telescope can only bring the surface of the moon to a distance from us of about 250 miles; and it need not be said that objects seen at such a distance on our earth present very deceptive appearances; so that we gain little information regarding the composition of the moon's crust, or exterior surface, simply from observation by the aid of large telescopes. Reasoning from analogy with our globe, we may infer that the exterior shell of the moon consists of crystalline volcanic matter of the highly silicated, or acid, varieties resting upon another of a denser description, rich in iron, and resembling basalt. This hypothesis is hazarded on the supposition that the composition of the matter of the moon's mass resembles in the main that of our globe. During the process of cooling from a molten condition, the heavier lavas would tend to fall inwards, and allow the lighter to come to the surface, and form the outer shell in both cases. Thus, the outer crust would resemble the trachytic lavas of our globe, and their pale colour would enable the sun's rays to be reflected to a greater extent than if the material were of the blackness of basalt.[4] So much for the material. We have now to consider the structure of the moon's surface, and here we find ourselves treading on less speculative and safer ground. All astronomers since the time of Schroter seem to be of accord in the opinion that the remarkable features of the moon's surface are in some measure of volcanic origin, and we shall presently proceed to consider the character of these forms more in detail. But first, and as leading up to the discussion of these physical features, we must notice one essential difference between the constitution of the moon and of the earth; namely, the absence of water and of an atmosphere in the case of the moon. The sudden and complete occultation of the stars when the moon's disk passes between them and the place of the observer on the earth's surface, is sufficient evidence of the absence of air; and, as no cloud has ever been noticed to veil even for a moment any part of our satellite's face, we are pretty safe in concluding that there is no water; or at least, if there be any, that it is inappreciable in quantity.[5] Hence we infer that there is no animal or vegetable life on the moon's surface; neither are there oceans, lakes or rivers, snowfields or glaciers, river-valleys or cañons, islands, stratified rocks, nor volcanoes of the kind most prevalent on our own globe. [Illustration: Fig. 38.--Photograph of the moon's surface (in part) showing the illuminated "spots," and ridges, and the deep hollows. The position of "Tycho" is shown near the upper edge, and some of the volcanic craters are very clearly seen near the margin.]