FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS.
The origin of volcanic cones with crater-shaped summits has been alluded to in the last chapter ([p. 368.]), and more fully explained in the "Principles of Geology" (chaps. xxiv. to xxvii.), where Vesuvius, Etna, Santorin, and Barren Island were described. The more ancient portions of those mountains or islands, formed long before the times of history, exhibit the same external features and internal structure which belong to most of the extinct volcanos of still higher antiquity.
The island of Palma, for example, one of the Canaries, offers an excellent illustration of what, in common with many others, I regard as the ruins of a large dome-shaped mass formed by a series of eruptions proceeding from a crater at the summit, this crater having been since replaced by a larger cavity, the origin of which has afforded geologists an ample field for discussion and speculation.
Fig. 455.
View of the Isle of Palma, and of the entrance into the central cavity or Caldera. From Von Buch's "Canary Islands."
Fig. 456.
