Dikes.—Dikes of vesicular and amygdaloidal lava are also seen traversing marine tuff or peperino, west of Palagonia, some of the pores of the lava being empty, while others are filled with carbonate of lime. In such cases, we may suppose the peperino to have resulted from showers of volcanic sand and scoriæ, together with fragments of limestone, thrown out by a submarine explosion, similar to that which gave rise to Graham Island in 1831. When the mass was, to a certain degree, consolidated, it may have been rent open, so that the lava ascended through fissures, the walls of which were perfectly even and parallel. After the melted matter that filled the rent in [fig. 468.] had cooled down, it must have been fractured and shifted horizontally by a lateral movement.
In the second figure ([fig. 469.]), the lava has more the appearance of a vein which forced its way through the peperino. It is highly probable that similar appearances would be seen, if we could examine the floor of the sea in that part of the Mediterranean where the waves have recently washed away the new volcanic island; for when a superincumbent mass of ejected fragments has been removed by denudation, we may expect to see sections of dikes traversing tuff, or, in other words, sections of the channels of communication by which the subterranean lavas reached the surface.
CHAPTER XXXI.
ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS—continued.
Volcanic rocks of the Older Pliocene period — Tuscany — Rome — Volcanic region of Olot in Catalonia — Cones and lava-currents — Ravines and ancient gravel-beds — Jets of air called Bufadors — Age of the Catalonian volcanos — Miocene period — Brown-coal of the Eifel and contemporaneous trachytic breccias — Age of the brown-coal — Peculiar characters of the volcanos of the upper and lower Eifel — Lake craters — Trass — Hungarian volcanos.
Older Pliocene period—Tuscany.—In Tuscany, as at Radicofani, Viterbo, and Aquapendente, and in the Campagna di Roma, submarine volcanic tuffs are interstratified with the Older Pliocene strata of the Subapennine hills, in such a manner as to leave no doubt that they were the products of eruptions which occurred when the shelly marls and sands of the Subapennine hills were in the course of deposition.
Catalonia.—Geologists are far from being able, as yet, to assign to each of the volcanic groups scattered over Europe a precise chronological place in the tertiary series; but I shall describe here, as probably referable to some part of the Pliocene period, a district of extinct volcanos near Olot, in the north of Spain, which is little known, and which I visited in the summer of 1830.
The whole extent of country occupied by volcanic products in Catalonia is not more than fifteen geographical miles from north to south, and about six from east to west. The vents of eruption range entirely within a narrow band running north and south; and the branches, which are represented as extending eastward in the map, are formed simply of two lava-streams—those of Castell Follit and Cellent.