154 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
Finally, from still greater depths the magma may rise and flow out as a con- 
tinuous mass, because it will have begun to move from the deeper part of the vol- 
canic chimney, with a still higher initial temperature, and with little or no loss of 
heat from the acquisition of water or excessive vesiculation or expansion of the latter. 
These facts, which I first discovered at Vesuvius,* but which can be seen 
in any volcanic region, and especially amongst basic rocks, where they are 
more clearly discernible, point, in my opinion, to the fact that the ‘explosive, 
expansible manifestations of volcanic action are a purely swzface phenomenon, 
and dependent on the porosity of the outer layers of the Earth’s crust, and thus 
they are more or less aquiferous. 
Unlike this surface voleanic action, which can be read from the rocks, the 
deeper volcanic phenomena are much more hypothetical. Whether we believe the 
Earth to possess a cooling crust, containing a fluid rock, or a cooling crust around 
a highly incandescent but solid nucleus, potentially fluid if under reduced pressure, 
we have conditions which will give rise to fracturing. Such tearing will take 
place in the inner cooling shells of contraction, and will extend upwards to the 
zone of no strain, and beyond it between the blocks or islands of compression. 
Any such fracture will, of course, be simultaneously filled by the fluid igneous 
paste beneath, or, if such does not exist, by a highly incandescent rock that would 
become fluid by the relief of strain set up in the neighbourhood of the fissure. 
Where the upper limits of such rifts reach aquiferous strata, filled by the liquid or 
liquified rock from below, the second series of conditions favourable to surface 
volcanic phenomena will take place. When a continuous communication exists in 
an active volcano, a state of relative equilibrium will ensue, by which, in con- 
sequence of the fairly regular secular cooling of the Earth’s crust, a uniform 
amount of the fluid nucleus will escape. In a general way we may say that the 
shrinking part of the crust will squeeze out through rifts in itself a fairly regular 
and uniform amount of fluid igneous paste. Such regularity and uniformity will 
be disturbed by the different variations in the state of the upper portion of this 
voleanic canal; and it is to those conditions that we will now direct our attention, 
as illustrated in this Vesuvian eruption. 
For a good many years the upper portion of the main canal of Vesuvius—that 
portion we may conventionally call the chimney of the voleano—has been occupied 
by a mass of igneous magma in a fluid state of ebullition. Let us for a moment 
consider what influences are at work upon such a mass, and what modifications it 
would undergo. 
There will be from the prime source below a fairly uniform accession of material, 
so that there will be a constant tendency for the lava to overflow from the summit 
of the cone. As amatter of fact, this did occur frequently. But the actual upward 
* Op. cit., Quart, Journ. Geol. Soc., vol. xl. 
