112 
THE GEOLOGIST. 
bodies belonging to our solar system have either already passed through a 
similar epoch, or are destined still to encounter it. With the exception of 
the polar ice of Mars, we have hitherto obtained no certain glimpse into the 
thermal or meteorological condition of the planets ; neither is the physical 
state of their surfaces accessible to our best telescopes. It is otherwise how- 
ever with the moon, whose distance is not too great to prevent the visibility 
of comparatively minute details, A careful observation of the lunar surface 
for more than a year with a silvered-glass reflector of 7 inches' aperture 
and of good defining power, had created in the speaker's mind an impres- 
sion that our satellite had, like its primary, also passed through a glacial 
epoch, and that several, at least, of the valleys, rills, and streaks of the 
lunar surface were not improbably due to former glacial action. jSTotwith- 
standing the excellent definition of modern telescopes, it could not be ex- 
pected that other than the most gigantic of the characteristic details of an 
ancient glacier-bed would be rendered visible. Under favourable circum- 
stances the terminal moraine of a glacier attains to enormous dimensions ; 
and, consequently, of all tlie marks of a glacial valley, this would be the 
one most likely to be first perceived. Two such terminal moraines, one of 
them a double one, appeared to him to be traceable upon the moon's sur- 
face. The first was situated near the termination of that remarkable 
streak which commences near the base of Tycho, and passing under the 
south-eastern wall of Bullialdus, into the ring of which it appears to cut, 
is gradually lost after passing crater 216 (Lubinietzky). Exactly opposite 
the last crater, and extending nearly across the streak in question, are two 
ridges forming the arcs of circles, whose centres are not coincident, and 
whose external curvature is towards the north. Beyond the second ridge 
a talus slopes gradually down northwards to the general level of the lunar 
surface, the whole presenting an appearance reminding the observer of the 
concentric moraines of the Hhone glacier. These ridges are visible for 
the whole period during which that portion of the moon's surface is illu- 
minated, but it is only about the third day after the first quarter and at 
the corresponding phase of the waning moon (when the sun's rays, fall- 
ing nearly horizontally, throw the details of this part of the surface into 
strong relief) that these appearances suggest the explanation now offered. 
The other ridge, answering to a terminal moraine, occurs at the northern 
extremity of that magnificent valky which runs past the eastern edge of 
Rheita, This ridge is nearly semicircular, and is considerably elevated, 
both above the northern termination of the valley and the general sur- 
face of the moon. It may be seen about four days after new and full 
moon, but the position of the observer, with regard to the lights and 
shadows, renders its appearance in the rays of the rising sun by far the 
most striking. 
With regard to the probability of former glacial, or even aqueous, 
agency on the surface of the moon, difiiculties of an apparently very for- 
midable character present themselves. There is not only now no evidence 
whatever of the presence of water, in any one of its three forms, at the 
lunar surface ; but, on the contrary, all seleniographic observations tend 
to prove its absence. Nevertheless, the idea of former aqueous agency in 
the moon is by no means new. It was entertained by Grruithuisen and 
others. But if water at one time existed on the surface of the moon, 
whither has it disappeared ? If we assume, in accordance with the ne- 
bular hypothesis, that the portions of matter composing respectively the 
earth and the moon once possessed an equally elevated temperature, it 
almost necessarily follows that the moon, owing to the comparative small- 
ness of its mass, would cool much more rapidly than the earth ; for whilst 
