Fig. 146.—Volcanic plug exposed by natural dissection of a volcanic cone in Colorado (U. S. G. S.).
Fig. 147.—A dike cutting beds of tuff in a partly dissected volcano of southwestern Colorado (after Howe, U. S. G. S.).
Not infrequently the beds of tuff composing the flanks of the volcano, upon dissection by the same process, bring to light walls of cooled lava standing in relief ([Fig. 147])—the filling of the fissure which gave outlet to the flanks of the mountain at the time of the eruption. Study of exposed dikes formed in connection with recent eruptions of Vesuvius has shown that in many instances they are still hollow, the lava having drained from them before complete consolidation.
Another agent which is effective in uncovering the buried structures of volcanoes is the action of waves on shores. Always a relatively vigorous erosive agency, the softer structures of volcanic cones are removed with especial facility by this agent. On the shores of the island of Volcano, the little cone of Vulcanello has been nearly half carried away by the waves, so as to reveal with especial perfection the structure of the cinder beds as well as the internal rock skeleton of the mass. Here the characteristic dips of lava streams, intercalated as they now are between tuff deposits and the lava which consolidated in fissures, are both revealed.
Fig. 148.—Map and general view of St. Paul’s Rocks, a volcanic cone dissected by waves.
In mid-Atlantic a quite perfect crater, the St. Paul’s Rocks, has been cut nearly in half so as to produce a natural harbor ([Fig. 148]).