Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 195 
magnetite seems to have proceeded right through the process, and the other 
groups of minerals overlap. 
The method of their deposition is what earlier authors referred to as sublimates. 
Of course a true sublimate is a material that has been heated until it reaches the 
gaseous state, and that again condenses into the solid form on any surface suffi- 
ciently cold to reduce the temperature of such gas or vapour to that at which the 
crystals of the substance can form. It is unquestionably certain that where such 
a temperature was reached, the same mineral species that have acted as condensers 
would at least have been fused. Perhaps the ¢achylyte infillings may, in fact, 
represent such a condition. But sublimation has been used by many writers in 
the sense of one vapour acting as a vehicle for others, and depositing by a slight 
reduction of temperature part of its burden as crystals. M. Lacroix affirms that 
these minerals were deposited by autopneumatolysis. This is a new word used of 
late years for an old and well-understood phenomenon. In the present instance 
these minerals were deposited by a deep fumarolic or emanational process, and 
the minerals we see are the result of the fractional condensation of the mixed vapour 
at that particular spot. We could admit the word pnewmatolysis, but autopneumato- 
lysis applies to the deposition of crystals in cavities, fissures, and vesicles of the 
magma from which the substances emanate, and not from extraneous sources. 
In the present instance, such is not the case, for the mineral-forming vapour 
came from a greater or less distance, and, in fact, we have to deal with a contact 
metamorphism. There has been little dialysis or osmotic interchange, as the 
active magma has approximately the same composition as the passive surrounding 
solid matrix. Even where, as we shall see later, the original minerals of the 
matrix have been changed, the mass analysis would prove to be much the same. 
In fact, dialysis seems to take place in the process of metamorphism mainly through fluid 
and solid media molecularly contiguous, in which the solid may be considered as a fluid with 
very high viscosity. The open brecciated character of the richest sublimate-charged 
ejecta shows the absence of such continuity. All that has taken place is a simple 
re-arrangement of the molecules to form new minerals or varieties of those already 
existing. 
No doubt a few additions have been made, as we shall see in discussing these 
changes; but such contributions are of the character of gifts rather than of 
exchanges. In fact, we have the first and most feeble grade of that process 
which I have described as the dialytic or osmotic theory of metamorphism. 
I laid down in considerable detail the general principles of the conditions 
under which certain rock-forming minerals came into existence in the magmas, in 
a paper I published over twenty years ago.* I there showed that leucite and 
* “The Relationship of the Structure of Rocks to the Conditions of their Formation.”—Scient. Proc. 
Roy. Dublin Soe., vol. v., 1886. 
TRANS. ROY. DUB. SOO., VOL. IX., PART VIII. 2H 
