52 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
form being accounted for by this mineral sometimes occurring in 
rhombic dodecahedrons *. 
Remarks.—Before proceeding further, it is necessary and conye- 
nient to speculate for a moment on this somewhat singular change 
in the character of the products of this volcano. 
It will be seen at a later period that there is not so much differ- 
ence in chemical composition between this pumice and the leucitic 
lavas as sometimes between different streams of the latter. 
Again, nearly all the minerals in the pumice are to be found also 
in the lavas. The only species of importance existing in the lava 
and absent from the pumice is leucite. 
Even this mineral we shall see to be a component of pumice of 
later date. The lavas, as a rule, have little if any vitreous base ; 
whereas in the present instance that amorphous material composes 
by far the largest part of the mass. 
We are struck, therefore, by the fact that the difference between 
these two forms of ejectamenta is rather of structure than of com- 
position. 
Pumice-stone is recognized to be the mechanical product of a 
vitreous rock, and the present instance is no exception to the rule. 
Up to the present date this rock has generally been classed amongst 
the acidic voleanic products. In this instance we have a pumice 
containing from 50 to 55 per cent. of silica, presenting all the cha- 
racters of a true vesicular obsidian. It is therefore necessary to 
search amongst the basic rocks for one that is vitreous. This we 
have in what is called tachylyte, a term probably under which 
vitreous varieties of many basic rocks have been included, such as 
some of the lavas of the Pacific islands. 
Closely related to the ancient lavas of Monte Somma, and in fact 
nothing more than a vitreous variety of them, is the thin layer, or 
saalband, of glassy matter sheathing the dykes. 
This proves to us that the leucitic rock, at a comparatively small 
depth from the surface, had not yet assumed its crystalline struc- 
ture when suddenly cooled by injection into a fissure. This fact 
strikes us the more when we remember the words of Messrs. Fouqué 
and Lévy rt, “‘ Toute cristallisation d'une roche ignée a commencé au 
sein d’un magma fondu. La composition minéralogique et la struc- 
ture d'une pareille roche ne peuvent done dépendre que de sa com- 
position chimique en bloc, et des conditions dans lesquelles s’est 
opéré le refroidissement.” 
We may, perhaps, extend the latter remark, and add some other 
influences that may be instrumental in bringing about changes, as 
shown in the table, pp. 54 & 55. 
Admitting the proximate identity in composition of these two 
products of Monte Somma, now under consideration,:Jet us attempt 
to discover the conditions necessary to bring about the difference in 
structure. 
The primitive vitreous magma, rising gradually from great depths, 
may possibly be modified in composition by the chemical reactions 
taking place between it and the surrounding rocks. 
* F. Rutley, ‘Study of Rocks,’ p. 153. 
t ‘Synthése des minéraux et des roches’ (Paris, 1882), p. 78. 
