204 WAEKEN UPHAM, M.A., E.G.S.A., ON THE NEBULAR AND 



adjustment of a cold crust to a cooling nucleus. . . . It is 

 therefore probable that the final shrinkage of nucleus was small, 

 and the antecedent storage of heat correspondingly small. During 

 the whole period of growth the body of the moon was cold." 



After thus stating the hypothesis of Gilbert for the origin of 

 the moon, in his own words, it is not needful to consider here in 

 detail the numerous arguments which favour vulcanism, instead 

 of impacts, as the cause of the moon's craters. The adoption 

 of Gilbert's explanation of the physiography and development 

 of the moon would go very far toward conclusive verification of 

 the planetesimal hypothesis ; but Chamberlin evidently thinks 

 that volcanic origin of the lunar craters is more probable. 



Gilbert considers the whole process of the moon's gathering 

 its formerly scattered material to have been completed at least 

 before the deposition of the earth's Paleozoic sediments, else 

 they would here and there reveal evidences of collision of some 

 of the portions of the previous ring matter, since these must 

 have fallen not only on the moon but in like manner on the 

 earth. Whether the craters of the moon resulted from meteoric 

 aggregation or from vulcanism, the very steep and high moun- 

 tains of the crater rims have doubtless remained through very 

 long ages unaffected by agencies of erosion, because of the 

 absence of atmosphere. 



Geologic antiquity, as hitherto studied, falls far short of 

 reaching back to the time of completion of the creation of these 

 companion globes, the earth and its satellite, in nearly the same 

 size and conditions which they have now. But in the new 

 views opened by the hypotheses noticed in this paper the range 

 of geologic inquiries and theories is extended almost inconceiv- 

 ably farther back, through the laying of "the foundation of the 

 earth." 



Discussion. 



Colonel Mackinlay. — I understand Mr. Gilbert tells us on page 

 202, that what we call volcanoes in the moon are masses roughly 

 comparable to the belt left on a wall when a snowball strikes it, and 

 not volcanoes at all. I must confess it has always struck me as a 

 very strange thing that the moon, which is so much smaller than 

 the earth, is so much richer in volcanoes, and that they should be so 

 very much larger than terrestrial ones. I never heard anyone give 



