In the section, [fig. 27], p. 104, copied from a drawing of a sea-cliff in the Island of Vulcano, a transverse section of a lava-stream is represented on a somewhat larger scale. The upper and under surface of the lava-stream is seen to have a scoriaceous structure, but the thick central mass is compact, and divided by regular joint-planes. This section also illustrates the fact that, before the lava-stream flowed down the sides of the mountain, a valley had been cut by meteoric agencies on the flanks of the volcano, the dykes which traverse the lower beds of tuff being abruptly truncated.
In mountain ravines, upon the slopes of ancient volcanoes, and in the cliffs of volcanic islands, we are often able to study the way in which these great mountain masses are built up of alternating lava-currents, beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting dykes. In [fig. 48], the features above described are illustrated by a section in the sides of the great volcano of Mont Dore.
Fig. 48.—Section seen at the cascade. Bains du Mont Dore.
Fig. 49.—Section in the Island of Ventotienne, showing a great stream of andesitic lava overlying stratified tuffs.
SECTIONS IN THE PONZA ISLANDS.
In figs. [49], [50], [51], and [52], we have given drawings of portions of the sea-cliffs in several of the Ponza Islands, a small volcanic group off the Italian coast.
Fig. 50.—Cliff on the south side of the Island of San Stephano.
a. Trachyte lava-stream, with a scoriaceous upper surface overlaid by stratified tuffs, b.