lO A fOMrARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



when the earth light falls upon the moon at a high angle, the effect must be due 

 to the angle of incidence of the rays on the shining surfaces. It should be noted 

 that the light bands in most instances diverge from more or less broad fields of 

 light color about the crater-like pits, fields which have the same habit of glowing 

 under a high illumination ; in fact, a large part of the surface of the moon, per- 

 haps near one-tenth of its visible area, becomes thus brilliant at full moon, 

 though it lacks that quality at the earlier and later stages of the lunar day. 



In the above considered statement concerning the visible phenomena of the 

 moon no account is taken of a great variety of obscure features which, though 

 easily seen with fairly good instruments, have received slight attention from 

 selenographers. As can readily be imagined, observers find it difficult to discern 

 obscure features which cannot be classed in any group of terrestrial objects. 

 Whosoever will narrowly inspect any part of the lunar surface, noting every- 

 thing that meets his eye, will find that he observes much that cannot be 

 explained by what is seen on the earth. It is evident, indeed, that while in the 

 earlier stages of development this satellite in good part followed the series of 

 changes undergone by its planet, there came a stage in which it ceased to con- 

 tinue the process of evolution that the parent body has undergone ; the reason 

 for this arrest in development appears to have been the essential if not complete 

 absence of an atmosphere and of water. 



The difference in height between the lowest and highest points on the lunar 

 surface is not determined. To the most accented reliefs, those of the higher 

 crater walls, elevations of more than twenty-five thousand feet have been assigned ; 

 it is, however, to be noted that all these determinations are made from the length 

 of the shadows cast by the eminences, with no effective means of correcting for 

 certain errors incidental to this method. It may be assumed as tolerably certain 

 that a number of these elevations have their summits at least twenty thousand 

 feet above their bases and that a few are yet higher. We do not know how 

 much lower than the ground about these elevations are the lowest parts of 

 the moon. My own observations incline me to the opinion that the difference 

 may well amount to as much as ten thousand feet, so that the total relief of the 

 moon may amount to somewhere between thirty and forty thousand feet. That 

 of the earth from the deepest part of the oceans to the highest mountain summits 

 is probably between fifty-five and sixty thousand feet ; so that notwithstanding 

 the lack of erosion and sedimentation which in the earth continually tends to 

 diminish the difference between the sea-floor and land areas, the surface of the 

 satellite has a much less range of elevation than the planet. If the forces which 

 have built the mountains and continents of the earth had operated without the 

 erosive action of water there is little doubt that the difference in height between 

 the highest and lowest parts would now be many times as great as it is on the moon. 



AGE OF THE EXISTING LUNAR SURFACE. 



Several of the most important problems to be considered in this writing in- 

 timately depend on a determination of the age of the moon's surface. If we 



