190 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
portion of the ejected blocks that Archangelo Scacchi extensively studied after 
the eruption of 1872. 
I shall only refer to them in a purely geological or vulcanological point of view 
—that is, in so far as they are documents bearing evidence for the elucidation of 
the phenomena of the last convulsion of Vesuvius. 
The minerals forming the principal constituents of the essential ejecta, namely, 
the lavas and scorize, have already been described, and show nothing of special 
interest with regard to this eruption. In three of my former papers* I have 
classified the different ejecta of Vesuvius, and explained their origin and distri- 
bution; and I must refer the reader to those papers for fuller details. 
In the great throes of the Somma volcano, gigantic craters were produced by the 
explosive eruptions, the apices of which craters were excavated down into the sub- 
volcanic platform. The sedimentary rocks and the metamorphic derivatives of 
these were ejected and spread around the mountain. The great craters were filled 
in with the ruins of the crater-edges and new volcanic ejecta; so that could we 
make a complete dissection of the mountain, it would appear in shape like a 
huge hob-nail. Paroxysmal eruptions that have occurred later than the explosive 
ones cannot reach the sub-voleanic platform, but can extend only more or less 
into the plug or neck that fills the largest crater of the explosive period. It is 
obvious, therefore, that such a paroxysm will only eject those materials that have been 
deposited there since the last explosive outburst. Such materials will consist of lava- 
flows, dykes, scoriz, lapilli, dust, and any sedimentary or metamorphic blocks that 
have fallen back from the crumbling edges of the crater where they had been 
deposited as earlier ejecta. 
It is obvious, therefore, that it is only fragments of these materials that 
we expect to encounter in the ejecta of such a paroxysm as that of April, 1906; 
and, in fact, that is just what we do, and they may be conveniently classified as 
follows :— 
1. Ancient lava and dyke rock :— 
(a) Fresh and unaltered. 
(2) Rocks such as (a) metamorphosed by prolonged heat under pressure 
near the voleanic chimney with infilling of cavities by sublimed 
silicates and other minerals, and by residual juice from the fluid 
lava of the chimney. 
(c) Ancient fumarolized rocks such as (a) that have been subject to meta- 
morphism as in (0). 
* “ Geology of Monte Somma and Vesuvius ’’—Quart. Journ. Geol. Soc., vol. xl., p. 35, 1884; ‘On 
the Fragmentary Ejecta of Volcanoes’’—Proc. Geol. Assoc., vol. ix., 1886; and ‘‘ Ejected Blocks of 
Monte Somma, Part 1.’,—Trans. Edinburgh Geol. Soce., vol. vi., 1898. 
