A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 33 



instance has the molten rock contained within a ring been observed to discharge 

 itself through the rampart, as is often the case in terrestrial volcanoes. It is per- 

 haps more likely that any communication with the maria was by fissures in the 

 walls which have since been closed, or, if remaining, are so narrow as to escape 

 observation. It may be said, however, that the great heat of the marial lavas and 

 their evident high fluidity would have enabled them to burrow through passages 

 not permeable to the viscous lavas of the vulcanoids. 



The evident fact that the order of succession in time of the vulcanoids is, in 

 a general way at least, in the order of succession of their size, the larger being 

 the more ancient, enables us approximately to determine at what stage in the 

 lunar surface the maria were formed. All of these several areas which have 

 originated independently one of another appear to have about the same sizes of 

 minor vulcanoids on their surfaces. The small craters apparently originated after 

 the greater rings had been formed, but certainly before the discharge of materials 

 from the interior had ceased. It is possible, however, that all the vulcanoids in 

 the maria, except those which were situated on such elevated ground that they 

 were not suffused by their lavas, owe their origin to boiling action within the 

 liquefied zone of the seas themselves. In this case it is possible that the time 

 when these fields were formed was after vulcanoids ceased to be produced on 

 other areas of the lunar surface. The general sharpness of these structures on 

 the maria is in favor of their relatively recent origin, though it affords no data 

 for a precise determination of their age. 



I have, in considering the origin of the maria, referred to what appears to 

 me to be evidence that the fluid of which they were originally composed had 

 extended upward along portions of and perhaps all of their shores, so as to pro- 

 duce a smudged effect on parts of the relatively low-lying ground. So far as I 

 have observed, this apparent effect is most evident on the southern shores of the 

 Mare Nubium and the Mare Humorum. (See plate xxi.) My observations 

 suggest that these apparently inundated fields lack craterlets, such as occur on 

 the areas of the distinct maria. If this observation should be confirmed, it would 

 make it likely that the seas were formed after the activity of the moon, as a whole, 

 had ceased, and that the craterlets of the maria were due, as just above suggested, 

 to boiling within their masses, and not to the internal fluid of the sphere. A careful 

 reckoning of the number of very minute craterlets on the maria, as compared with 

 those on other parts of the moon, will probably show that they are on the average 

 more numerous on them than on some other fields of higher ground, and also 

 that they are of prevailingly smaller size. As a group they appear to me to 

 grade less distinctly into the flat-bottomed craters than do those of the high- 

 lands. My observation on these points are, however, not sufficient to more than 

 suggest these possibilities. Anything like a determination of them demands 

 better seeing than is to be had at the Harvard College Observatory and bet- 

 ter sight than is now mine. Should these variations really exist, they would 

 tend to show that the maria had developed their vulcanoids from their own ma- 

 terials. In further inquiries concerning these pits on the maria, it will be well to 



