634 GEOLOGY. 



dominant product. Prodigious quantities of this covered the sea about 

 Krakatoa after its tremendous explosion in 1883. Judd estimates that 

 the volume of included steam involved in the inflation of the pumice 

 examined by him, was from three to five times that of the rock, and that 

 the amount involved in exploding the lava into the fine dust that floated 

 in the upper atmosphere for months, was presumably much greater. 



If the sudden flashing into steam of bodies of water in external contact 

 with hot lava be rejected as only an incidental source of explosion, it 

 remains to be considered whether waters permeating the rock and 

 becoming converted into steam may not be absorbed into the rising 

 lava, become diffused through it, and ascending with it, explode at 

 the surface. So far as access through fissures and cavities large enough 

 to be entered by lava are concerned, it may safely be concluded that 

 since the hydrostatic pressure of the lava must be greater than that of 

 the water in the fissures, or else it could not rise, the lava will enter them, 

 forcing back the water or the steam generated from it, and, having 

 penetrated as far as accessible, will sohdify as a dike, and plug up the 

 avenue of contact between the ground-water and the portion of the lava 

 still remaining molten. The numerous dikes that attend volcanic 

 necks testify to the prevalence of this action. The capillary pores of 

 the wall-rock, which cannot be thus bodily occupied by the lava, must 

 doubtless become filled with steam, and this, following the principles 

 of Daubree's experiment, will force itself into contact with the lava and 

 be absorbed by it, but whether this will be in sufficient quantity, and 

 will become sufficiently diffused through the body of the lava-column 

 to produce the observed effects, is an open question. The increasing 

 testimony of deep mining is that relatively little water flows through 

 the deeper horizons. It is urged that the water remaining in solidified 

 lavas is very unequal in distribution, as though due to unequal access 

 and partial diffusion. The argument seems strong, but to make it 

 thoroughly good, it must be shown that this inequality is not due to 

 irregularity of discharge of the gases during and after eruption, rather 

 than to irregularity of original accession. There is, perhaps, as much 

 ground for assigning differences in the degree of parting with the included 

 gases, as in acquiring them. Doubtless those lavas that boiled and 

 seethed for a long period in the caldron were more fully deprived of 

 their gases than those that were more promptly disgorged and cooled 

 with less convection and surface exposure. 



