“MECHANISM OF VOLCANIC ACTION—JOHNSTON-LAVIS. 313 
In many cases, however, the tension of the magma will steadily 
rise as it acquires more and more volatile matter from the surrounding 
rocks... A moment will be reached when the tension has gradually 
attained such intensity that the earth’s crust is rent by an extension 
upward of the fissure. This fissure may reach the surface and make 
a new volcano, or the obstructing plug of an old one may be cleared 
away. In either case an explosive eruption will result. 
Now the first portion to issue will be that part of the magma at the 
top of the fissure that has been in contact for the longest time with 
the more aquiferous rocks, and consequently will be richer in acquired 
volatile materials than that below. It may be a pure glass, or a 
certain number of crystals may have individualized under the intra- 
telluric conditions of slow cooling. Once free from compression 
H,O, ete., will separate from the nonvolatile silicates and oxides as 
bubbles, undergo enormous expansion, escape in great part, and afford 
the explosive agent in the ejection of the remaining fluid-froth still 
holding much gas in the vesicles. This sudden expansion means a 
tremendous loss of heat energy, and the vitreous matter is so rapidly 
cooled that it has no time to individualize into microliths or crystals, 
or the crystals already existing to grow in size. 
We know from the effect of pumice on combustible substances, as 
wood, bread, cloth, etc., as at Pompeii, that nearly all the latent heat 
has been used up in this expansion, so that only partial roasting has 
resulted. Thus has been produced the first white light pumice of an 
explosive eruption by the extremely rapid expansion and cooling of 
a pure or nearly pure glass. 
As the upper contents of the volcanic canal blows out, that part of 
the magma lower down follows. This next portion has been a shorter 
time probably in contact with aquiferous rocks; these latter, being 
deeper, are usually poorer in H,O. The consequence is that this 
second batch of magma will contain less volatile matter, expansion 
will be slower, there will be less loss of heat required for expansion, 
so that there will be time and other more favorable conditions for 
part of the glass to individualize into microliths. (Compare 1, 2, 5, 6, 
with 3, 4, 7, 8, pls. 2 and 3.) This second batch of magma further- 
more will have lost less heat energy in consequence of having had 
less H,O to dissolve, but also being deeply seated and in hotter rocks 
it will have lost less heat by conduction. 
“T use H,O not to assume any special physical state of that substance. I also 
refer to it specially as being the principal volatile material of igneous magma 3 
but I fully appreciate the salts and gases derived from their decomposition, as 
likewise the rarer materials that were acquired by and can be separated from 
igneous rocks by a high temperature, to which Monsieur Brun and others have 
furnished us with such interesting details by their studies. 
