332 FROM NEBULA TO NEBULA 



to be distinguishable; in some the enclosed plain is as 

 level as a floor, in others it is studded with cones whose 

 peaks rise as high as the rampart itself ; in some the wall 

 is practically complete and of fairly uniform height, 

 while in the case of a near neighbor the very reverse may 

 be the case. In those craters where the walls are very 

 tall, they are terraced on the inside, while, in those whose 

 ramparts are low, or of moderate altitude, the inside wall 

 is usually sharply precipitous. There are not a few in- 

 stances where two, or even several, crater walls impinge, 

 and one appears to mount higher on the ruins of the 

 other or others. On the other hand, there are some iso- 

 lated craters that have great breaches in their bastions, 

 breaches that cleave clear down to the ground and con- 

 tinue thence as canyons for scores and hundreds of miles 

 through a most rugged country, not following the valleys, 

 mark you, but straight through the highest mountains. 



Yet, though all the foregoing enumerated details are 

 severally of great importance, and should be rehearsed 

 by the conscientious student with attentive care, the two 

 most significant, because explicatory, facts about these 

 craters are, first, that the deeper the crater the more 

 does the bottom of its pit extend down below the general 

 level of the surface, much as though some giant had dug 

 out a well and piled the excavated earth around the brim, 

 thereby making the shaft doubly deep ; and, second, that, 

 steep as may be the inner wall, the outer invariably slopes 

 gently away to nothing, unless some obviously independ- 

 ent formation rises to obstruct its normal trend. 



WHITE RAYS. "The most puzzling feature of the 

 surf ace ", says Professor E. W. Brown ( Americana) f 

 " consists in a series of white rays or streaks which radi- 

 ate from a few of the principal craters in every direction. 

 In their brightness they mask all other shades of tint on 

 the surface and seem to continue their course, sometimes 

 for hundreds of miles, quite independently of the nature 

 of the country they cross. Professor W. H. Pickering, 

 however, who has studied the systems carefully, con- 

 siders that their actual length has been much exag- 



