216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



it is probable that the atmosphere was maintained until at least 

 the close of vulcanism. Then the traces of water action may either 

 have been obliterated by the agents of degradation previously enu- 

 merated, or have been destroyed by a later phase of vulcanicity. The 

 first explanation scarcely seems probable, since innumerable fine 

 markings exist on the moon's surface which should have been oblit- 

 erated if river courses had been hidden by the effects of degradation. 

 The second explanation points to revolutions at a date subsequent 

 to the loss of the moon's atmospheric envelope. The gaseous enve- 

 lope temporarily provided by these phases of vulcanicity was appar- 

 ently insufficient to impress upon the moon's surface the phenomena 

 of water action. 



Taylor's suggestion " that the moon was acquired by capture dur- 

 ing the early stages of the Tertiary, offers another explanation of 

 the freshness of many of the moon's features. Then this can be 

 explained by supposing that the moon has but recently arrived 

 from a part of space where meteors do not exist. Again, if the 

 moon were in a part of space far removed from any star, its sur- 

 face temperature would be in the region of the absolute zero; nor 

 would any temperature changes due to an external source be ex- 

 perienced to degrade its features. Taylor seeks to explain the 

 terrestrial Tertiary orogenesis by the capture of our satellite, but 

 fails to advance a cause for the equally important periods of dias- 

 trophism of earlier geological ages. However, even if this theory 

 be adopted, we have still to explain the lack of signs of marked 

 contraction, and the evidence of extensional tendencies beneath the 

 moon's surface. 



According to Jeffreys,^^ orogenesis is probably due to horizontal 

 crushing stress developed in the outer layer of the earth. This outer 

 layer can undergo no further cooling and contraction, and is there- 

 fore too large to fit the contracting substrata, which lie between the 

 crust and the region at the center of the earth where no appreciable 

 change of temperature takes place, and therefore no change of 

 volume. 



We have seen that evidences of compression are visible upon the 

 moon, but are not developed to that degree which would be expected 

 if any consideration of its radioactive content were neglected, as has 

 previously been the case. In this paper an attempt has been made to 

 demonstrate that there are good foundations for believing that a far 

 greater proportion of the moon's volume is composed of radioactive- 

 rich materials than is the earth. The foregoing arguments induce 

 the supposition that due to this high proportion of heat-generating 



"Theory of Continental Drift. Tulsa, Okla. 1928. p. 175. (A symposium on Wege- 

 ner's hypothesis.) 



IS " The Karth," by Harold Jefifreys. Cambridge University Press. 1929. p. 279. 



