206 R. A. Daly — Abyssal Igneous Injection. 



of a single tidal period, for example, it will, in certain lines 

 appropriately oblique to the earth's equator, tend to wrench 

 apart the crust even down through its viscous bottom layer. 

 To such a powerful fluid as that composing the substratum, 

 this viscous layer, suddenly sheared or broken, is relatively a 

 solid mass ; to the searching fluid a plane of shearing in the 

 viscous layer is virtually a crack. Into that plane the tidal 

 pulsations will pump the fluid, which instantly exerts its lateral 

 hydrostatic and expansional pressures on a shell already prone 

 to recoil because of the real though mild tension residual in 

 the bottom of the shell. As the fluid thus works its way 

 upward, it encounters rock which is increasingly more rigid 

 and increasingly charged with accumulated tension and cooling 

 cracks. In fact, if we conceive that the viscous bottom layer 

 is once completely penetrated, it is easy to believe that the 

 abyssal dike will be rapidly injected toward the top of the 

 shell of tension. The shearing-in of the solid rock opposes 

 the continued opening of the potential fissure, but this shearing, 

 as the level of no strain is approached, becomes . slower and 

 slower and thus more and more powerless to check the rapidly 

 acting wedge of expanding fluid. 



The injection might conceivably (following Fisher's idea) be 

 aided by the local removal of the viscous basement through 

 the special attack of upward convection currents in the sub- 

 stratum ; for it is clear that the injection is most difficult at 

 its very beginning. Fisher has suggested that water-gas given 

 off from the substratum when it is already injected into a 

 downwardly-opening "chasm" may contribute force tending 

 to widen the crack. However, such gas could not, on this 

 hypothesis (the gas being dissolved in the magma according to 

 Henry's law), segregate except by release of pressure. The 

 release is a slow process. As a means of injection through the 

 viscous layer the solution of magma and water-gas would be 

 more effective than the compressed anhydrous magma, but 

 their activities would be of the same kind. Fisher assumes 

 that the magma is saturated under a pressure of 12,000 atmos- 

 pheres. This implies that the more rapid the outflow of lava 

 at a vent, the more imposing would be the explosive phenom- 

 ena. It is obvious that this is not the case in nature. The 

 exceedingly small amount of water in the lavas of fissure-erup- 

 tions and of the Hawaiian calderas seems, indeed, to show that 

 the abyssal fluid is essentially anhydrous.* The water actually 

 found in lava and that accompanying explosions of the Yesu- 

 vian type may be all or nearly all derived from the shell of 

 rock-fracture. We conclude that it is wisest to find the posi- 

 tive penetrating force of the magma in its own elastic expan- 



*Cf. J. D. Dana, Characteristics of Volcanoes, New York, 1891, p. 197. 



