Fig. 7.
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.
Fig. 8.
Fig. 9.
A long-kept shrivelled apple affords an apt illustration of this wrinkle theory; another example may be observed in the human face and hand, when age has caused the flesh to shrink and so leave the comparatively unshrinking skin relatively too large as a covering for it. We illustrate both of these examples by actual photographs of the respective objects, which are reproduced on [Plate II]. Whenever an outer covering has to accommodate and apply itself to an interior body that has become too small for it, wrinkles are inevitably produced. The same action that shrivels the human skin into creases and wrinkles, has also shrivelled certain regions of the igneous crust of the earth. A map of a mountainous part of our globe affords abundant evidence of such a cause having been in action; such maps are pictures of wrinkles. Several parts of the lunar surface, as we shall by-and-by see, present us with the same appearances in a modified degree.
To the few primary causes we have set forth in this chapter—to the alternate expansion and contraction of successive strata of the lunar sphere, when in a state of transition from an igneous and molten to a cooled and solidified condition, we believe we shall be able to refer well nigh all the remarkable and characteristic features of the lunar surface which will come under our notice in the course of our survey.
PLATE II.
BACK of HAND & SHRIVELLED APPLE.
TO ILLUSTRATE the ORIGIN of CERTAIN MOUNTAIN RANGES BY SHRINKAGE of the GLOBE.