[Illustration: Fig. 5. A A. The solidified crust cooling, contracting, and cracking; the cracking action enhanced by the expansion of the substratum of molten matter, B B B, which, expanding as it approaches the point of solidification, injects portions of the molten matter up through the contractile cracks, and results in producing craters, mountains of exudation, and districts flooded with extruded lava, C C C. The nucleus of intensely hot molten matter.]
That an upheaving and ejective force has been in action with varying intensity beneath the whole of the lunar surface is manifest from the aspect of its structural details, and we are impressed with the conviction that the principle we have set forth, namely the paroxysms of expansion which successively occurred as portions of its molten interior approached solidification, supply us with a rational cause to which such vast ejective and upheaving phenomena may be assigned. Many features of terrestrial geology likewise require such an expansive force whereby to explain them; we therefore venture to recommend this source and cause of ejective action to the careful consideration of geologists.
When the molten substratum had burst its confines, ejected its superfluous matter, and produced the resulting volcanic features, it would, after final solidification, resume the normal process of contraction upon cooling, and so retreat or shrink away from the external shell. Let us now consider what would be the result of this. Evidently the external shell or crust would become relatively too large to remain at all points in close contact with the subjacent matter. The consequence of too large a solid shell having to accommodate itself to a shrunken body underneath, is that the skin, so to term the outer stratum of solid matter, becomes shrivelled up into alternate ridges and depressions, or wrinkles. In its attempt to crush down and follow the contracting substratum, it would have to displace the superabundant or superfluous material of its former larger surface by thrusting it (by the action of tangential force) into undulating ridges as in Fig. 6, or broken elevated ridges as in Fig. 7, or overlappings of the outer crust as in Fig. 8, or ridges capped by more or less fluid molten matter extruded from beneath, as indicated in Fig. 9, a class of action which might occur contemporaneously with the elevation of the ridge or subsequently to its formation.