In still other instances we may thank the volcano itself for opening up the interior of the mountain for our inspection. The eruption in 1888 of the Japanese volcano of Bandai-san, by removing a considerable part of the ancient cone, has afforded us a section completely through the mountain. The summit and one side of the small Bandai was carried completely away, and there was substituted a yawning crater eccentric to the former mountain and having its highest wall no less than 1500 feet in height ([Fig. 149]). In two hours from the first warning of the explosion the catastrophe was complete and the eruption over.
Fig. 149.—Dissection by explosion of Little Bandai-san in 1888 (after Sekiya).
The eruption of Krakatoa in 1883, probably the grandest observed volcanic explosion in historic times, left a volcanic cone divided almost in half and open to inspection ([Fig. 150]). Rakata, Danan, and Perbuatan had before constituted a line of cones built up round individual craters subsequent to the partial destruction of an earlier caldera, portions of which were still existent in the islands Verlaten and Lang. By the eruption of 1883 all the exposed parts and considerable submerged portions of the two smaller cones were entirely destroyed, and the larger one, known as Rakata, was divided just outside the plug so as to leave a precipitous wall rising directly from the sea and showing lava streams in alternation with somewhat thicker tuff layers, the whole knit together by numerous lava dikes.
Fig. 150.—The half-submerged volcano of Krakatoa in the Sunda Straits before and after the eruption of 1883 (after Verbeek).
In order to carry our dissecting process down to levels below the base of the volcanic mountain, it is usually necessary to inspect the results of erosion by running water. Here the plug or chimney, instead of being surrounded by tuff, is inclosed by the country rock of the region, which is commonly a sedimentary formation. Such exposed lower sections of volcanic chimneys are numerous along the northwestern shores of the British Isles. Where aligned upon a dislocation or noteworthy fissure in the rocks, the group of plugs has been referred to as a scar or cicatrice ([Fig. 151]). Associated with the plugs of the cicatrice are not infrequently dikes, or, it may be, sheets of lava extended between layers of sediment and known as sills.
Fig. 151.—The cicatrice of the Banat (after Suess).
If we are able to continue the dissection process to still greater depths, we encounter at last igneous rock having a texture known as granitic and indicating that the process of consolidation was not only exceedingly slow but also uninterrupted. This rock is found in masses of larger dimensions, and though generally of more or less irregular form, no one dimension is of a different order of magnitude from the others. Such masses are commonly described as bosses, or, if especially large, as batholites ([Fig. 152]). Wherever the rock beds appear as though they had been forced up by the upward pressure of the igneous mass, the latter takes the form of a mushroom and has been described as a laccolite ([Figs. 479-481], [pp. 441-442]). Evidence seems, however, to accumulate that in the greater number of cases the molten rock has fused its way upward, in part assimilating and in part inclosing the rock which it encountered. This process of upward fusion has been likened to the progress of a red hot iron burning its way through a board.