x THE STARRY HEAVENS 381 



of our mountain chains. Yet no one can look 

 at a map of the Moon without being struck 

 with the very rugged character of its moun- 

 tain scenery. This is mainly due to the 

 absence of air and water. To these two 

 mighty agencies, not merely " the cloud- 

 capped towers, the gorgeous palaces, the 

 solemn temples," but the very mountains 

 themselves, are inevitable victims. Not 

 merely storms and hurricanes, but every 

 gentle shower, every fall of snow, tends to 

 soften our scenery and lower the mountain 

 peaks. These agencies are absent from the 

 Moon, and the mountains stand to-day just 

 as they were formed millions of years ago. 



But though we find on our own globe (see, 

 for instance, Fig. 21) volcanic regions closely 

 resembling those of the Moon, there are other 

 phenomena on the Moon's surface for which 

 our earth presents as yet no explanation. 

 From Tycho, for instance, a crater 17,000 

 feet high and 50 miles across, a number of 

 rays or streaks diverge, which for hundreds, 

 or in some cases two or three thousand, miles 

 pass straight across plains, craters, and moun- 



