I would, in conclusion, invite those who have the requisite leisure to a careful study of the distribution of various orders of lunar marking. It would be well if the moon's surface were isographically charted, and the distribution of the seas, mountain ranges, and craters of different dimensions and character, of rills, radiating streaks, bright and dark regions, and so on, carefully compared _inter se_, with the object of determining whether the different parts of the moon's surface were probably brought to their present condition during earlier or later periods, and of interpreting also the significance of the moon's characteristic peculiarities. In this department of astronomy, as in some others, the effectiveness of well-devised processes of charting has been hitherto overlooked. FOOTNOTES: [Footnote 8: It would still be somewhat denser, because under the circumstances it would be somewhat cooler.] [Footnote 9: It is thus, and not by the effects due to increasing pressure (effects which probably do not increase beyond a certain point), that we are to explain the fact that the earth's density as a whole is about twice the mean density of the matters which form its solid surface. It may be that this consideration, supported by the results of recent experimental researches, may give a significance hitherto not noted to the relatively small mean density of the moon.] [Footnote 10: I have occasion to make some remarks at this stage to avoid possible and (my experience has shown me) not altogether improbable misconception, or even misrepresentation. The theory enunciated above will be regarded by some, who may have read a certain review of my Treatise on the Moon, as totally different from what I have advocated in that work, and, furthermore, as a theory which I have borrowed from the aforesaid review. I should not be particularly concerned if I had occasion to modify views I had formerly expressed, since I apprehend that every active student of science should hope, rather than dread, that as his work proceeds he would form new opinions. But I must point out that earlier in my book I had advocated the theory urged above. After describing the radiations from Tycho and other craters, I proceed as follows in chapter iv.--'It appears to me impossible to refer these phenomena to any general cause but the reaction of the moon's interior overcoming the tension of the crust, and to this degree Nasmyth's theory seems correct; but it appears manifest, also, that the crust cannot have been fractured in the ordinary sense of the word. Since, however, it results from Mallet's investigations that the tension of the crust is called into play in the earlier stages of contraction, and its power to resist contraction in the later stages,--in other words, since the crust at first contracts faster than the nucleus, and afterwards not so fast as the nucleus,--we may assume that the radiating systems were formed in so early an era that the crust was plastic.