118 PHOTOGRAPHY OF THE LUNAR SURFACE. 



Although it is easy iudeed to find on the surface of the moon regions 

 similar in their aspects both to the low plains and the elevated plateaus 

 of the earth, the differences become accentuated on comparing the 

 mountainous regions, properly so called; that is to say, those in which 

 differences in level of several thousand meters appear short distances 

 apart. Wherever this condition is encountered on our globe it may be 

 observed that the relief of the surface has been totally transformed by 

 the action of water, and the inspection of sedimentary deposits shows 

 that our present mountain chains are but a small residue of the primi- 

 tive formations. Formations rising boldly above the general level 

 have undergone a progressive erosion, especially their contour, and the 

 direction of their greatest elongation is to-day marked by a crest or a 

 divide the more marked the higher the mountains. 



The causes which lead to the appearance of these divides also give 

 rise to the formation of secondary ridges. Between these ridges the 

 valleys appear, which deepen and enlarge gradually from their source 

 to their junction with the plains below. Some of them, deepened by 

 numerous rivers, conduct to a common destination all the waters 

 of a vast region, and not infrequently acquire important dimensions. 

 There is often a distance of 10 to 15 kilometers between the ridges on 

 either side. Vast basins would also be easily recognized under an 

 oblique illumination on the present scale of lunar photographs, and 

 we would be able to verify without any difficulty their general charac- 

 ter of convergence. 



The evidences which we to-day possess cause the lunar mountains 

 to appear in an entirely different aspect. Rarely is a sharply defined 

 watershed found, and even in this case it is easily recognized that it 

 is the edge of a plateau of which only one side shows a marked decliv- 

 ity extending for a considerable distance and marked differences in 

 level. From this standpoint, at least, we should have conditions most 

 favorable for indicating the action of flowing water. We find, however, 

 that there are no large basins formed such as abound among the high 

 mountains of our globe. The depressions there encountered are iso- 

 lated and closed from all sides. They show no progressive extensions, 

 no tendency to ramify toward the ridges or to converge in descending 

 toward the plains. 



These facts taken together appear to us irreconcilable with the idea 

 that there was ever an important circulation of water on the moon. 

 It is hardly necessary to point out that if there were any bodies of 

 water at present they would be revealed by their aspects and by their 

 power of reflecting solar light. 



X. — THE PRESENCE OF ICE ON THE SURFACE OF THE MOON. 



It might be thought that if water has not accomplished any impor 

 taut mechanical changes on our satellite this might be due to the rapid 

 cooling of the lunar globe, the water having thus passed in a short 



