Dr. Johnston- Lavis — Mechanism of Volcanic Action. 435 



globe. Some hold that it is like an egg — a solid shell with a fluid 

 interior ; others maintain that by the increase of gravity as the centre 

 is approached there is a solid nucleus which is potentially fluid were 

 it not for this gravitational condensation, so that there would be a solid 

 nucleus, a solid crust, and a stratum of liquid rock separating them. 

 Finally, there is a third school who holds that the highly heated 

 nucleus, although potentially fluid, is really solid in consequence of 

 pressure or, more correctly, gravitational condensation. 



No known rock that we are acquainted with gives the conception 

 of having sufficient tensile strength to be capable of exerting any 

 really contractile or squeezing power on fluid enclosed within it or 

 suiTounded by it. There will be a tendency as the inner shells 

 contract to split by fissures. Such fissures would extend from 

 within outwards, and would be top-shaped in section, with the edge 

 extending up to the neutral zone of no contraction, and their lower 

 limit at the inner surface of the lowest shell (Fig. 1, E, F). Such 

 a fissure might be simultaneously filled by the fluid rock-paste beneath. 



How this filling will take place requires consideration. As there is 

 reason to disbelieve in any considerable constricting power of the inner 

 cooling shells, and that even if such constricting power did exist it 

 would be annulled by the development of fissures within its mass, it 

 is evident one must look to other causes. The welling up into the 

 fissure of the fluid rock, if we admit a fluid nucleus or a stratum or 

 shell of such fluid, might be due to the settling down by gravitation of 

 the cooled blocks ^ of crust limited by the fissures. If, on the contrary, 

 we admit the immediate contact of the lowest cooling shell with 

 a highly incandescent nucleus (P), solid by pressure, but potentially 

 fluid when this pressure is removed, we can well see what would take 

 place. As soon as the fissures and therefore fluid in the inner cooling 

 shells begin to form, their location and their edges will represent 

 a site of diminished pressure. The subjacent and neighbouring but 

 potentially fluid rock will in consequence liquefy and expand and 

 fill the fissure. As the fissure broadens and extends so will the 

 expansion and liquefaction \nQ,ve?i?,Q pari passu. 



Liquid rock may thus reach up to the neutral zone of no contraction, 

 but its extension further must be a matter of chance. It is evident 

 that if the shells of compression were in every part homogeneous and 

 coherent, then no up ward -pointed fissure could be formed. In practice 

 neither of these conditions is fulfilled. It is obvious that the 

 crowding and crushing will be most complete in the shells of 

 compression (Fig. 1, C) where these are carried on a continuous block 

 of contraction (Fig. 1, A). I mean by a block a portion bounded by 

 fissures formed in the contracting part of the earth-crust. Where the 

 shells of contraction are fissured (E, F) there the crowding of the 

 superincumbent masses of cooled rock will not take place. As a result, 

 each block or island of contractile crust, with its compressed burden, 

 will tend to tear away from the adjoining blocks or islands, so that the 

 limiting fissures in the contractile joints will extend up into the 

 compression al shells (G, H). 



' Tliese blocks are quite different to the blocks referred to by some recent writers 

 on terrestrial mechanics. 



