JoHNnsron-Lavis.—The Eruption of Vesuvius in April, 1906. 163 
distribution of the phenocrysts and microliths is different. In fact, it 1s those 
differences that make the areola visible. In the most extreme case this areola 
does not exist from its refusion after its first formation. 
The most striking examples that prove my original explanation of the origin 
of these bodies were some of these bombs or dumplings containing masonry. 
In fig. 18, Pl. XII., we have a very large bomb.on the lava some half-way between 
Boscotrecase and Torre Annunziata, which contained a piece of a wall with a 
well-formed adherent crust of lava. Most of this cracked off when the dumpling 
was struck with my hammer; but it is very clearly seen in the photograph, 
especially on the top and to the reader’s left side. 
Petrographical and Chemical Characters of the Lavas.—The lavas (figs. 22, 238, 
24, 25, Pl. XVII.) are of a very common type of those ejected from Vesuvius 
during the last two or three centuries, and would be almost indistinguishable from - 
a good many. In clean section they are bluish-grey, fairly uniform in texture, 
exhibiting scattered augites of bottle-green colour, and sparsely scattered crystals, 
and crystal groups of leucite. The augites range up to 5 mm. or more zn length; 
but the most conspicuous are from 3 to 5 mm. ‘The diameter of the leucites is 
about the same. These last have that peculiar greasy, glassy-grey lustré of the 
fresh mineral, so that they are not easily distinguishable in colour from the matrix. 
In fact, it is only in old lavas that leucites look more prominent, when the first 
traces of kaolization begin to give them a whiter colour. Here and there it is 
possible to pick out an olivine or biotite crystal, so that these are not so common 
as in many Vesuyvian lavas. Other minerals can occasionally be met with, visible 
to the eye, but can only be considered as accidental constituents of the rock.* 
Where quarrying operations have in a few places opened up the massive part 
of a flow, it appears as a fairly compact and uniform rock, and, no doubt, in the 
future will take a fair place for industrial uses. In some spots it is of course fairly 
vesicular. In the cavities of the vesicles, and in the spaces left along the shear 
planes of flow, an extremely fine geodic lining has been deposited. The crystals 
are so extremely small, that they are practically incapable of being scraped off. 
By comparison, however, with the cavity linings of other Vesuvian lavas, I think 
we can refer them chiefly to sodalite and neochrysolite, fayalite, and probably a 
little breislakite. | Atl 
On microscopical examination the augites are seen to be well-formed crystals, 
with sharp edges, very clean, and containing the usual apatite microliths, and a 
few well-formed crystals of iron ore. ‘These augites seem to be of two tints, though 
* T do not propose to describe here the already well-known petrographical details of Vesuvian lavas 
in general, and reference is made only to those ‘variations that afford some association with the dynamic 
manifestations of the recent events at Vesuvius. 4 
” 
TRANS. ROY. DUB. SO0., VOL, IX., PART VIII, ig: 2D 
