VOLCANO. 



VOLCANO. 



608 





and predominates towards the close of the eruption, and even, by the 

 aid of the residual internal heat operating on sulphur, is evolved for 

 centuries after the volcanic violence is spent. 



The evolution of chlorine is easily traced to a double decomposition 

 of sea-salt ; carbonic acid is supposed to rise from calcined limestone 

 rocks ; and specular iron-ore (fer otiyiste) is a product of sublimation. 

 Thus, in the opinion of Dr. l)aubeny, all the main phenomena conco- 

 mitant upon volcanic action seem to admit of explanation if we suppose, 

 first, sea-water, and afterwards atmospheric air, admitted to consider- 

 able masses of metals, metalloids, sulphur, &c., the basis of the whole 

 speculation being the abundant decomposition of water at a moderate 

 depth below the surface of the earth. The views of Dr. Daubeny have 

 been controverted by very eminent writers (aa Dr. Davy, Prof. Bischof, 

 and others) on particular points ; but we are not aware of any attempt 

 upon other assumptions quite so satisfactory as this of Dr. Daitbeny, to 

 explain generally the chemical products of volcanic eruptions in the 

 order of their occurrence. 



Perhaps, therefore, we may concede to this hypothesis the proba- 

 bility that in the interior of the earth the metalloids exist in quantity 

 sufficient to cause an abundant decomposition of water, and thus 

 originate a given series of chemical changes such as are witnessed in 

 volcanic eruptions. But before we accept it as a general explanation of 

 volcanic disturbances, other classes of data than those furnished by 

 chemical analysis must be brought into the reasoning. 



An important circumstance in the general theory of volcanoes is the 

 connection and reciprocal activity which exists underground between 

 volcanic regions entirely separated on the surface, as between Sicily 

 and Naples; between the Mediterranean volcanic region, taken gene- 

 rally, and the region of extinct (or long suspended) volcanic action in 

 Asia. To this we must add another and larger series of facts regarding 

 the extent and distribution of volcanic action on the surface. 



VrJ,.unic Xeyion and their Connection. Volcanic regions, if estimated 

 by the area over which the lava and ashes have been dispersed, con- 

 stitute but a small portion (perhaps less than in jjth part) of the surface 

 of the globe, but in a survey of these regions we must include not only 

 the active vents and extinct craters, but also " large intermediate spaces 

 where there is abundant evidence that the subterranean fire is at work 

 continuously, for the ground is convulsed from time to time by earth- 

 quakes ; gaseous vapours, especially carbonic acid gas, are disengaged 

 plentifully from the soil ; springs often issue at a very high tempera- 

 ture, and their waters are usually impregnated with the same mineral 

 matters as are discharged by volcanoes during eruptions. (Lyell, ' Princ. 

 of Geology," book ii., ch. ix.) 



To describe these districts would be entirely foreign to the purpose 

 of this essay, but we may by a simple classification show how much of 

 the grandest features of physical geography is due to volcanic dis- 

 turbance. 



European Volcanic Ditricti. The Icelandic volcanoes, remarkable 

 for abundant lava streams ; the Azores, amongst which new islands 

 have been thrown up ; Sicily, including ^tua and the vanished Gra- 

 ham's Island, often called Sciacca, which is properly only the name of 

 the opposite point of Sicily ; the Lipari Isles, with Stromboli always 

 burning ; the Neapolitan tract, including Vesuvius ; Ischia and the 

 . Isles ; Sautorini and some neighbouring islets. The above are 

 all considered as active volcanic centres, and have been subject to 

 eruptions in historical times. The extinct volcanic systems of Europe 

 are the trachytic domes in the centre of France (Auvergne, [AUVERGNE, 

 in GEOO. Div.] the Vivarais, Ac.), the Eifel country, the Seven Moun- 

 tains, and other trachytic and basaltic parts along the Rhine ; the 

 Westcrwald, Vogelsgebirge,Rhongebirge, together with the Kaiserstuhl; 

 and many other scattered basaltic hills in the middle of Germany; 

 Hungary, Transylvania, both remarkably rich in trachytes and the 

 vitreous lavas called pearlstones; the Gleichenberg in Styria. In 

 Italy, the Euganean hills and other smaller points appear in the north 

 of Italy; while between Rome and Naples large ancient craters occur, 

 and connect Mount Albano with Vesuvius. (Daubeny.) 



Asiatic Volcanic Liiitrictt. Mixed active and extinct volcanic moun- 

 tains occur about the Persian Gulf, the Red Sea, and the Dead Sea, in 

 the vicinity of Smyrna, in the Caucasus (especially in Mount Ararat), 

 and in the Elburz Mountains, including Demavend. These are, or 

 appear, detached points of more or less decayed, though once powerful, 

 action, fed by inland seas. On the southern and eastern shores of 

 Asia the subterranean energies are still unsubdued, and constitute a 

 long chain of lofty islands and promontories from Barren Island, in 

 the Bay of Bengal, through the length of Sumatra and Java, by Bally, 

 Su.T'awa (already noticed), Flores, Celebes, Sangir, Mindanao, Kugo, 

 Lucoi. ''onnosa, Loochoo, Japan, the Kurilian Isles, the magnificent 

 in'mntaii.s of Kamtvhatka, examined by Erman, and the line of the 

 Aleutian Isles, to Alaschka, on the western coast of North America. 



