6O A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



must be judged to be the result of actions essentially like those termed by geolo- 

 gists solfataric, i. e., due to the escape of vapors from a heated sphere, which 

 have colored or coated the surface on which they lie. The considerations which 

 lead us to believe that this internal heat of the moon vanished long before the 

 earth's surface became frozen over are very strong. Accepting the view that the 

 light streaks on the moon are of exceeding antiquity, the question arises as to 

 why they have not been obscured by the fall of meteoric matter upon the surface 

 of that sphere. It is a well-known fact that some hundred thousand, if not some 

 million, meteoric bodies come upon the earth each day. It is true that nearly all 

 of these bodies are so small that they are burned by their friction in the atmos- 

 phere, and are added to our planet only as dust that descends in the rain or as 

 gases contributed to the air ; but on the lunar surface, which, apparently, should 

 receive, per unit of area, quite as many of these fragments as the earth, there is 

 not, and probably never has been, an atmosphere sufficient to decompose these 

 wanderers so that they should have attained its surface unchanged. 



Estimating the average diameter of the meteorites that come into our atmos- 

 phere at only a millimeter, which, in view of the light they afford, is probably too 

 small, it is evident that even in a hundred thousand years they would, if gathered 

 on the surface of an airless sphere, be sufficient to form a coating such as would 

 give a common hue to all its features, and in a geologically brief time the mass 

 would attain a considerable depth. Yet we have evidence in the ample grada- 

 tions of light reflected from the moon that very ancient features of color are as 

 undimmed by foreign matter as newly fallen snow. In other words, we seem to 

 be compelled to. the opinion, either that there has been no such in-falling of mete- 

 oric matter on the moon as has of late taken place on the earth, or that the 

 whole scheme of coloring on the lunar surface has been formed within a few 

 thousand years. That the latter of these suggestions is not true is clearly indi- 

 cated by sundry considerations. It is, in the first place, to be noted that there 

 is much to show the absence of any accumulation of fragmental matter since the 

 oldest of the lunar features were formed. A meteoric rain such as comes upon 

 the earth for even a million years would have masked a host of objects which, 

 though presumably very old, are still manifestly unaffected by any such sheet of 

 dust as would have enwrapped the lunar sphere. Thus the exemption from 

 meteoric contributions appears to have been from a very remote time. More- 

 over, as before noted, the rays of different systems are of diverse ages, yet there 

 is no indication that the newer are very much brighter than the older. 



As for the other possible explanation, i. e., that the moon has not long 

 received meteoric material in the manner in which it now comes upon the earth, 

 there appear to be at first sight but two diverse ways that may have brought 

 about this condition. In the first place, the earth and moon alike may, until 

 very recent times, have been exempt from such contributions. In the second 

 place, it may be that the matter which falls on the earth is in whole or in large 

 part limited to materials which have been ejected from the planet by volcanic 

 action. The first of these suppositions must be regarded as possible, though 



