ea. v.] PLANETARY EVOLUTION. 379 



occulted by the moon, they disappear instantaneously," — 

 which would not be the case had the moon an appreciable 

 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 

 originally 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 terrestrial 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 implying the former presence of water. 1 

 If this view be correct, there must in all probability have 

 been winds to excite the erosive movements of the water 

 which caused this sedimentation. For tidal action upon the 

 moon cannot be regarded as a considerable factor in the 

 erosion, unless we go back to that enormously remote period 

 when the earth's tidal pull was still employed in dragging 

 the moon's rotation into synchrony with its revolution. 



Here there is an apparent discrepancy, which will dis- 

 appear, however, when we inquire further into the past 

 career of the moon as indicated by the present condition of 

 its surface. To a great extent the lunar surface is made up 

 of huge masses of igneous rock, through which at short 

 intervals yawn enormous volcanic craters, whose fires seem 

 to be totally extinguished. The giant forces required to 

 bring about such a state of things are now quiescent. And 

 this implies that the moon is a dead planet. It implies that 

 the thermal energies which were once instrumental in raising 

 those huge cones, Tycho, Copernicus, and the rest — quaintly 

 named after our terrestrial heroes of science — and which once 

 drove up fiery streams of molten lava through their ample 



1 Moreover, " it is not to be forgotten that, so far as terrestrial experience 

 is concerned, water is absolutely essential to the occurrence of volaaDic 

 action." Proctor, The Moon, p. 353. 