African Vulcanic Diitritti. The African islands are nearly all volcanic, 

 though, aa in St. Helena, the action has long been extinct ; or, as in 

 the Canary Isles, the localities once devastated now enjoy immunity 

 through the great safety-valve of Teneritfe. (Von Buch.) On the 

 continent, traces of volcanic action appear in the chain of the Atlas ; 

 while in the northern aa well as in the equatorial portions of the 

 " Mountains of the Moon," now known through Dr. Beke to be a 

 meridional range parallel to the coast, and extending from the north- 



eastern to the south-eastern regions of the continent, are uiany active 

 volcanoes. An account of an eruption of one of these, Jebel Dubbeh, 

 in May last (1861), by Captain R. L. Playfair, R.N., was recently com- 

 municated to the Geological Society. 



The late M. Daussy, geographer to the French Board of Longitude, 

 collected observations of earthquake-shocks received by vessels at sea 

 at various periods, but all within a given limited area, which, according 

 to Mr. Mallet, render the existence almost certain of a vast active 

 volcanic suboceanic area in the basin of the Atlantic, nearly midway 

 between Cape Palmas on the west coast of Africa, and Cape St. Koque 

 on the east coast of South America, or in the narrowest part of the 

 ocean between these continents. This vast disturbed and perhaps 

 partially igneous ocean-floor can be no less than nine degrees, or above 

 620 miles, in length from west to east, and from three to four degrees, 

 or between 200 and 300 miles, in breadth from north to south. We 

 have thus a submarine volcanic tract in activity beneath the Atlantic, 

 as large in area as Great Britain, and where the bottom of the ocean is 

 rendered uneven in the extreme, immense protrusions, that is, eleva- 

 tions of laud, whether persistent or temporary, taking place in deep 

 water. 



American Volcanic Districts. "Along the north-west coast of the 

 American continent," Sir John Herschel states, " the chain of newer 

 igneous formations is almost continuous, and in Oregon attains an 

 immense development; nor are active volcanoes of great magnitude 

 wanting, but only thine parts of the volcanic zone which lie upon the coast- 

 line contain such, namely, Mount Regnier and St. Helen's, at the 

 mouth of the Columbia river." The Rocky Mountains show many 

 marks of ancient volcanic action, and serve incompletely to connect the 

 long Asiatic line just described with another enormous volcanic system 

 running through California and Mexico, interrupted at the isthmus of 

 Darien, but continued through Pasto, Popayan, Quito, Peru, and Chili, 

 to Tierra del Fuego ; in the last locality, however, though there are trap- 

 rocks, there are no active volcanoes. This mighty range of mountains is 

 everywhere parallel to the sea, being only crossed by the line of Mexican 

 volcanoes, which includes the new mountain of Jorullo,and passes perhaps 

 from the West Indies to the Revillagigedo Isles. The volcanic vents are 

 unequally distributed along the great Cordillera : one in California, 

 five in Mexico, and above twenty between this and the isthmus of 

 Darien. South of this point the volcanoes are few, but mostly of 

 prodigious grandeur and frequent activity, the fire issuing from one or 

 other of the mountains, which, according to Humboldt and Darwin, 

 are all parts of one grand swollen-up mass supporting Cotopaxi, 

 Antisana, Tunguragua, and other huge cones. Only one active volcano 

 occurs in Peru, but nineteen are active at frequent intervals in Chili, 

 and one (Villarica) burns almost uninterruptedly. Here also is the 

 highest volcano in the world. Aconcagua, measuring 23,910 feet. Most 

 of the West Indian islands are volcanic, or partly volcanic and partly 

 calcareous, the limestone being mostly due to the growth of corals, 

 perhaps on the craters or round the slopes of volcanic mounds. A 

 similar view appears applicable to the numerous groups of islands in 

 the Pacific Ocean, in some of which, as the Ladroue Isles and Hawaii, 

 are lofty and active volcanoes. 



In general the Banda Isles, New Guinea, New Britain, Norfolk 

 Island, and St. Philip, the Society, and the Sandwich Islands, are princi- 

 pally of volcanic origin. The low lagoon islands, described by Mr. 

 Stutchbury ^' Journal of the British Institution') as deriving this form 

 from the growth of coral, have been thought to be so many points of 

 volcanic mounds ; but it has been suggested by Darwin (' Trans. Geol. 

 Soc.') that they are points of subsided land, on which the zoophyta 

 attached themselves. Western Australia contains basaltic and other 

 volcanic accumulations. 



" The east coast of Australia offers no active volcano, but is marked 

 along its whole extent, from north to south, with evidences of former 

 igneous activity, 'occurring (in striking resemblance with what prevails 

 on the opposite coast in South America) among the crystalline and 

 transition (primary and palaeozoic) rocks which constitute the general 

 seaboard. But the subterranean fires would seem here to have shifted 

 their ground, and taken up a new line of action to seaward, at an 

 interval of from 1000 to 1200 geographical miles from the coast, but 

 still conforming to its curvature, prolonging the series through the 

 Solomon Islands, New Hebrides, and Friendly Islands, to New Zealand. 

 (Herschel, ' Phys. Geog.') 



On comparing this synopsis of the geographical distribution of 

 volcanoes with a good map of the world, two remarkable features of 

 that distribution will be obvious : these are, in the words of the phi- 

 losopher last cited, ' Their tendency to a linear arrangement when 

 insular," and " their constant association with coast lines." 



In reference to these features, and to the phenomena described in the 

 preceding review of volcanic regions and their connection, Sir J. Herschel 

 resumes : " It seems impossible to disconnect this obviously systematic 

 arrangement with the general evidence we have, from other sources, of 

 the tendency to continued elevation of the coast line of the Andes, 

 and, indeed, of the whole continent of South America, on the one hand, 

 and of the depression over a large portion of the bed of the Pacific on 

 the other alternations which would naturally result from a change in 

 the incidence of pressure on the general substratum of liquefied matter 

 which supports the whole. The bed of an ocean supported on a yield- 

 ing substratum may be depressed without a corresponding depression 



