A COMPARISON' OF THE FEATURES OF THE EARTH AND THE MOON. 65 



many structures, such as the cracks, displaced faults, and the smaller vulcanoids, 

 all of which must, on any apparently valid supposition as to the moon's history, 

 be many million years old, we are led to believe this view inadmissible. 



In this connection attention is due also to the fact that on the unilluminated 

 part of the moon various observers have, from time to time, noted patches of 

 light which they have believed to indicate volcanoes in activity. I have elsewhere 

 suggested (see p. 53) that these objects may have been highly reflecting parts 

 of the lunar surface illuminated by the earth-shine. It is barely possible, how- 

 ever, that in some instances they can be explained on the supposition that con- 

 siderable meteorites had recently fallen at the point where the light was noted. 

 So also it seems possible that the vapors which W. H. Pickering and others 

 have thought they observed floating in the manner of clouds on the illuminated 

 area may be in this way accounted for : a large meteorite penetrating deeply into 

 the crust might give rise to vapors which would continue to pour forth for months 

 or years after it fell. The difficulty with this hypothesis is to see how vapors 

 could ^i?^/' and remain in the form of a cloud in the conditions of essential vacuum 

 which e.xist on the surface of the moon. Granting the possibility of such action, 

 which in the present state of our knowledge seems improbable, I should much 

 prefer to account for these vapors by meteoric action than to seek their explana- 

 tion in true volcanic activity. 



EROSIVE ACTION ON THE LUNAR SURFACE. 



Those who are familiar with the lunar surface as it is exhibited by a good 

 telescope, cannot help acquiring the impression that there is some agent which 

 has operated on the moon in a way partly to break down the more ancient topo- 

 graphical features. There is an evident difference of aspect between the walls of 

 the older vulcanoids and those of newer formation. Apart from the distortions 

 of the ancient structures and the breaches of their ramparts, which may be fairly 

 accounted for in other ways, there are a rounding of their steeps and a general ap- 

 pearance of having been smoothed over by some erosive agency which are evident 

 in proportion to their antiquity. It is indeed a general fact which has been re- 

 marked by many observers, that the newer vulcanoids have an appearance of 

 freshness that is never found in the earliest formed. It is therefore important 

 to discover, if we may, what are the actions by which such changes may be 

 brought about. 



On the surface of the earth there are four agents of erosion, all of which, 

 cooperating with gravitation, serve to bring about more or less considerable 

 changes. These are chemical alterations, which loosen the structure of rocks ; 

 the direct action of the wind, which removes their lighter particles when they are 

 not protected by vegetation ; the action of moving water by waves, streams, and 

 glaciers ; and last, and by far the least, the expansion and contraction of materials 

 arising from changes of temperature. The essential effect of all these agents is 

 to deliver fragments of rocks to the more or less free action of gravitation. They 



