44 Condition of the Moon's Surface. [January, 



to the fused nucleus ; yet without water there can be no volcano. 

 More recent investigation on the part of mathematicians 

 has been supposed to prove that the earth's crust is not 

 thin." He proceeds to show that, without attaching any- 

 great weight to these mathematical calculations, there are 

 other grounds for believing that the solid crust of the earth 

 is of great thickness, and that " although there is evidence 

 of a nucleus much hotter than the crust, there is "no cer- 

 tainty that any part of it remains liquid ; but if so, it is in 

 any case too deep to render it conceivable that surface-water 

 should make its way down to it. The results of geological 

 speculation and of physico-mathematical reasoning thus 

 oppose each other; so that some source of volcanic heat 

 closer to the surface remains to be sought. The hypothesis 

 to supply this, proposed by Hopkins and adopted by some, 

 viz., of isolated subterranean lakes of liquid matter, in 

 fusion at no great depth from the surface, remaining fused 

 for ages, surrounded by colder and solid rock, and with (by 

 hypothesis) access of surface-water, seems feeble and un- 

 sustainable." 



Now in some respects this reasoning is not applicable to 

 the moon, at least so far as real evidence is concerned; 

 though it is to be noticed that, if a case is made out for any 

 cause of volcanic action on the earth, we are led by 

 analogy to extend the reasoning (or at least its result) to 

 the case of the moon. But it may be remarked that the 

 solidification of the moon's crust must have proceeded at a 

 more rapid rate than that of the earth's, while the proportion 

 of its thickness to the volume of the fused nucleus would 

 necessarily be greater for the same thickness of the crust. 

 The question of the access of water brings us to the diffi- 

 culty already considered, — the inquiry, namely, whether 

 oceans originally existed on the moon. For the moment, 

 however, we forbear from considering whether Mallet's 

 reasoning must necessarily be regarded as inapplicable to 

 the moon if it should be admitted that there never were any 

 lunar oceans. 



We come now to Mallet's solution of the problem of 

 terrestrial volcanic energy. 



We. have been so long in the habit of regarding volcanoes 

 and earthquakes as evidences of the earth's subterranean 

 forces, — as due, in fact (to use Humboldt's expression), to 

 the reaction of the earth's interior upon its crust, — that the 

 idea presents itself at first sight as somewhat startling, that 

 all volcanic and seismic phenomena, as well as the formation 

 of mountain ranges, have been due to a set of cosmical 



