A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 9 



instances are continued as branching cracks for a hundred miles or more. The 

 characteristic rills are far more abundant than the valleys, there being many 

 scores already described ; the slighter are evidently the more numerous ; a cata- 

 logue of those visible in the best telescopes would probably amount to several 

 thousand. (See plates xn, xxi, and xxn.) 



It is a noteworthy fact that in the case of the rills and in great measure also 

 in the valleys the two sides of the fissure correspond so that if brought together 

 the rent would be closed. This indicates that they are essentially cracks which 

 have opened by their walls drawing apart. Curiously enough, as compared with 

 rents in the earth's crust there is little trace of a change of level of the two sides 

 of these rills only in one instance is there such a displacement well made out, 

 that known as the Strait Wall, where one side of the break is several hundred 

 feet above the other. (See plate xxi.) 



In the region outside of the maria much of the general surface of the moon 

 between the numerous crater-like openings appears in the best seeing with power- 

 ful telescopes to be beset with minute pits, often so close together that their 

 limits are so far confused that it appears as honeycombed, or rather as a mass of 

 furnace slag full of holes if greatly magnified, through which the gases developed 

 in melting the mass escaped. (See plates ix, xm.) 



Perhaps the most exceptional feature of the lunar surface, as compared with 

 that of the earth, is found in the numerous systems of radiating light bands, in 

 all about thirty in number, which diverge from patches of the same hue about 

 certain of the crater-like pits. These bands of light-colored material are gen- 

 erally narrow, not more than a few miles in width ; they extend for great dis- 

 tances, certain of them being over a thousand miles in length, one of them 

 attaining to one thousand seven hundred miles in linear extent. In one instance 

 at least, in the crater named Saussure, a band which intersects the pit may be 

 seen crossing its floor, and less distinctly, yet clearly enough, it appears on the 

 steep inside walls of the cavity. In no well-observed case do these radiating 

 streaks of light-colored material coincide with the before-mentioned splits or 

 rifts. Yet the assemblage of facts, though the observations and the theories 

 based upon them are very discrepant, lead us to believe that they are in the 

 nature of stains or sheets of matter on the surface of the sphere, or perhaps in 

 the mass of the crust. At some points the rays of one system cross those of 

 another in a manner that indicates that the one is of later formation than the 

 other. (See plates vi, xvi, and xix.) 



Perhaps the most puzzling feature of the radiating streaks, where everything 

 is perplexing, is found in the way they come into view and disappear in each 

 lunar period. When the surface is illuminated by the very oblique rays of the 

 sun they are quite invisible ; as the lunar day advances they become faintly 

 discernible, but are only seen in perfect clearness near the full moon. The 

 reason for this peculiar appearance of these light bands under a high sun has 

 been a matter of much conjecture ; it is the subject of discussion in a later 

 chapter of this memoir, where it is shown that inasmuch as these bands appear 



