THE MOON 391 



upon whether they were originally submerged, and if so 

 to what depth. 



Just as there may be peaks within craters so there 

 may be craters within craters. The difference consists in 

 this, that whereas the peaks are solid columns mounted 

 on islands from snow supplied by the vapors risen from 

 surrounding pools, the craters are hollow chimneys 

 grounded upon the margins of enclosed wells sunken into 

 the floor of the main basin. That is to say, some pools, 

 both large and small, when baled out, exhibit not only 

 protuberances on their floors, but also pit holes that still 

 remain filled with water, and these sub-pools by going 

 through the same modus operandi as did the original one 

 give birth to sub-craters. In fine, every dent in the 

 moon's surface that normally would be filled with water 

 becomes a crater by the conversion of its contents into 

 snowflakes and the settling of the latter on the brims. 



The very largest craters on the moon are, literally, 

 the maria themselves, for here was the chief source of the 

 snow supply. Around their borders on all sides, there- 

 fore, stand high snow cliffs, some of them very precipi- 

 tous ; while at other places there are terraces, or perhaps 

 gentle slopes masking the elevated character of the mesas 

 or plateaus at their backs. These differences are due to 

 the varying natural depth of the ocean bed at the shore 

 line. If the offset is abrupt, the gathering water from 

 the summer thaws washes and undermines the base of the 

 glacial cliffs, keeping them sharply trimmed, whereas if 

 there is a natural beach, a different order prevails. 



When snow is piled too high, its own weight causes 

 it to collapse or telescope upon itself, resulting in two 

 things ; first, the top layer flattens out and overhangs the 

 base, forming a lip, which in time loses its balance and 

 falls, forming a terrace or talus; this event being per- 

 haps followed by another similar avalanche farther on, 

 provided the pile be sufficiently deep ; and, secondly, the 

 increased weight finally reaches an extreme limit, where 

 It liquefies and squashes out the undermost layer into 

 the flanking pools. These two processes, major though 



