(_i._) _Structure of the Neapolitan Campagna._--But before giving a description of this terrific outburst of volcanic energy, it may be desirable to give some account of the physical position and structure of this mountain, by which the phenomena of the eruption will be better understood. Vesuvius and the Neapolitan Campagna are formed of volcanic materials bounded on the west by the Gulf of Naples, and on the east and south by ranges of Jurassic limestone, a prolongation of the Apennines, which send out a spur bounding the bay on the south, and forming the promontory of Sorrento. The little island of Capri is also formed of limestone, and is dissevered from the promontory by a narrow channel. The northern side of the bay is, however, formed of volcanic materials; it includes the Phlegræan Fields (Campi Phlegræi), 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.)]