(f.) Volcanic Groups.—Besides the volcanic vents arranged in lines, of which we have treated above, there are a large number, both active and extinct, which appear to be disposed in groups, or sporadically distributed, over various portions of the earth's surface. I say appear to be, because this sporadic distribution may really be resolvable (at least in some cases) into linear distribution for short distances. Thus the Neapolitan Group, which might at first sight seem to be arranged round Vesuvius as a centre, really resolves itself into a line of active and extinct vents of eruption, ranging across Italy from the Tyrrhenian Sea to the Adriatic, through Ischia, Procida, Monte Nuovo and the Phlegræan Fields, Vesuvius, and Mount Vultur.[12] Again, the extinct volcanoes of Central France, which appear to form an isolated group, indicate, when viewed in detail, a linear arrangement ranging from north to south.[13] Another region over which extinct craters are distributed lies along the banks of the Rhine, above Bonn and the Moselle; a fourth in Hungary; a fifth in Asia Minor and Northern Palestine; and a sixth in Central Asia around Lake Balkash. These are all continental, and the linear distribution is not apparent.
[1] For an interesting account of this range of volcanic islands see Kingsley's At Last. The grandest volcanic peak is that of Guadeloupe, rising to a height of 5000 feet above the ocean, amidst a group of fourteen extinct craters. But the most active vent of the range is the Souffrière of St. Vincent. In the eruption of 1812 this mountain sent forth clouds of pumice, scoriæ and ashes, some of which were carried by an upper counter current to Barbados, one hundred miles to the eastward, covering the surface with volcanic dust to a depth of several inches.
[2] An excellent, and perhaps the most recent, map of this kind is that given by Professor Prestwich in his Geology, vol. i. p. 216. One on a larger scale is that by Keith Johnston in his Physical Atlas.
[3] Memoir on the Physical Geology and Geography of Arabia Petræa, Palestine, etc., published for the Committee of the Palestine Exploration Fund (1886), p. 113, etc.
[4] This mountain was ascended in 1837 by Mr. Taylor Thomson, who found the summit covered with sulphur, and from a cone fumes at a high temperature issued forth, but there was no eruption.—Journ. Roy. Geographical Soc., vol. viii. p. 109.
[5] Humboldt, Atlas der Kleineren Schriften (1853).
[6] Ascended by Whymper June 29, 1880. He found the elevation to be 16,515 feet.
[7] The arrangement of the volcanoes of Peru and Bolivia is also suggestive of a double line of fissure, while those of Chili suggest one single line. The volcanoes of Arequipa, in the southern part of Peru, are dealt with by Dr. F. H. Hatch, in his inaugural dissertation, Ueber die Gesteine der Vulcan-Gruppe von Arequipa (Wien, 1886). The volcanoes rise to great elevations, having their summits capped by snow. The volcano of Charchani, lying to the north of Arequipa, reaches an elevation of 18,382 Parisian feet. That of Pichupichu reaches a height of 17,355 Par. feet. The central cone of Misti has been variously estimated to range from 17,240 to 19,000 Par. feet. The rocks of which the mountains are composed consist of varieties of andesite.
[8] D. Forbes, "On the Geology of Bolivia and Southern Peru," Quarterly Journal of the Geological Society, vol. xvii. p. 22 (1861).
[9] Darwin, Structure and Distribution of Coral Reefs, second edition, p. 186.