The diameter of the ring is 60 miles, but the inclosure sinks only about half as deep beneath the crest of the wall, as is the rule with formations of similar outline. A central peak, a group of mountains, may be buried there. “It is within this ring of Plato that some of the strongest evidences of continued change, and possibly of continued life upon the moon, have been found. Prof. William H. Pickering, after long and careful studies of this remarkable plain, says of it: “‘Plato is, I believe, more active [in a volcanic sense] than any area of similar size upon the earth. There seems to be no evidence of lava, but the white streaks indicate apparently something analogous to snow or clouds. There must be a certain escape of gases, presumably steam and carbonic acid, the former of which probably aids in the production of the white markings.’ “The white marks to which Professor Pickering refers are but faintly indicated in the photograph before us, but with the telescope, when the illumination is favorable, they are plainly seen. There are a number of very small crater pits scattered over the floor of Plato, and around these changes of color occur which have been ascribed to the emission of some substance from the pits and to the presence of vegetation, nourished by the gases and vapors, and springing into renewed life every time the sun rises upon the plain. Broad areas of the inclosure gradually change color as the sun rises, and again as the sun sets, and these phenomena have also been ascribed to the presence of vegetation. You may, if you wish, regard Plato as a kind of mountain-ringed prairie, covered with something analogous to prairie grass and shrubs, which depends for its existence, partly, upon the supply of gases spreading over the surface from the crater pits.” “So this, then, is your ‘lunar grass’?” “Yes, but not all of it. Mark, I do not aver that it actually exists; I only say that it has been suspected to exist. On some of the _mares_ similar appearances are seen, as I have already told you, on a much more extensive scale, and I may again quote Professor Pickering, who says that some of his observations ‘point very strongly to the existence of vegetation upon the surface of the moon in large quantities at the present time.’” “Does this vegetation resemble that of the earth?” “I cannot tell you.” “But where vegetation exists animal life is possible, is it not?” “Yes, it is _possible_.” “What forms would it have?” “I cannot tell you. But I certainly should not expect to find manlike creatures there.” “Oh, men are not _necessary_ everywhere,” said my friend, laughing. “I am content if you admit that there may be living creatures of some kind. Henceforth I shall never forget Plato and the other places on the moon where such significant changes are seen.”