(Campi Phlegraei), and terminates in the
promontory of Miseno. Lying in the same direction are the islands of
Procida and Ischia, also volcanic. Hence it will be seen that the two
horns of the bay are formed of entirely different materials, that of
Miseno on the north being volcanic, that of Sorrento on the south being
composed of Jurassic limestone, of an age vastly more ancient than the
volcanic rocks on the opposite shore. (Map, p. 52.)

The general composition of the Neapolitan Campagna, from which the
mountain rises, has been revealed by means of the Artesian well sunk to
a depth of about 500 metres (1640 feet) at the Royal Palace of Naples,
and may be generalised as follows:--

{ Recent beds of volcanic tuff
(1) From surface to depth of { with marine shells, and containing
715 feet { fragments of trachytic
{ lava, etc. (_Volcanic Beds_).

{ Bituminous sands and marls
(2) From 715 to 1420 { with marine shells of recent
{ species(?) (_Pre-Volcanic Beds_).

(3) From 1420 to 1574 { EOCENE BEDS. Micaceous sandstone
{ and marl (_Macigno_).

(4) From 1574 to bottom { JURASSIC BEDS. Apennine
{ Limestone.

[Illustration: Fig. 8.--Map of the district bordering the Bay of
Naples, with the islands of Capri, Ischia, and Procida.]

From the above section, for which we are indebted to Mr. Johnston-Lavis,
the most recent writer on Vesuvius, it would appear that the first
volcanic explosions by which the mountain was ultimately to be built up
took place after the deposition of the sands and marls (No. 2), while
the whole Campagna was submerged under the waters of the Mediterranean.
By the accumulation of the stratified tuff (No. 1), the sea-bed was
gradually filled up during a period of volcanic activity, and afterwards
elevated into dry land.[9]

[Illustration: Fig. 9.--View of Vesuvius from the Harbour of Naples at
the commencement of the eruption of 1872.--(From a sketch by the
author.)]

(_j._) _Present Form and Structure of Vesuvius and Somma._--The outer
cone of Vesuvius, or Monte di Somma, rises from a circular platform by a
moderately gentle ascent, and along the north and east terminates in a
craggy crest, with a precipitous cliff descending into the Atria del
Cava