PLANETARY EVOLUTION 



from the lunar surface was long since proved by 

 the fact that " when stars are occulted by the 

 moon, they disappear instantaneously," — which 

 would not be the case had the moon an appre- 

 ciable atmosphere ; and spectroscopic evidence 

 has confirmed this conclusion. Nor are there 

 any signs of the presence of liquid oceans, or 

 of running water. Yet if the moon was origi- 

 nally formed from an equatorial zone of the 

 earth, it would seem that it ought to contain 

 the same materials which have from the oldest 

 times constituted a considerable part of the ter- 

 restrial surface. Besides this, the vast plains on 

 the moon which the old astronomers supposed 

 to be seas, and named as such, are now held to 

 be areas underlaid by sedimentary rocks im- 

 plying the former presence of water. 1 If this 

 view be correct, there must in all probability 

 have been winds to excite the erosive move- 

 ments of the water which caused this sedimen- 

 tation. For tidal action upon the moon cannot 

 be regarded as a considerable factor in the ero- 

 sion, unless we go back to that enormously re- 

 mote period when the earth's tidal pull was still 

 employed in dragging the moon's rotation into 

 synchrony with its revolution. 



1 Moreover, "it is not to be forgotten that, so far as ter- 

 restrial experience is concerned, water is absolutely essential 

 to the occurrence of volcanic action." Proctor, The Moon, 

 P- 353- 



283 



