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



vulcanoids were in activity is that it was in a state of essential fluidity with a 

 relatively thin crust. This fluidity may not have been that of terrestrial lavas ; it 

 may have been, and apparently was, more viscous or pumiceous. That such was 

 the case is suggested by the behavior of the extruded lavas ; it is further sup- 

 ported by the form of those other extrusions which occur in the so-called moun- 

 tains, as will be further noted in the study of those structures. Thus the crust, 

 despite its being of greater weight than the interior lavas, may have attained a 

 considerable thickness ; it may have had a depth of some miles. Yet it is hard 

 to believe that it would have formed a sufficiently rigid enclosure of the interior 

 fluid to have caused the sphere to remain undeformed by the earth's attraction to 

 the extent necessary to bring about a great up-and-down play of the lava in the 

 passages leading to the surface. It is furthermore to be noted that no trace of 

 tidal action has been observed in terrestrial volcanoes though this fact may be 

 accounted for by the difference in the nature of their origin. 



I have already, in preparation for the study of the maria, considered the argu- 

 ments against the supposition that the vulcanoids are due to the in-falling of 

 meteoric bodies, the main point being that they fail to exhibit any trace of the 

 great melting due to the collision of bolides of sufficient size to make such pits. 

 The maria being, according to my view, due to such in-fallings, showing all the 

 evidences of a vast and sudden development of very fluid material of high tem- 

 peratures, it follows on this hypothesis that the vulcanoids cannot be due to like 

 action. The objection to this explanation in the case of all the crateriform open- 

 ings seems to me to be so insuperable that it may not be further discussed. 



It is important to consider the group of vulcanoids which have been formed 

 on the surface of the maria since the lavas of the maria were produced. We note, 

 at the outset, that these openings are all of relatively small size. Leaving out many 

 doubtful cases, where it is not easy to determine whether the structure was in age 

 antecedent to the maria in which it lies or no, these vulcanoids, so far as I have 

 observed, never exceed ten miles in diameter, and even those of such width lie in 

 positions where the covering of lava proper to the mare may be thin. It is there- 

 fore possible that they are due to actions occurring beneath this marial sheet 

 which have manifested themselves on the new surface. The only vulcanoids 

 which may be with some confidence regarded as having their origin in the lavas 

 of the maria are the numerous small craters and craterlets, those in general of 

 less than a mile in diameter, which are abundantly found scattered over their fields, 

 though they are there less numerous than on certain other parts of the lunar 

 surface. 



It may here be noted once again that in certain instances the likeness of 

 color and the relation of height of the lavas of the maria and those of large nearby 

 craters leans to the suggestion that the igneous fluid from the neighboring mare 

 passed under the ring wall, or through clefts since effaced, into the area it encloses. 

 This view is most distinctly suggested in the case of Plato and Grimaldi, but 

 there are other instances to which it would be applicable. Such a passage of lavas 

 by underground ways is made doubtful by the fact before adverted to, that in no 



