PLANETARY EVOLUTION 



by this stoppage of further lunar consolidation 

 must immeasurably exceed anything within ter- 

 restrial experience. It may well have been great 

 enough to freeze all the lunar oceans, and even 

 to liquefy, or perhaps to solidify, the gases of 



crust must shrink to their least dimensions — not all equally 

 (in this we find a circumstance increasing the energy of the 

 disintegrating forces), but each according to the quality which 

 our physicists denominate the coefficient of expansion. Then 

 comes on the long lunar day, at first dissipating the intense 

 cold, then gradually raising the substance of the lunar crust to 

 a higher and higher degree of heat, until (if the inferences of 

 our most skilful physicists, and the evidence obtained from our 

 most powerful means of experiment can be trusted) the surface 

 of the moon burns (one may almost say) with a heat of some 

 500 F. Under this tremendous heat all the substances which 

 had shrunk to their least dimensions must expand according to 

 their various degrees ; not greatly, indeed, so far as any small 

 quantity of matter is affected, but to an important amount 

 when large areas of the moon's surface are considered. Re- 

 membering the effects which take place on our earth, in the 

 mere change from the frost of winter to the moderate warmth 

 of early spring, it is difficult to conceive that such remarkable 

 contraction and expansion can take place in a surface presum- 

 ably less coherent than the relatively moist and plastic sub- 

 stances comprising the terrestrial crust, without gradually effect- 

 ing the demolition of the steeper lunar elevations. When we 

 consider, further, that these processes are repeated not year by 

 year, but month by month, and that all the circumstances at- 

 tending them are calculated to render them most effective be- 

 cause so slow, steadfast, and uniform in their progression, it 

 certainly does not seem wonderful that our telescopists should 

 from time to time recognize signs of change in the moon's 

 *ace." Proctor, The Moon, pp. 380-382. 



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