196 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
augite were minerals, when intratelluric in origin, which were produced at no 
great depth; whereas their competitors for the same chemical constituents, as 
potash, hornblende, orthoclase, and mica, could only come into existence im a 
magma under abyssal conditions. 
Now, although such minerals as specially hornblende, orthoclase, and biotite 
usually require abyssal conditions when crystallizing out from solution in other fused 
silicates, as In a paste or magma, they can be formed at a temperature of an 
ordinary kitchen oven, when they are deposited by ‘‘ sublimation” or pneumatolysis. 
In another paper of mine* I drew attention to very extensive deposition of 
large crystals of biotite and hornblende, on a quaternary fossil bone that still 
retained a large part (five-and-a-half per cent.) of its organic matter. When this 
bone, covered with these silicates, usually abyssal in origin, was heated, it 
blackened and gave off the odour characteristic of roasted or burnt bone. Now, 
at Faiano, in the ‘pipernoid tuff” from which these bones were removed, 
the commonest of the above-mentioned silicates are augites, nepheline, and 
fluorite—in fact, the same companions as we have in these drusy blocks 
of the late eruption of Vesuvius. ‘These facts, I think, go to prove that the 
mineral-lined blocks ejected from Vesuvius owe their enclosed minerals to pro- 
bably volatile fluorides, chlorides, &c., as the vehicle that carried them from the 
magma and deposited them in the rock in which we now find them. 
These old lavas, scorize, breccias, &c., containing these new minerals in their 
cavities, have, as has already been touched upon, undergone a certain amount of 
metamorphism, or alteration. Lacroixt has described in great detail these 
changes, which have been recorded by several observers, and which I have verified 
by slices I have cut and have examined. ‘The most interesting are the loosely 
agglomerated masses of old fragmentary materials of the cone which are only 
just soldered together at their points of contact, leaving many spaces in which 
the neogenic minerals have been deposited. We find that on cutting sections of 
the old angular fragments, the constituent minerals of the rock have been altered 
and modified to a greater or less distance from the surface. The chief changes 
are the bleaching of the augites and the gradual conversion of them to egirine, 
whilst the leucite is partly converted into a microsommite and sometimes 
into orthoclase. 
It is obvious that if these breccias remained as a relatively incoherent mass, 
the molecular contiguity between the active magma and passive matrix could 
practically only be through gases; in other words, transmission could not be 
through the solid, but only through the gasiform infillings of the intervening spaces. 
* “On the Formation at Low Temperatures of certain Fluorides, Silicates, Oxides, &c.”’—Geol. Mag. 
Dec. tv., vol. ii., p. 309. July, 1895. 
+ Op. cit. 
