io6 ROUGH WAYS MADE SMOOTH. 



the greater part of the heat to which volcanic phenomena 

 are due. If this is so in the earth's case at present, how 

 tremendous must have been the heat evolved by the far 

 more rapid contraction of the moon's mass in the remote 

 era we are considering, when probably her heat passed into 

 space unchecked by the action of a dense moisture-laden 

 atmosphere ! We can well understand that enormous 

 volumes of heated gas would be formed including steam, 

 for there is good reason to believe that water is present in 

 large quantities in the moon's interior. The imprisoned gas 

 would find an outlet at points of least resistance, the centres, 

 namely, of the great radiating systems of streaks. These 

 centres would certainly be regions of outlet But they would 

 not be sufficient. We can understand then why every ray 

 system extends from a great crater, though that crater was 

 really formed after the system of radiating streaks ; and we 

 can equally understand why these central craters are not 

 the only or even the chief of the great craters in the moon. 

 Here again I would suggest that possibly the careful study 

 of American geology might disclose features illustrating the 

 great lunar craters. 



When we pass to the smaller craters, ranging in diameter 

 from seven or eight miles to less than a quarter of a mile, 

 even if there be not some far smaller and beyond the range 

 of the most powerful telescopes man can construct, we find 

 ourselves among objects resembling those with which the 

 study of our own earth has rendered us familiar. When 

 Sartorius's map of Etna and the surrounding region was first 

 seen at the Geological Society's rooms, many supposed that 

 it represented lunar features. The Vesuvian volcanic region, 

 again, is presented side by side with a lunar region of similar 

 extent in Nasmyth's fine treatise on the moon, and the re- 

 semblance is very close. Considering the part which water 

 plays in producing terrestrial volcanic phenomena, it may 

 reasonably be doubted whether there is in reality so close a 

 resemblance as a superficial comparison (and we can make 

 no other) would suggest There are those, indeed, who be- 



