In seeking an explanation of these cracks, two possible causes suggest themselves. One is the expansion of subsurface matter, already suggested as explanatory of the bright streaks; the other, a contraction of the crust by cooling. We doubt not that both causes have been at work, one perhaps enhancing the other. Where, as in the cases we have pointed out, there are cracks which are so connected with craters as to imply relationship, we may conclude that an upheaving or expansive force in the sublunar molten matter has given rise to the cracks, and that the central craters have been formed simultaneously, by the release, with ejective violence, of the matter from its confining crust. The nature of the expansive force being assumed that of solidifying matter, the wide extent of some chasms indicates a deep location of that force. And depth in this matter implies lateness (in the scale of selenological time) of operation, since the central portions of the globe would be the last to cool. Now, we have evidence of comparative lateness afforded by the fact that in many cases the cracks have passed through craters and other asperities which thus obviously existed before the cracking commenced; and thus, so far, the hypothesis of the expansion-cracking is supported by absolute fact. It may be objected that such an upheaving force as we are invoking, being transitory, would allow the distended surface to collapse again when it ceased to operate, and so close the cracks or chasms it produced. But we consider it not improbable that in some cases, as a consequence of the expansion of subsurface matter, an upflow thereof may have partially filled the crack, and by solidifying have held it open; and it is rational to suppose that there have been various degrees of filling and even of overflow—that in some cases the rising matter has not nearly reached the edge of the crack, as in Fig. 44, while in others it has risen almost to the surface, and in some instances has actually overrun it and produced some sort of elevation along the line of the crack, like that represented sectionally in Fig. 45. It is probable that some of the slightly tumescent lines on the moon’s surface have been thus produced. [Illustration: PLATE XIX. GLASS GLOBE CRACKED BY INTERNAL PRESSURE.] [Illustration: Fig. 44.] [Illustration: Fig. 45.]