On the Agency of Water in Volcanic Eruptions. 171 



from the influence of some slight disturbances — sufficient, however, 

 for the purposes of our contention. 



For the geological argument, neither a perfectly fluid substratum 

 nor a molten nucleus are required. The hypothesis of a central solid 

 nucleus seems to me to be the only one compatible with geological 

 phenomena. All that is required for the conditions of geological 

 phenomena is that on this solid nucleus there should be a molten 

 yielding envelope — not fluid, but viscid or plastic ; nor is it necessary 

 that it should be of any great thickness, but a thin crust is, geologi- 

 cally, an essential condition. The relative proportions of the two are, 

 however, questions for physicists. The late M. Roche did attempt a 

 solution based on the astronomical and physical conditions of the 

 problem. Assuming the earth to consist of a solid centre with a 

 density of about 7'0, and of an outer layer consisting of a fluid sub- 

 stratum with a solid crust, and having a mean density of 3*0, he 

 found that the outer layers should have a maximum thickness equal 

 to one-sixth of the earth's radius, or 660 miles, but he left the question 

 of their possible minimum thickness open to other considerations. 



The considerations I have already urged, in conjunction with the 

 results of other independent physical investigations, would lead me 

 to assign lesser dimensions to this combined thickness of the outer 

 layers. On geological grounds the solid crust at all events need not 

 have a thickness of even 20 miles ; while, on the same grounds, the 

 dimensions required for the underlying molten layer to place it in con- 

 cordant relation with the ascertained mobility of the crust during the 

 later geological periods, is that it should be a mass sufficiently large 

 for the play of movements such as would come within the compass of 

 continental (not mountain) elevations and depressions ; and for this 

 object and for the purposes of vulcanicity a molten layer, having a 

 thickness measured not by hundreds, but by tens of miles, would 

 fulfil the necessary conditions. 



It is quite possible, as suggested by Scrope, that owing to pressure 

 the fusion-point of lava at great depths is so much higher than at 

 the surface, that the lava may, and possibly does, exist at depths in a 

 viscid or plastic state, and only becomes fluid as it rises to the surface 

 and the pressure is removed. This state of viscidity accords with the 

 excessively slow rate of movement and steadiness of the great con- 

 tinental elevations and depressions, — changes in close relation one 

 with another, and which may arise from the slow transference from 

 one area to another of a partially resisting plastic medium within con- 

 fined limits. 



I cannot conceive such a transference to be effected unless that the 

 molten layer were of moderate thickness, so that when locally com- 

 pressed between the outer solid crust and the inner solid nucleus, that 

 portion of the magma subjected to pressure would expand laterally 



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