On the Agency of Water in Volcanic Eruptions. 131 



the lava in the duct would not on the contrary increase the pressure 

 in the volcanic foci in which the occluded vapour is present ? 



We have already pointed out the difficulty of accounting for the 

 introduction of water into the volcanic foci. Even supposing it to be 

 introduced and to cause a boiling over, that ebullition would go on 

 so long as any of the imprisoned vapour remained in the lava; 

 but when the expulsion of one or the other was effected, then the 

 introduction of fresh materials from the outside, as in the case 

 of the water in the Geyser pipes, would become necessary, or the 

 boiling over of the lava would cease for want of supplies. If the 

 water were present in combination with the lava in the volcanic foci, 

 there is no reason why the passage to the exterior once formed, the 

 eruption should cease until all the mass susceptible of boiling over 

 should be expelled, in which case each eruption would be of longer or 

 shorter duration, and a volcano would become extinct after one 

 eruption. If, on the other hand, the expulsion were due to the access 

 of water from the exterior, whether by fissures or by permeation, it is 

 difficult to imagine such an influx of water without the previous 

 action of some disturbing cause whereby the existing equilibrium 

 under which the descent of the water is stayed, would be destroyed. 



The only logical hypothesis on which I can conceive the vapour of 

 water of gases to be present in the fluid magma of the volcano is the 

 one suggested by Dr. Sterry Hunt, who considers that the magma is 

 not part of the original molten anhydrous nucleus of the earth, but an 

 intermediate layer derived from the first outer crust of old surface 

 rocks which had been exposed to meteorological agencies, and retained, 

 when fused under pressure, the water with which they had become 

 permeated when on the surface. He supposes the original nucleus to 

 have gradually become solid by pressure and loss of heat, and an 

 outer crust to have formed. As that crust became thicker and 

 covered by sedimentary strata accumulated upon it, its under surface, 

 owing to the rise of the isothermal bars, was gradually remelted, 

 forming an intermediate fluid layer between the solid nucleus and the 

 solid outer crust. 



Or else that of Mr. Fisher, who, from investigations in which he com- 

 pares the existing inequalities of the earth's surface with such as could 

 possibly have arisen from secular cooling, concluded that the interior 

 of the earth had shrunk more than mere cooling alone would account 

 for, and suggested that this was due to the presence of superheated 

 water in large quantities in the original nucleus, and that the blowing 

 off of this water during volcanic eruptions might have contributed 

 materially to the diminution of the volume of the magma.* In a sub- 

 sequent workj Mr. Fisher has applied this hypothesis more particularly 



* " Trans. Cambridge Phil. Soc," vol. xii, p. 414. 



f " Physics of the Earth's Crust," chap, xv, p. 185 et seg. 



K 2 



