The actual process by which a lunar crater would be formed would differ from that pertaining to a terrestrial crater only to the extent of the different conditions of the two globes. We can scarcely accept Scrope’s term “basal wrecks” (of volcanic mountains that have had the summits blown away) as applicable to the craters of the moon, for the reason that the lunar globe does not offer us any instance of a mountain comparable in extent to the great craters and whose summit has _not_ been blown away. Scrope’s definition implies a double, or divided process of formation: first the building up of a vast conical hill and then the decapitation and “evisceration” of it at some later period. There are grounds for this inferred double action among the terrestrial volcanoes, since both the perfect cone and its summitless counterpart are numerously exemplified. But upon the moon we have no perfect cone of great size, we have no exception whereby the rule can be proved. It is against probability, supposing every lunar crater to have once been a mountain, that in every case the mountain’s summit should have been blown away; and we are therefore compelled to consider that the moon’s volcanic craters were formed by one continuous outburst, and that their “evisceration” was a part of the original formative process. We do not, however, include the central cone in this consideration: that may be reasonably ascribed to a secondary action or perhaps, better, to a weaker or modified phase of the original and only eruption. [Illustration: Fig. 20.] [Illustration: Fig. 21.] [Illustration: PLATE IX. THE LUNAR APENNINES, ARCHIMEDES &c., &c.]