enormous magnifying power,
nothing could be more disappointing as regards the appearance of our
satellite. The sheen and lustre of the surface was now observed no
longer; the mountains and valleys, the circular ridges and hollows were,
indeed, wonderfully defined and magnified, but the matter of which they
seemed to be constituted resembled nothing so much as the pale plaster
of a model. One could thus fully realise the fact that the moon's light
is only derivative. Still we must recollect that the most powerful
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