!S73-] Condition of the Moon's Surf ace. 43 



of our satellite. But it is manifest that the processes which 

 brought the moon's surface to its present condition must 

 have occupied many ages, during which the craters formed 

 earliest would be exposed to the effects of denudation, and 

 to other processes of which no traces can be recognised. 

 It is not likely, however, that the withdrawal of the lunar 

 oceans into the moon's cavernous interior can have taken 

 place suddenly ; up to a certain epoch the entry of the 

 waters within the moon's mass would be impossible, owing 

 to the intense heat, which, by maintaining the plasticity of 

 the moon's substance, would prevent the formation of cavi- 

 ties and fissures, while any water brought into contact with 

 the heated interior would at once be vaporised, and driven 

 away. But when once a condition was attained which ren- 

 dered the formation of cavities possible, the contraction of 

 the moon's substance would lead to the gradual increase of 

 such cavities, and so, as time proceeded, room would be 

 found for all the lunar oceans. - 



We are next led to the inquiry whether the contraction of 

 the moon's substance may not have played the most important 

 part of all, in producing those pkenomena of disturbance 

 which are presented by the moon's surface. Quite recently 

 the eminent seismologist Mallet has propounded a theory 

 of terrestrial volcanic energy, which not only appears to 

 account — far more satisfactorily than any hitherto adopted — 

 for the phenomena presented by the earth's crust, but sug- 

 gests considerations which may be applied to the case of the 

 moon, and in fact are so applied by Mallet himself. It be- 

 hoves us to inquire very carefully into the bearing of this 

 theory upon the subject of lunar seismology, and therefore 

 to consider attentively the points in which the theory differs 

 from those hitherto adopted. 



Mallet dismisses first the chemical theory of volcanic 

 energy, because all known -facts tend to show that the 

 chemical energies of the materials of our globe were almost 

 wholly exhausted prior to the consolidation of its surface. 

 This may be regarded as equally applicable to the case of 

 the moon. It is difficult to see how the surface of the moon 

 can have become consolidated while any considerable portion 

 of the chemical activity of her materials remained un- 

 exhausted. 



" The mechanical theory," proceeds Mallet, " which finds 

 in a nucleus still in a state of liquid fusion a store of heat 

 and of lava, &c, is only tenable on the admission of a very 

 thin solid crust ; and even through a crust but 30 miles 

 thick, it is difficult to see how surface-water is to gain access 



