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60 at one time, they might have been very numerous. Successive upheavals, however, and various denuding influences have obliterated the distinctive features, and it is only, in a few cases, that we can trace the typical form. Monte Venere, at Rome, presents a very fair specimen, being the central cone of a large crater.

As in the morphology of plants, we detect, amidst diversity, the same typical form of the leaf, so Tve find in the moon endless repetitions of the typical crater with the central cone. There are, for example, walled plains of vast extent, some of them being as much as 150 miles in diameter. They differ from the typical crater only in this, that the enclosed part is a plain, instead of a concavity, and that there is no central peak. Again, extensive ranges of mountains assume a semicircular form, and when the vast dark plains were regarded as seas, these semicircular forms were called bays or gulfs. But this semicircular form evidently points to the circular typical form. Again, some of the craters are without any walls or rims, and, in others, the floor of the crater is convex, though, in all cases, it is sunk below the level of the surrounding country. But it is not merely the more prominent features that conform to the crater type. On minute inspection, we find that the whole surface has a crateriform structure. When you take a large crystal of calc spar, and break it into numerous pieces, you find that the large rhomboid is made up of innumerable