Look again at the border of the “Sea of Serenity”—what a name for such a scene!—and observe how it has been rent with almost inconceivable violence, the wall of the colossal crater Posidonius dropping vertically upon the ancient shore and obliterating it, while its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow the sea itself. A scene like this makes one question whether, after all, those may not be right who have imagined that the so-called sea bottoms are really vast plains of frozen lava which gushed up in floods so extensive that even the mighty volcanoes were half drowned in the fiery sea. This suggestion becomes even stronger when we turn to another of the photographs of Mr Ritchey’s wonderful series, showing a part of the _Mare Tranquilitatis_ (“Sea of Tranquility”!). Notice how near the center of the picture the outline of a huge ring with radiating ridges shows through the sea bottom; a fossil volcano submerged in a petrified ocean! This is by no means the only instance in which a buried world shows itself under the great lunar plains. Yet, as the newer craters in the sea itself prove, the volcanic activity survived this other catastrophe, or broke out again subsequently, bringing more ruin to pile upon ruin. Yet notwithstanding the evidence which we have just been considering in support of the hypothesis that the “seas” are lava floods, Messrs. Loewy and Puiseux, the selenographers of the Paris Observatory, are convinced that these great plains bear characteristic marks of the former presence of immense bodies of water. In that case we should be forced to conclude that the later oceans of the moon lay upon vast sheets of solidified lava; and thus the catastrophe of the lunar world assumes a double aspect, the earliest oceans being swallowed up in molten floods issuing from the interior, while the lands were reduced to chaos by a universal eruption of tremendous volcanoes; and then a period of comparative quiet followed, during which new seas were formed, and new life perhaps began to flourish in the lunar world, only to end in another cataclysm, which finally put a term to the existence of the moon as a life-supporting world.