i8 93 . THE SURFACE OF THE MOON. 53 



ment of a cold crust to a cooling nucleus. The topography is due to 

 sculpture, not to wrinkling ; the smooth floors of the maria have 

 anticlinal and monoclinal forms, but of so gentle a slope that they 

 are seen very near the terminator, and can represent but a minute 

 amount of arc shortening. 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. With regard to the age of the moon, Professor 

 Gilbert asks two questions. (1.) Does the earth exhibit impact 

 craters ? If not, then erosion and sedimentation have destroyed them, 

 and the Cenozoic era did not witness the building of the moon. (2.) 

 Is any horizon of stratified rocks generally or widely characterised 

 by molten disjecta ? If not, then the moon was already a finished 

 planet in Palaeozoic times. 



