18 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
The nucleus is usually a larger grain, an empty vesicular space with 
smooth walls, or in some cases there seems to be nothing apparent 
as the special centre of concretion. The preparation of a section for 
microscopic purposes is attended with almost insuperable difficulties 
from the fragility and pulverent structure. The principal mass is 
seen to be composed of fragments of the different minerals of the 
pumice and lava, together with a small quantity of some intersti- 
tial silicate (?), which would require careful chemical analysis for its 
determination. 
This pisolitic tufa is not easily to be distinguished from other de- 
posits that overlie it, and which may also present the same structure, 
so that its thickness is not easily determined, though it rarely seems 
to average more than one metre. The peculiarity of pisolitie con- 
eretions seems not to be confined to this bed, since we have already 
seen it earlier, in the case of PHasz VI., Period 3. In fact, whenever 
we find a fine dusty ash of pumiceous origin, of whatever age, either 
directly deposited as an ash, or collected as fine alluvium, this 
structure is apt to develop itself. The reason for considering this 
pisolitic ash to be derived from the same eruption as the pumice 
is quite evident when we compare this outburst with the subsequent 
Plinian one, which produced very similar materials. 
The broken-up fragments of ancient leucilitic lava-streams and 
scoria, which form the principal secondary or accessory components 
of the lower and middle subdivisions, are of the same type as in 
earlier ejectamenta. Many are moderate-sized lapilli; but large 
blocks are abundant enough. Naturally the varieties of structure 
are very numerous, as they have no doubt been derived from the 
breaking up of lava-fiows of all ages. The most remarkable, how- 
ever, are those which were originally vesicular in structure, but are 
now filled with zeolitic and other minerals, the principal amongst which 
are phillipsite, thompsonite, analcime, calcite, aragonite, and selenite. 
From what we know of these minerals we should expect that lavas 
in which they were deposited were derived from some of the most 
ancient streams situated deep in the bowels of the mountain, where 
water at a high temperature transported and deposited in their 
present sites these hydrous silicates, sulphates, and carbonates. 
The accidental or tertiary constituents of the ejectamenta consist 
ot altered tufa (few), fossiliferous marls, mudstones, and sandstones 
already described with earlier ejectamenta, and a peculiar series of 
altered limestones and metamorphic rocks most variable in structure 
and composition; in fact, so different are their characters that it 
seems a most difficult matter to attempt any general classification, 
even such as is within the scope of this paper. 
In my collection I have over four-hundred separate varieties, and 
each excursion adds new ones. From this we see that the study 
of these ejected blocks would require years of patient labour, and 
can yield but insignificant results until the study of the whole geo- 
logy of the neighbourhood gives us an idea of the kind and quality 
of the rocks that underlie Monte Somma. 
One of the commonest varieties generally included in this group 
