248 Br. H. J. Johnston-Lams — On Volcanic Action. 



separation of the sides at the zone of maximum cooling, with one 

 extremity reaching on one side to the uncooling nucleus, and on the 

 other to the zone of no-strain. But the compression exerted on the 

 nucleus hy the shells beneath the no-strain zone would cause any 

 part of it sufficiently fluid to rise and inject this fissure. In con- 

 sequence of this, we should have a mass of igneous matter that 

 would have become more fluid by diminished jiressure connected by 

 a narrow neck below at the lowermost shell of cooling, but its 

 maximum bulk at the zone of maximum cooling, whilst its most 

 excentric or uppermost prolongation would reach the zone of 

 no-strain. 



Now the shrinking shells beneath the no-strain layer will tend to 

 draw to and within themselves the water squeezed out of the crushed 

 shells above the no-strain layer, and this would be particularly 

 marked at the zone of no -strain, and just below it where we have 

 seen the outermost limits of the fissure injected with igneous matter 

 reaches. In other words, we have highly heated magma in contact 

 with the aquiferous strata, the conditions required in the memoirs 

 referred to to explain the whole of the remaining or superficial eruptive 

 phenomena. The compression of the shells above the no-strain zone 

 will tend to prevent the progress of the magma in the upper part 

 of the fissure towards the surface as it increases in tension by the 

 gradual absorption of water from the surrounding rocks. This re- 

 sistance is at first slight, because the lowermost shells near the 

 no-strain zone are interested where the compression or crushing is at 

 its minimum, as, however, the magma gradually bursts its way by 

 prolonging the fissure further and further towards the surface, the 

 increased resistance is counterbalanced by the increased supply of 

 water and consequent solution thereof, so that the tension of the 

 magma may rise more rapidly, and also by the crushing and com- 

 pression being resolved into crumpling and shearing at the free 

 surface of the earth. 



I do not for one moment suppose that every fissure that forms 

 becomes eventually a chain of volcanoes for conduction, convection, 

 etc., balanced against initial temperature of magma, and many 

 other circumstances may be such as to bring about solidification. 



This constant compression of the terrestrial nucleus would have 

 the effect on the one hand to tend to its solidification. Even were the 

 whole of the earth's interior solid, the moment the tearing asunder 

 took place in the lowermost shells underlying cooling and con- 

 traction, the relief of pressure on the nucleus in the neighbourhood 

 of the newly-formed fissure would no doubt be sufficient to allow 

 the solid to pass into a fluid state. Individually, I cannot persuade 

 myself that even if the uncooling nucleus of the earth is not fluid, 

 or its outer surface fluid, yet there must be large intratelluric reser- 

 voirs of fluid rock. One of the principal reasons of this is that 

 over large areas we find some dominant chemical character of the 

 essential volcanic products, the only sure guide to a common origin 

 of igneous rocks. No more striking example could be cited than 

 Italy, where potash is practical] y always the dominant alkali in all 



