HI OUR MOLTEN GLOBE 53 



many difficulties in this view. Others, with whom is 

 Mr. Fisher, think that they form an essential constituent 

 of the primeval globe, and that, instead of being derived 

 from the ocean, it is more probable that the ocean itself 

 has been derived from the vapours which have been always 

 escaping from the interior. Leaving this question as one 

 of comparatively little importance for the present discus- 

 sion, we have now to point out how the facts, that the fluid 

 substratum is saturated with water-vapour and other 

 gases, and is also subject to convection-currents continually 

 bringing superheated matter up to the lower surface of 

 the crust, enable us to explain the special difficulties as to 

 volcanic eruptions alluded to in the early portion of this 

 chapter. 



The first of these difficulties is, that neighbouring 

 volcanoes of very different heights act quite independently, 

 a fact which is supposed to be inconsistent with the idea 

 that both are in connection with the same molten interior. 

 It seems, however, to have been assumed that a mere 

 fissure or other aperture extending from the surface to 

 the substratum, or from the substratum to the surface, 

 would necessarily be followed by an outflow of lava, even 

 though the opening terminated at the summit of a moun- 

 tain many thousand feet above the sea level. But it is 

 evident that on the theory of a molten interior, with a 

 crust of somewhat less specific gravity resting upon it in 

 hydrostatic equiUbrium, nothing of the kind would happen. 

 When a hole is bored through an extensive ice-field, 

 whether on a lake or in the Arctic Ocean, the water does 

 not spout up through the aperture, but merely rises to the 

 same level as it would reach on the sides of a detached 

 block of floating ice, or on the outer margin of the ice- 

 field itself. The facts that the fluid on which the crust 

 of the earth rests is intensely heated, and that the crust 

 is continuous over its whole surface, can make no differ- 

 ence in the behaviour of the fluid and the solid, so as to 

 cause the molten rock to rise with great violence thousands 

 of feet above its mean level whenever an aperture is made ; 

 and this is the more certain when we take account of the 

 fact, which may now be considered to be well established. 



