210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



solved when one applies to the moon Joly's^ well-known theories of 

 radioactivity, which seek to account for geological periodicities upon 

 the earth. 



We have accepted the assumption that the moon was originally 

 derived from the outer layers of the earth. Therefore it must have 

 the same radioactive constituents as our globe. Furthermore, radio- 

 activity should be greater in the moon than in the earth, since the 

 former contains a larger proportion of sial, and the lighter acidic 

 rocks are more radioactive than the denser basic rocks. Density 

 considerations lead us to expect that whereas the inner core of the 

 moon is sima, a large part of its volume down to a very considerable 

 depth beneath its crust will be of sial. Joly has advanced the hy- 

 pothesis that in the earth, heat accumulations due to radioactivity 

 beneath the continental blocks will escape when the substratum is 

 liquefied, and the heated areas are brought beneath the thin ocean 

 beds, through the crust being rotated over the liquid substratum by 

 tidal drag. He has also shown that under this theory a continuous 

 belt of land around the equatorial regions of the globe would be an 

 unstable arrangement, for then no amount of slipping of the crust 

 over the molten interior would provide any means of escape for the 

 heat. The continuous belt would ultimately be broken up by melting. 



On the moon, however, not only is there a continuous belt of sial 

 around the equatorial regions, but the whole surface of the sphere 

 is underlain by a thick stratum of that material. Obviously heat 

 accumulated within the moon due to radioactivity, can not escape by 

 the means advocated for its dispersion in the earth. Only a period of 

 cataclysmic vulcanicity marked by violent eruptions, rending of the 

 crust and outpourings of lava can release the pent-up heat. It is 

 to such revolutions that the surface phenomena of the moon may ten- 

 tatively be ascribed, the state of excellent preservation of the younger 

 features pointing to comparatively recent date for their formation. 



An examination of the moon's features appears to lend very sub- 

 stantial support to this conception. Everywhere there are traces of 

 exceptionally violent eruptions, craters occupying a very large 

 proportion of the surface. That the largest craters as a general rule 

 appear to have been formed first, confirms the expectation that the 

 initial outburst of the compressed lavas would be of great violence. 

 Concurrently the great rifts, clefts, and fractures were formed, as the 

 thin skin of unmelted crust failed to extend to cover the now greater 

 volume of the sphere. For, as heat accumulated within the crust, so 

 would the constitutents of the interior tend to become liquefied and 

 less dense. Only the intense pressure would keep them from lique- 

 faction. However, a time would come when the crust was so far 



6 The Surface-History of the Earth, by John Joly, Oxford University Press, 1025. 



