Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 173 
Casoria* found soluble salts in the cocoa-coloured dust equalling 3:219 per 
cent., and in the grey dust 2°626 per cent. 
Surely, the amount of soluble saline contents of the lava must vary according 
to the number and size of fissures, by which some of these substances could have 
volatilized off. ‘To repeat what I stated in detail twenty years ago, the compo- 
sition of a magma in the upper part of the chimney of a volcano, over and above 
any effects due to osmosis or diffusion from the walls of the canal, will vary more 
or less through the loss of some of its own more volatile constituents. If a 
column. of lava has been in a state of ebullition for. months in the upper part 
of the open volcanic chimney, surely there will be a much larger amount of 
alkalies fied in the solid and insoluble part of the resultant rock, representing 
the bases retained in the magma, whilst the acids have escaped as HCl, SO;, HF, 
&c., in the vast volumes of the smoke continuously evolved from the volcano. In 
a lava-stream undergoing slow cooling we have the sublimation of chlorides and 
sulphates in abundance, as probably the temperature is too low for SiO, to drive 
out the HCl, SO;, &e., in the alkaline chlorides and sulphates.t 
Not unlikely, the rapid evolution of H,O im the essential magma of the 
paroxysmal or explosive phase of eruption will carry with it the chlorides, 
sulphates, &c., as such. The purging of the magmais so rapid, and the coincident 
cooling also, that no time is given to the SiO, to perform its function as it would 
do in a mass of magma undergoing slow cooling at a high temperature. 
I do not attribute the differences in composition between the lava and the 
essential scoria to any one of these causes; but I do maintain that until we have 
a very extensive and detailed investigation of this point, we have no right to 
generalize on a slight increase or diminution of alkalies in the products of different 
phases of activity of a volcano, or to increase a superfluous nomenclature on such 
slight variation of composition. Still more is this the case when, as pointed out 
above, due credit has not been given to aerial sorting in showers of fragmentary 
ejecta, or a definite estimate of the soluble and insoluble components in an analysis. 
The dusts, either grey or chocolate-coloured, are composed of fragments of 
the usual mineral constituents of lavas and scorize of Vesuvius, and represent the 
triturated components of the summit and heart of the great cone, which was 
removed by this eruption. Chemically, as shown by the analysis at p. 166, 
* «Sulla Composizione Chimica delle Cenere Vesuyiane cadute a Portici nei Giorni 9 e 10 Aprile, 
1906.” Portici, Della Torre, 1906. 
+ Recently M. Albert Brun has made some interesting studies (‘‘ Quelques Recherches sur le 
Volcanisme.”’—Arch. des Sc. Phys. et Nat. de Genéve, Mai, Juin, 1905, et Novy., 1906) on the volatile 
components of volcanic products ; and though I do not accept many of his conclusions, still there is much 
of value in his researches. His laboratory researches for volatile elements in lavas refer, to my mind, 
only to the rarer, less detachable, and therefore residual substances. 
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