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Doubtless at the close of the eruption a cone of fragmental matter and lava of some slight elevation was built up, and, if so, was subsequently destroyed; for, as we shall presently see by the testimony of the Abate Guilio Cesare Braccini, who examined the mountain not long before the great eruption of A.D. 1631, there was no central cone to the mountain at that time; and the mountain had assumed pretty much the appearance it had at the time that Spartacus took refuge within the walls of the great crater.
No evidence appears that the molten rock ever reached the surface, and if volcanic eruptions ever took place either in lava flows or fragmental materials, all traces of them have been effaced. The rock of the intrusive masses is coarsely crystalline, and no doubt solidified slowly under the pressure of vast thicknesses of overlying rock, now mostly removed by erosion.
The whole mass of the mountain consists of volcanic rock, varieties of andesite; there is no trace of a crater, nor of any fragmental materials, such as are usually ejected from a volcanic vent of eruption. Lava Crater-Cones. A third form of volcanic mountain is that which has been built up by successive eruptions of basic lava, such as basalt or dolerite, when in a molten condition.
"Take that!" roared my opponent; and the bread-basket, with its fragmental cargo of biscuits, came full in my face, very considerately putting bread into my mouth for his supposed injury. "Take that!" said I, seizing the rum-bottle. "No, he sha'n't," said Pigtop, the master's mate, laying hold of the much-prized treasure, "let him take anything but that." So I flung the water-jug at his head.
A second groan, shaken out of its continuity into another comical shower of fragmental coughs, closed this dreary but silent soliloquy. The bishop proceeded: "You have been inveigled, young man, by the charms of a deceitful and heretical syren, for the purpose of alienating you from the creed of your forefathers."
Sedimentary rocks are divided into the fragmental rocks which are made of fragments, either coarse or fine and the far less common rocks which are constituted of chemical precipitates. The sedimentary rocks are divided according to their composition into the following classes: 1. The calcareous, or lime rocks, including limestone and a soft white rock formed of calcareous powder known as chalk.
Long-buried glassy lavas devitrify, or pass to a stony condition, under the unceasing action of underground waters; but their flow lines and perlitic and spherulitic structures remain to tell of their original state. Ancient tuffs are known by the fragmental character of their volcanic material, even though they have been altered to firm rock.
I shall have occasion further on to describe more fully the structure of this remarkable mountain; so that it will suffice to say here that this old prehistoric crater, the walls of which enclose the modern cone of Vesuvius, is seen to be formed of irregular beds of ash, scoriæ, and fragmental masses, traversed by numerous dykes of lava, and sloping away outwards towards the surrounding plains.
The former is composed of a species of trachyte; the latter of ashes and fragmental matter which have been blown out of their respective vents of eruption into the air, and piled up and around in a crateriform manner with sides of gradually diminishing slope outwards, thus giving rise to the characteristic volcanic curve.
The inundation of fluid rock which poured from it is the largest of historic record, reaching a distance of forty-seven miles and covering two hundred and twenty square miles to an average depth of a hundred feet. At the present time the fissure is traced by a line of several hundred insignificant mounds of fragmental materials which mark where the lava issued.
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