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Page:Popular Science Monthly Volume 19.djvu/63

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53
ORIGIN AND STRUCTURE OF VOLCANIC CONES.

opposite view, which will be immediately explained, claimed as their adherents Sir Charles Lyell, Poulett Scrope, and others.

In Figs. 3 and 4 are diagrammatic representations of the two theories.

The upheavalists believed that the earth-crust actually surrounding the vent was bodily lifted up by the subterranean igneous forces into a dome-shaped or bubble-like mass, thus forming the main mass of the

PSM V19 D063 Crater of upheaval.jpg
Fig. 4.—Crater of Upheaval.

cone, of which the center was the point of fracture, and therefore the vent. The ejecta were therefore considered to form only a thin superficial crust covering this. The subjacent rock which had been elevated would thus have a quaquaversal or periclinal dip away on all sides from the chimney (Fig. 4).

The opponents to this view attribute the entire bulk of the mountain to the ejecta, as seen in Fig. 3, the only change in the basement beds being those produced by pressure and excavation, both of which tend to make them dip toward the vent, thus producing quite a converse effect to the former.

This latter view certainly seems the most feasible, and, after a careful examination of many of the old craters brought forward by the upheavalists as evidence, one becomes satisfied that they have wrongly interpreted facts, which the more advanced state of knowledge at the present day and the collected experience of subsequent observers make easy to our perception. On the other hand, it would be undoubtedly rash to conclude that all craters were formed entirely on one or the other model. Jorullo, in Mexico, for instance, has many points about it to support the upheaval theory. David Forbes, that clear observer, mentions many facts about South American volcanoes that should deter us from admitting the formation of cones and craters by the deposition of ejecta only.

The rapidity with which a volcanic cone may be raised is a point of great interest. We hear every now and then of some small island appearing and again disappearing below the sea almost as rapidly as it rose. Probably, however, the best illustration is that of Monte Nuovo, four hundred and fifty-six feet high, situated in the Campi Phlegraci, about eight miles west of Naples. This was raised from a