92 H, J. JOHNSLON-LAVIS ON THE GEOLOGY 
men has been separated out. The large crystals of leucite are 
excessively rich in minute glass-cavities; a few, however, are large, 
and may contain as many as six or more vacuoles. 
Very long and slender microlithic rods traverse the crystal in every 
direction, the larger of which seem to be amphibole. The micro- 
liths and small amphibole crystals often traverse large cavities of 
glass of slightly brown colour, which may at the same time contain 
vacuoles. 
Remarks.—The rock under consideration is the more highly erys- 
talline successor of that of the Plinian eruption. The structure is 
much the same, except that each component has been carried to a 
much higher state of development, so that the leucite, for instance, 
has passed from the microlithic state to crystals of considerable dimen- 
sions. The facts would lead us to conclude that the igneous matter 
has been exposed for a time, before its ejection, to exactly the same 
conditions as its predecessor. If we judge by the limited distri- 
bution of this deposit, the eruption that ejected it could not have 
been a very forcible one. 
After the Plinian eruption had terminated, the crater, as is always 
the case, became partly filled up by the crumbling in of its walls, 
so as to form a plain over its bottom. This we are led to conclude 
from what is generally known to follow a paroxysmal eruption, and 
certainly is confirmed by the account of Galen *, of which Prof. 
Phillips remarks, “From this account we may conclude that 
nothing like the modern cone of Vesuvius existed.” 
The result of such an eruption as the present, bursting through 
the crater-plain, and being so feeble that with difficulty it hoisted 
its products only just sufficiently to deposit a portion on the great 
crater-rim, would be to build up a cone or crater-ring around its 
own vent. Such a cone of eruption would be a representative of 
the present Vesuvius, but was, no doubt, in part, if not wholly, de- 
stroyed by the great eruption of a.p. 472. 
This eruption, immediately subsequent to that of a.p. 79, so far 
as stratigraphical evidence is afforded, would correspond with that 
recorded by Dion Cassius and Galen to have happened in the year 
203, and to have continued for eight days. 
Puase VII., Period 3. 
General Description.—Reposing upon the deposit of pumice just 
described we see, in the section of the Canale di Arena, 2} metres 
of coarse breccia, composed of large blocks of lava and lapilli. This 
is interlaminated at various points by bands of a pumice, to all 
intents and purposes undistinguishable from the last, except perhaps 
by aslightly more perfected development of formed materials, besides 
larger crystallization; but the actual measure of this can hardly be 
obtained, except as a rough appreciation. 
It is interesting to notice that many of the larger blocks of lava 
contain various zeolites, probably as the result of their exposure in 
© Mb. Sevie + ‘Vesuvius,’ p. 41. 
