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. Notwith- 

 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 the 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 Rhone 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 valley 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, difficulties 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 



