THE PAST HISTORY OF OUR MOON". 95 



surface would be everywhere the same), but would belong to 

 independent fluid masses, irmeed it may be noticed that 

 the very nature of the case requires us to adopt this view, 

 since no other will acount for the variety of level observed 

 in the different lunar crater-floors. If these ceased to be liquid 

 at different times, the independence of the fluid masses is 

 by that very fact established ; and if they ceased to be liquid 

 at the same time, they must have been independent, since, 

 if communication had existed between them, tiiey would 

 have shown the uniformity of surface which the laws of 

 hydrostatics require. 1 



The next effect which would follow from the gradual re- 

 treat of the nucleus from the crust (setting aside the with- 

 drawal of- lunar seas) would be the formation of corrugations, 

 in other words, of mountain-ranges. Mallet describes the 

 formation of mountain-chains as belonging to the period 

 when * the continually increasing thickness of the crust re- 

 mained such that it was still as a whole flexible enough, or 

 opposed sufficient resistance of crushing to admit of the up- 

 rise of mountain-chains by resolved tangential pressures/ 

 Applying this to the case of the moon, I think it is clear that 

 with her much smaller orb and comparatively rapid rate of 

 cooling the era of the formation of mountain-chains would 

 be a short one, and that these would therefore form a less 

 important characteristic of her surface than of the earth's. 

 On the other hand, the period of volcanic activity which 

 would follow that of chain-formation would be relatively 

 long continued ; for regarding this period as beginning when 

 the thickness of the moon's crust had become too great to 

 admit of adjustment by corrugation, the comparatively 

 small pressure to which the whole mass of the moon had 

 been subjected by lunar gravity, while it wo'uld on the one 

 hand cause the period to have an earlier commencement 

 (relatively), would on the other leave greater play to the 



1 It is important to notice that we may derive from these considera- 

 tions an argument as to the condition of the fluid matter now existing 

 beneath the solid crust of the earth. 



