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34 but this is an extraneous circumstance, to be presently described, having no bearing on the age of the crater. It is not probable that new craters would be formed anywhere but in the dark areas of the Moon.

The hypothesis is frequently maintained that the maria were originally covered with water. There seems to me no sufficient evidence to be found in support of such a conclusion. It is probable that the lunar atmosphere was never very dense, perhaps never exceeding one inch in pressure. Under these circumstances the evaporation from any large body of water exposed to the Stm's rays would be very rapid, and the condensation at night equally so. This more than tropical precipitation could hardly have failed to leave very conspicuous evidence of its former existence in denuded slopes and deep ravines. Moreover, the maria, although frequently in communication, are placed, as just noted, at very different levels. As the water gradually dried up, or was absorbed, and the upper seas emptied into the lower ones, channels would have been cut connecting them. Nothing of the sort is seen, however, although these would be the most favourable regions in which to observe them. In short, the evidence of the action of water upon the Moon is certainly very much less marked than upon the Earth, and seems quite inadequate to support the hypothesis that the maria were ever flooded.

The Itmar craters may be divided according to their appearance into six classes, distributed in two periods. Craters of the first period. These all have bright walls. (a) Unaltered craters. These have high, sharp walls with rough interiors. Copernicus, 11A [2.5, 6.9], Tycho, 10E [2.1, 6.6], Arzachel[1.3, 3.9]. (b) Partially submerged craters. These have sharp walls but smooth interiors, which may be either dark or light. Plato, 9A [2.3, 2.2], Archimedes [2.1, 4.5], Ptolemy, 10E [1.3, 2.9], Schickard, 14B [1.9, 5.5]. (c) Softened craters. The whole crater floor and walls seem to have been softened and flattened. Posidonius, 5A [1.6, 4.5], Cassini, 7A [2.2, 3.5], Gassendi, 14A [1.9, 3.9]. (d) Submerged craters. Only portions of the rims of these craters show above the surface of the maria. Large nameless crater near Flamsteed, 13E [2.8, 8;4]. Numerous craters in nearly all the maria. Craters of the second period. All craters having dark walls belong to this class, (e) Dark craters shaped like Copernicus, but on a much smaller scale, rising in several of the maria. Lambert, 11E [1.6, 4.3], and other craters in Mare Imbrium. (f) Craters with smooth floors and walls, the interiors being much deeper than the exteriors. Most of the smaller craterlets found in the maria belong to this type. Helicon, 11E [1.6, 2.8], and Bessel, 5E [2.6, 4.8], are among the larger examples.