The study of the varying brightness or reflective power of different regions and spots of the lunar disc leads us to the consideration of the relative antiquity of the surface features; for it is hardly possible to regard these variations attentively without being impressed with the conviction that they have relation to some chronological order of formation. We cannot, in the first place, resist the conviction that the brightest features were the latest formed; this strikes us as evident on _primâ facie_ grounds; but it becomes more clearly so when we remark that the bright formations, as a rule, overlie the duller features. The elevated parts of the crust are brighter than the “seas” and other areas; and it is pretty clear that the former are newer than the latter, upon which they appear to be super-imposed, or through which they seem to have extruded.[13] The vast dusky plains are in every instance more or less sprinkled with spots and minute craters, and these last were obviously formed after the area that contains them. One is almost disposed to place the order of formations in the order of relative brightness, and so consider the dingiest parts the oldest and the brightest spots and craters the newest features, though, in the absence of an atmosphere competent to impair the reflective power of the surface materials, we are unable to justify this classification by suggesting a cause for such a deterioration by time as the hypothesis pre-supposes. As we have entered upon the question of relative age of the lunar features, we may remark that there are evidences of various epochs of formation of particular classes of details, irrespective of their condition in respect of brightness, or, as we may say, freshness of material. As a rule, the large craters are older than the small ones. This is proved by the fact that a large object of this class is never seen to interfere with or overlap a small one. Those of nearly equal size are, however, seen to overlap one another as though several eruptions of equal intensity had occurred from the same source at different points. This is strikingly instanced in the group of craters situated in the position 35-141 on our map, the order of formation of each of which is clearly apparent. The region about Tycho offers an inexhaustible field for study of these phenomena of overlapping or interpolating craters, and it will be found, with very few exceptions, that the smaller crater is the impinging or parasitical one, and must therefore have been formed after the larger, upon which it intrudes or impinges. There are frequent cases in which a large crater has had its rampart interrupted by a lesser one, and this again has been broken into by one still smaller; and instances may be found where a fourth crater smaller than all has intruded itself upon the previous intruder. The general tendency of these examples is to show that the craters diminished in size as the moon’s volcanic energy subsided: that the