MOON AND RADIOACTIVITY FORBES 217 



elements, our satellite has attained only that early stapfe of cooling 

 where the surface layers aie too small to (it the interior, and therefore 

 are faulted and rifted. Future evolution will move in the direction 

 of coolinjx and contraction of the substrata, until the crust becomes 

 too lar<;e to fit the interior. Horizontal crushiui; stresses will then 

 be developed in the crust, causinjj diastrophism. Periodical revolu- 

 tions resulting in a hydrostatic condition of the upper laj'crs of the 

 substrata, will temporarily alFord a relatively rapid means of escape 

 of heat b}' convection from the lower layers. Absence of marked 

 contraction is thus ascribed to the present hi<}:h internal temperatures 

 of the moon, and the lack of well-develoj)ed oro<renic belts of 

 terrestrial type is tentatively accounted for under the main hypothesis 

 adopted in this paper. 



