Fig. 51.—The headland of Monte della Guardia, in the Island of Ponza.
a. Columnar trachyte. b. Stratified tuffs. c. Pumiceous agglomerates. d. Dyke of rhyolite.
Fig. 52.—Western side of the same headland, as seen from the north side of Luna Bay.
a. Trachyte lava. b. Stratified tuffs. c. Dykes of rhyolite, with their edges passing into pitchstone. d. Pumiceous agglomerate.
Fig. 53.—Sea-cliff at Il Capo, the north-east point of Salina showing stratified agglomerates traversed by numerous dykes, the whole being unconformably overlaid by stratified aqueous deposits.
[Fig, 53] represents a cliff-section in the island of Salina, one of the Liparis, exhibiting evidence that a series of volcanic agglomerates traversed by dykes of Andesite have been denuded and covered by a recent stratified deposit.
PART PLAYED BY DYKES IN CONE-BUILDING.
In the formation of these great composite cones, a minor but by no means insignificant part is played by the dykes, or lava-filled fissures, which are seen traversing the mass in all directions. That dyke-fissures often reach the surface of a volcanic cone, and that the material which injects them then issues as a lava-stream, is illustrated by [fig. 54]. The formation of these cracks in a volcanic cone, and their injection by liquid lava, must of course distend the mountainous mass and increase its volume. If we visit the great crater-walls of Somma in Vesuvius, and of the Val del Bove in Etna, we shall find that the dykes are so numerous that they make up a considerable portion of the mass. When the loose scoriæ and tuffs are removed by denudation, these hard dykes often stand up prominently like great walls, as represented in [fig. 55]. Even in such cases as these, however, it is doubtful whether the bulk of all the dykes put together exceeds one-tenth of that of the lavas and fragmentary materials.
