MECHANISM OF VOLCANIC ACTION—JOHNSTON-LAVIS. 309 
Liquid rock having thus reached a considerable way to the surface, 
either as simple dikes (pl. 1, I, 1), laccolites, or sills (pl. 1, L, L), 
and so forth, is now in a situation suitable for the second series of 
phenomena, constituting what I call surface volcanic action, to come 
into play. 
Surface volcanic phenomena.—Two schools of vulcanologists have 
held opposed views as to the origin of the volatile constituents con- 
tained in fluid igneous rock. One class of writers maintain that the 
gaseous contents are primordial, and have been contained in the 
igneous paste from the time that our globe condensed from the 
nebulous state. Others attribute all the volatile matter still retained 
in cooled igneous rock or evolved at voleanic mouths and fumaroles 
to the volatilization of water met by the igneous rock in its journey 
toward the surface Probably both are right, but I propose to bring 
before you a series of my observations that point incontrovertibly 
to the fact that by far the major part of the volatile constituents of 
a magma are acquired by it on its journey toward the surface. 
As condensation took place in our planet from a nebulous state, 
as each layer or shell of rock materials passed from the gaseous to 
the liquid state, and probably solid, it is evident that those most 
volatile would be the last to change their physical state. That some 
of the more volatile ones were entangled or held in solution by the 
less volatile is quite likely, but the amount must have been small. 
Another possible source of volatile matter in the deep-seated 
igneous matter may well be due to a slow osmosis or diffusion ex- 
tending over vast periods of time and directed by the varying affini- 
ties of one class of matter for the other. 
A quarter of a century ago, as a result of a careful and detailed 
study of Vesuvius” and other volcanoes, I was able to show that a 
voleano the more continuously active it was in the emission of 
igneous material the more tranquil was the character of emission, 
and that practically under such conditions lava was the only product. 
I showed also that the longer were the intermissions in the extrusive 
efforts of a voleano the more the ejecta tended to issue in a broken 
up and fragmentary condition, from the larger and more violent 
evolution of volatile or gaseous materials. We thus had the whole 
gamut of products—scoria, pumiceous scoria, scoriaceous pumice, 
pumice, and pumice dust—bearing a distinct ratio to the time that 
any volcano had been in a condition of “ repose.” 
Two explanations offered themselves to my mind for this state of 
things. One was that the persistent evolution of volatile materials 
primordially stored up in the original voleanic paste escaping con- 
«“’Mhe Geology of Monte Somma and Vesuvius;” Q. J. G, S., 1884, vol. 40, 
pp. 35-119, 
