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) voleanic 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- 
