at the same rate, with the fall of temperature, as when undergoing the process of solidification, and the result would be that the surface of the Moon, instead of being too small, would now be too large for its interior. When this "critical epoch" occurred, the formation of craters would for a time almost cease.
We have already seen that large cavities are usually found under the craters formed by the contraction and cooling of iron slag. In the case of the Moon, where such craters are numerous, these cavities would in many cases unite, and would thus cover considerable areas, leaving large empty spaces between the outer crust and the hot and viscous material of the Moon's interior. Should the partition walls between some of these cavities give way, large areas of the outer crust might thus be precipitated upon the partly liquid mass beneath, and since by the contraction due to solidification the specific gravity of the outer crust would now be greater than that of the fluid interior, the descending material would sink and melt, thus forming a fresh lunar surface in those places. This sinking and melting of the solid suface crust is frequently observed in the Hawaiian volcanoes. Whether the subsidence was cataclysmal or gradual is of little consequence; the subsident areas would be in general of circular form and of large area, as compared to the individual craters. This process is suggested as the probable method of evolution of the maria or seas upon the Moon. Conspicuous examples are the Maria Imbrium Serenitatis and Crisium, Frontispiece [4.2, 2.2], [2.5, 2.5] and [o.&, 3.1]. The darker colour of their floors would seem to indicate that they were formed from another kind of material, which, coming from a considerable depth, had united and mixed with the lighter-coloured molten matter which had formed the original surface. In these maria we often see the outlines of old crater rings which have been partially melted down and absorbed in the subsequent eruption of melted matter from the interior. Since in all cases the melting progresses outward from a centre, we see why it is that these large seas, like the smaller craters, all retain approximately the circular shape. In some cases where the original crust has subsided it has melted in the thinnest places only, such as the bottoms of the deeper craters. Thus Plato, Frontispiece [3.9, 1.4], probably had originally an interior like that of Copernicus [4.6, 3.3], but the melting process which destroyed the bottom was not carried far enough to ruin its walls also, as was partially done in the case of many of the older craters [5.6, 4.3]. The elevation of the surfaces of the maria probably indicates their relative age, the lower ones, since the interior of the Moon was constantly contracting, being formed last.
