We thus see that the classes of features observable upon the moon are not great in number: they may be summed up as _craters_ and their central cones, _mountain chains_, with occasional isolated peaks, _smooth plains_, with more or less of irregularity of surface, and _bright radiating streaks_. But when we come to study with higher powers the individual examples of each class we meet with considerable diversity. This is especially the case with the craters, which appear under very numerous variations of the one order of structure, viz., the ring-form. A higher telescopic power shows us that not only do these craters exist of all magnitudes within a limit of largeness, but seemingly with no limit of smallness, but that in their structure and arrangement they present a great variety of points of difference. Some are seen to be considerably elevated above the surrounding surface, others are basins hollowed out of that surface and with low surrounding ramparts; some are merely like walled plains or amphitheatres with flat plateaux, while the majority have their lowest point of hollowness considerably below the general level of the surrounding surface; some are isolated upon the plains, others are aggregated into a thick crowd, and overlapping and intruding upon each other; some have elevated peaks or cones in their centres, and some are without these central cones, while the plateaux of others again contain several minute craters instead; some have their ramparts whole and perfect, others have them breached or malformed, and many have them divided into terraces, especially on their inner sides. In the plains, what with a low power appeared smooth as a water surface becomes, under greater magnification, a rough and furrowed area, here gently undulated and there broken into ridges and declivities, with now and then deep rents or cracks extending for miles and spreading like river-beds into numerous ramifications. Craters of all sizes and classes are scattered over the plains; these appear generally of a different tint to the surrounding surface, for the light reflected from the plains has been observed to be slightly tinged with colour, The tint is not the same in all cases: one large sea has a dingy greenish tinge, others are merely grey and some others present a pale reddish hue. The cause of this diversity of colour is mysterious; it has been supposed to indicate the existence of vegetation of some sort; but this involves conditions that we know do not exist.