or fused into maria. But what makes this region particularly interesting is that we have here a clear view of a section of the surface some two miles in depth, cut by the melted lava that formed the Sinus Iridum. An examination of this section shows a nearly vertical wall, in places perhaps overhanging, apparently composed chiefly of objects like huge boulders, measuring several thousand feet in diameter, but separated from one another here and there by interstices forming caves of surprising dimensions. As seen under favourable conditions, at Arequipa, the appearance was not unlike that of a piece of wood broken squarely across the grain. The structure of the wall is entirely different from that of the Apennines, which bound the Mare Imbrium on the west. Neither does it resemble the smooth, sloping terraced interiors of the larger craters.
Among the more noteworthy of the minor lunar features may be mentioned the following formations. To the north and also to the south of Copernicus, 11A [2.5, 6.9], are found a number of spindle-shaped cuts in the surface, like very elongated craters. They are on the average about two miles long by half a mile broad, and very much resemble
