Part 2 (1/2)

(_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 Phlegraean 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 Souffriere of St. Vincent. In the eruption of 1812 this mountain sent forth clouds of pumice, scoriae 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 Petraea, 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.

[10] Erman, _Reise um die Welt_.

[11] Milne, ”Cruise amongst the Kurile Islands,” _Geol. Mag._, New Ser.

(August 1879).

[12] See Daubeny, _Volcanoes_, Map I.

[13] Sir A. Geikie has connected as a line of volcanic vents those of Sicily, Italy, Central France, the N. E. of Ireland, the Inner Hebrides and Iceland, of which the central vents are extinct or dormant, the extremities active.

CHAPTER IV.

MID-OCEAN VOLCANIC ISLANDS.

_Oceanic Islands._--By far the most extensive regions with sporadically distributed volcanic vents, both active and extinct, are those which are overspread by the waters of the ocean, where the vents emerge in the form of islands. These are to be found in all the great oceans, the Atlantic, the Pacific, and the Indian; but are especially numerous over the central Pacific region. As Kotzebue and subsequently Darwin have pointed out, all the islands of the Pacific are either coral-reefs or of volcanic origin; and many of these rise from great depths; that is to say, from depths of 1000 to 2000 fathoms. It is unnecessary here to attempt to enumerate all these islands which rise in solitary grandeur from the surface of the ocean, and are the scenes of volcanic operations; a few may, however, be enumerated.

[Ill.u.s.tration: Fig. 4.--The Peak of Teneriffe (Pic de Teyde) as seen from the ocean.--(From a photograph.)]

(_a._) _Iceland._--In the Atlantic, Iceland first claims notice, owing to the magnitude and number of its active vents and the variety of the accompanying phenomena, especially the geysers. As Lyell has observed,[1] with the exception of Etna and Vesuvius, the most complete chronological records of a series of eruptions in existence are those of Iceland, which come down from the ninth century of our era, and which go to show that since the twelfth century there has never been an interval of more than forty years without either an eruption or a great earthquake. So intense is the volcanic energy in this island that some of the eruptions of Hecla have lasted six years without cessation.

Earthquakes have often shaken the whole island at once, causing great changes in the interior, such as the sinking down of hills, the rending of mountains, the desertion by rivers of their channels, and the appearance of new lakes. New islands have often been thrown up near the coast, while others have disappeared. In the intervals between the eruptions, innumerable hot springs afford vent to the subterranean heat, and solfataras discharge copious streams of inflammable matter. The volcanoes in different parts of the island are observed, like those of the Phlegraean Fields, to be in activity by turns, one vent serving for a time as a safety-valve for the others. The most memorable eruption of recent years was that of Skaptar Jokul in 1783, when a new island was thrown up, and two torrents of lava issued forth, one 45 and the other 50 miles in length, and which, according to the estimate of Professor Bischoff, contained matter surpa.s.sing in magnitude the bulk of Mont Blanc. One of these streams filled up a large lake, and, entering the channel of the Skapta, completely dried up the river. The volcanoes of Iceland may be considered as safety-valves to the region in which lie the British Isles.

(_b._) _The Azores, Canary, and Cape de Verde Groups._--This group of volcanic isles rises from deep Atlantic waters north of the Equator, and the vents of eruption are partially active, partially dormant, or extinct. It must be supposed, however, that at a former period volcanic action was vastly more energetic than at present; for, except at the Grand Canary, Gomera, Forta Ventura, and Lancerote, where various non-volcanic rocks are found, these islands appear to have been built up from their foundations of eruptive materials. The highest point in the Azores is the Peak of Pico, which rises to a height of 7016 feet above the ocean. But this great elevation is surpa.s.sed by that of the Peak of Teneriffe (or Pic de Teyde) in the Canaries, which attains to an elevation of 12,225 feet, as determined by Professor Piazzi Smyth.[2]

This great volcanic cone, rising from the ocean, its summit shrouded in snow, and often protruding above the clouds, must be an object of uncommon beauty and interest when seen from the deck of a s.h.i.+p. (Fig.

4.) The central cone, formed of trachyte, pumice, obsidian, and ashes, rises out of a vast caldron of older basaltic rocks with precipitous inner walls--much as the cone of Vesuvius rises from within the partially encircling walls of Somma. (Fig. 5.) From the summit issue forth sulphurous vapours, but no flame.