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With the Santorin group we conclude our account of the active European volcanoes. It may be observed, however, that from some cause not ascertained the volcanic districts of the Mediterranean and its shores are confined to the north side of that great inland sea; so that as regards vulcanicity the African coast presents a striking contrast to that of the opposite side.
In most of these cases the secondary phenomena of vulcanicity are abundantly manifest; but the great exhibitions of igneous action, when the plains were devastated by sheets of lava, and cones and craters were piled up through hundreds and thousands of feet, have for the present, at least, passed away. Geol.-topographischer Atlas von Neu-Seeland, von Dr. Ferd. von Hochstetter und Dr. A. Petermann.
As a matter of fact, the "elevatory theory," or the "elevation-crater theory," as it is called by Scrope, has been almost universally abandoned by writers on vulcanicity. Principal Varieties of Volcanic Mountains as regards Form.
On the contrary, these more prominent features of vulcanicity over the surface of the ground have been removed by the agents of denudation, and our observations are confined to the phenomena presented by extensive sheets of lava and beds of ash, or the stumps and necks of former vents of eruption, together with dykes of trap by which the plateau-lavas are everywhere traversed or intersected.
In the vast enclosure that was originally the floor of the crater-mountain Catharina, several crater rings, only a third, a quarter, or a fifth as great in diameter, have broken forth, and these in turn have been partially destroyed, while in the interior of the oldest of them yet smaller craters, a nest of them, mere Etnas, Cotopaxis, and Kilaueas in magnitude, simple pinheads on the moon, have opened their tiny jaws in weak and ineffective expression of the waning energies of a still later epoch, which followed the truly heroic age of lunar vulcanicity.
Together with these materials are sheets of lava extruded in a molten condition from the sides or throat of the crater itself. Professor J. W. Judd, F.R.S., in his able work entitled, Volcanoes: What they are, and what they teach, has furnished the student of vulcanicity with a very complete manual of a general character on the subject.
Lastly, turning our attention to the European area, notwithstanding the still active condition of Etna, Vesuvius, and a few adjoining islands, we see in all directions throughout Southern Italy evidences of volcanic operations of a past time, such as extinct crater-cones, lakes occupying the craters of former volcanoes, and extensive deposits of tuff or streams of lava all concurring in giving evidence of a period now past, when vulcanicity was widespread over regions where its presence is now never felt except when some earthquake shock, like that of the Riviera, brings home to our minds the fact that the motive force is still beneath our feet, though under restrained conditions as compared with a former period.
The far greater effects of lunar vulcanicity, as compared with those of our globe, may be accounted for to some extent by the consideration that the force of gravity on the surface of the moon is only one-sixth of that on the surface of the earth.
Some of the earlier writers of modern times on the subject of vulcanicity such as Sir Humphrey Davy and Leopold von Buch maintained that the conical form was due to upheaval by a force acting from below at a central focus, whereby the materials of which the mountain is formed were forced to assume a quâ-quâ versal position that is, a position in which the materials dip away from the central focus in every direction.
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