OF MONTE SOMMA AND VESUVIUS. 53 
The aqueous matter contained within the rocks in immediate 
vicinity of the column of incandescent magma, must be raised to a 
very high temperature. We are acquainted with the solubility of 
silica and silicates in water at a high temperature and pressure. 
The escape of vast volumes of vapour from lava must convince us of 
the presence of aqueous matter combined chemically with the fused 
silicates, or, at any rate, intimately diffused or mixed with them. 
It is probable that the aqueous matter is assimilated by the 
igneous matter from the water-bearing strata it traverses at a very 
high temperature and pressure. 
It is evidently the manner of formation and escape of this vapour 
upon which is dependent the difference of structure in the Somma 
lavas and pumice. 
This mass of aquiferous magma, in proportion as it approaches 
the surface, passes from a high to a lower pressure, at the same time 
being reduced in temperature in the same ratio as expansion takes 
place. A moment arrives when the pressure no longer balances the 
tension: just as we see the carbonic acid begin to form minute 
bubbles in seltzer-water when the cork is removed, so does the 
aqueous vapour develope minute globules which, coalescing, form 
bubbles distributed throughout the still fluid mass. 
If cooling takes place immediately before the escape of the vapour, 
a spongy or, more properly, vesicular rock or pumice will result. 
It is necessary that the magma should come from great depths, 
that it should have been retained for long periods in contact with 
aquiferous strata, and that it should be expelled rapidly before the 
vapour-vesicles can unite together as bubbles and escape. We 
conclude, therefore, that pumice in a basic rock so tar appears to be 
characteristic of Scrope’s ‘ Paroxysmal Eruptions.’ 
The question arises, how is it that in a basic rock, which is always 
very fluid, the vapour-vesicles had not time to escape? If we 
imagine a fluid mass throughout which the amount of volatile matter 
is disseminated in large quantity and under high pressure, the re- 
moval of this latter will result in the conversion into a gaseous form 
of the volatile matter, so bringing about a corresponding loss of 
heat. If this descent reaches sufficiently far to render the mass 
solid or even viscid, the vapour, unable to escape, will remain 
behind involved in the magma as so many vesicles or bubbles. The 
process is analogous, in a mechanical point of view, to bread rising 
as a plastic mass, which is solidified by baking. The spaces occu- 
pied by the vapour nearest the surface of a detached mass will burst 
and leave a coating of compact matter, as in the crust of bread and 
the hard envelope of the pumice. 
From the rapidity of cooling, the process of crystallization will be 
arrested, and the structure remain vitreous. 
Such, therefore, seems to me the true explanation of this some- 
what complex phenomenon. 
In the case of Monte Somma a certain amount of the vapour must 
have escaped and formed the element of explosion or ejection. The 
endeavour has been to show that rocks apparently of totally dif- 
ferent type may have a common origin. 
