124 FROM NEBULA TO NEBULA 



sary to sustain a taller column. As a matter of fact 

 the third block has much the broadest top of any, and 

 though still shorter than either the first or second must 

 in time, a* the result of future processes of evaporation 

 and snowfall grow to a great height, yet, being itself 

 limited also, can never reach the altitude of the encir- 

 cling rampart, which by now has risen to the imposing 

 altitude of possibly thirty feet, and will continue rising, 

 given the necessary supply of snow, until it telescopes 

 upon itself by its own weight. 



I hope the reader will understand that I have not 

 introducer! the stair as representing nature, or in order 

 to account for the "terraces." The terraces are actu- 

 ally produced by collapses of the top-heavy main walls 

 falling in on the lake margins, as these latter .gradu- 

 ally heroine exposed owing to the steady lowering of 

 the water level by evaporation. Eventually a small pit 

 is left that (father* water every recurring thaw, and this 

 'water it is which perpetually trims the bases of the 

 ramparts and keeps their faces 'sheer by constant wider - 

 mining, or "swilling." 



Of course natural lakes are not round like cisterns, 

 nor their brims even, and many have broad straits con- 

 necting them with other lakes, or with the ocean itself. 

 AH of these conditions are reflected in the moon's 

 glacial features. Several cases occur where craters 

 overlap, and in part obliterate the outlines of their 

 neighbors, a phenomenon which readily yields to the 

 supposition that one of the lakes is deeper than its com- 

 panion and goes on building after the water supply of 

 the first has been exhausted. 



Some may argue that if the moon be covered with 

 snow, its albedo should be much higher than the tests 

 show. If the snow surface stayed as flat as the planet's 

 natural one, the point were well taken. But the moun- 

 tains not only cast great shadows, but cause the solar 



