[Illustration: PLATE I. CRATER OF VESUVIUS. 1865.]
Let us now consider what would be the effect produced upon a spherical mass of molten matter in progress of cooling, first under the action of the above described expansion which precedes solidification, and then by the contraction which accompanies the cooling of a solidified body. The first portion of such a mass to part with its heat being its external surface, this portion would expand, but there being no obstacle to resist the expansion there would be no other result than a temporary slight enlargement of the sphere. This external portion would on cooling form a solid shell encompassing a more or less fluid molten nucleus, but as this interior has in its turn, on approaching the point of solidification, to expand also, and there being, so to speak, no room for its expansion, by reason of its confinement within its solid casing, what would be the consequence?—the shell would be rent or burst open, and a portion of the molten interior ejected with more or less violence according to circumstances, and many of the characteristic features of volcanic action would be thus produced: the thickness of the outer shell, the size of the vent made by the expanding matter for its escape, and other conditions conspiring to modify the nature and extent of the eruption. Thus there would result vast floodings of the exterior surface of the shell by the so extruded molten matter, volcanoes, extruded mountains, and other manifestations of eruptive phenomena. The sectional diagram (Fig. 5) will help to convey a clear idea of this action. Basing our reasoning on the principle we have thus enunciated, namely, that molten telluric matter expands on nearing the point of solidification, and which we have endeavoured to illustrate by reference to actual examples of its operation, we consider we are justified in assuming that such a course of volcanic phenomena has very probably occurred again and again upon the moon; that this expansion of volume which accompanies the solidification of molten matter furnishes a key to the solution of the enigma of volcanic action; and that such theories as depend upon the agency of gases, vapour, or water are at all events untenable with regard to the moon, where no gases, vapour, or water, appear to exist.