478 0. FISHER ON MR. MALLEl's THEORY OP VOLCANIC ENERGY. 



attraction of the sun and moon, and remarked that now there could 

 be no precisely definite line of demarcation between the fluid interior 

 and the solid crust, and that the crust probably was so thick and 

 rigid as to press on the fluid interior, and effectually prevent any 

 such tidal action as was described. He admitted that great oscil- 

 lations have taken place, but thought that all continental areas 

 existed as such in very early geological times, and he illustrated 

 this opinion by discussing the geological history of Europe. With 

 regard to the notion that pressure due to shrinkage might produce 

 fusion of rocks, he remarked that he had suggested the possibility 

 of such being the case in his Presidential Address to the Geological 

 Section of the British Association at its Meeting in Nottingham 

 in 1866. 



Mr. Blake remarked that as all agreed that the earth was a cool- 

 ing, and therefore a contracting body, the pressures and strains 

 arising from unequal contraction must do work of some kind. It is 

 impossible that there should be any sudden descent of an exterior 

 shell on account of the shrinking of an inner one, as it would involve 

 a vacuum within the earth ; but the more probable account would 

 appear to be this : — There are continual strains going on within the 

 earth ; and these, when long continued in a gradual manner, may 

 give rise to contortions in rocks, as shown by the experiments of 

 Mr. Miall, the heat produced being gradually dissipated ; but when 

 a mass of rock breaks down under accumulated pressure, the heat 

 developed suddenly may be too great to be dissipated, and the rock 

 may be melted by the heat, the suddenness of the breaking down 

 being an essential element in the process. 



Mr. Fisher, in reply, said that he had read Leconte's papers, and 

 did not think there was much in that writer's views beyond what 

 had been referred to in the paper. He thought that the influence 

 of attraction was not sufficiently understood, and that there was no 

 doubt the attraction of such a body as the moon might be capable of 

 producing change in the form of the earth. He could not agree 

 with Prof. Bamsay in thinking that the present continents had 

 always been continental ; but the fundamental Laurentian rocks 

 being themselves aqueous deposits was an answer to that assertion. 

 Mr. Mallet's period of a thin crust must have been antecedent to 

 these. 



