Under these circumstances we conceive the upcasting and excavating of a normal lunar crater to have been primarily caused by a local manifestation of the force of expansion upon solidification of the subsurface matter of the moon, resulting in the creation of a mere “star” or crack in and through the outermost and solid crust. As we shall have to rely upon diagrams to explain the more complicated features, we give one of this elementary stage also as a commencement of the series; and Fig. 20 therefore represents a probable section of the lunar surface at a point which was subsequently the location of a crater. From the vent thus formed we conceive the pent-up matter to have found its escape, not necessarily at a single outburst, but in all probability in a paroxysmal manner, as volcanic action manifests itself on our globe. The first outflow of molten material would probably produce no more than a mere hill or tumescence as shewn sectionally in Fig. 21; and if the ejective force were small this might increase to the magnitude of a mountain by an exudative process to be alluded to hereafter. But if the ejective force were violent, either at the moment of the first outburst or at any subsequent paroxysm, an action represented in Fig. 22 would result: the unsupported edges or lips of the vent-hole would be blown and ground or fluxed away, and a funnel-formed cavity would be produced, the ejected matter (so much of it as in falling was not caught by the funnel) being deposited around the hollow and forming an embryo circular mountain. The continuance of this action would be accompanied by an enlargement of the conical cavity or crater, not only by the outward rush of the violently discharged material, but also by the “sweating” or grinding action of such of it as in descending fell within the hollow. And at the same time that the crater enlarged the rampart would extend its circumference, for it would be formed of such material as did not fall back again into the crater. Upon this view of the crater-forming process we base the sketch, Fig. 23, of the probable section of a lunar crater at one period of its development. [Illustration: Fig. 22.]