Page:The Moon (Pickering).djvu/55

Rh The characteristic difference between the maria and the larger craters, like Clavius, 10A [2.1, 7.4], Ptolemy, A [1.4, 2.3], and Schickard, 14E [1.9, 5.3], is that every crater is surrounded by a high, more or less continuous, mountain ridge or ring. In the case of the older and larger craters, when not too far destroyed by subsequent melting, the interior of the walls show a terraced structure due to the past tidal action. The maria, on the other hand, are usually surrounded by low shores. When they are high, as in the case of Crisium, lA [2.4, 6.7], and Humorum, 14A [1.7, 4.6], the shore is either a plateau or a broken series of mountain peaks, rather than a continuous ring. The northern boimdary of the Sinus Iridum, 11E [2.2, 2.4], is distinctly a plateau. The same was apparently true of the Apennines, 9A [2.0, 5.7], but the western side has sunk, leaving now a mountain range with very steep slopes on the east and a gradual decline on the western side. According to this view, therefore, the Apennines are merely the. curved edge left to the original solid surface of the Moon by the molten flood, which, welling up from the interior, melted and destroyed everything before it, as long as the supply of heat lasted. Portions of the original surface, only partially destroyed, are seen in the regions about Archimedes, 9A [2.1, 4.4], and Autolycus [1.5, 3.9].

A considerable number of large and medium-sized craters, such as Aristoteles, 5A [3.3, 2.7], and Hercules [1.4, 2.9], show a single, well-defined internal terrace. In order to exhibit it the crater must be rather deep. Copernicus, 11E [1.7, 6.2], shows traces of several terraces, while Tycho, 10A [2.2, 6.2], which in many respects resembles Copernicus, scarcely shows any at all. Among large craters the single terrace is well shown in the case of Clavius, 10A [2.1, 7.4].

As soon as the surfaces of the maria had solidified, fresh compression of their molten interiors began, and a second era of crater formation was inaugurated. These craters necessarily differed from those of the earlier period in one respect, however, and that was that while the walls of the earlier craters were light-coloured, those of the secondary period were of the same colour as the material of the surronding mare. Thus, of two craters located side by side in a mare, one may be light and the other dark. In such a case the former one survived the flood, the latter was formed subsequently to it. The craters of this second era are all of them of small dimensions. This secondary period seems to have extended to the present time, for, as we shall later see, small craters are even now being constantly produced, as in the case of the interior of Plato, 9A [2.3, 2.2]. Some craters, like Aristarchus, 13A [2.4, 5.3], and certain small craterlets, are intensely brilliant;