Volcanic Districts. — TheAfrican islands are nearly all volcanic, though, as in St. lIelena, 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 Buck) On the continent, traces of volcanic action appear in the chain of the Atlas; while in the northern as 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 many 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. Roque 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 land, whether persi4tent 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 those 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 Pasta, 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 duo 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 Ladrone 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 beeu 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 cast 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 palmozoic) 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, ' l'hys. 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 of its surface, by the simple laying on of material, whether abraded from the land, or chemically abstracted from the sea itself. That matter is in process of abrasion and transportation from the land into the ocean at every instant, and along every coast lino, we know as a matter of fact,. We know, too, that all existing strata. however enor mous their thickness, hare been formed at the bottom of the sea, and it is, therefore, no hypothesis, but a perfectly legitimate assumption, that the same process is still in progress, no matter how slowly, from this cause, at least iu the vicinity of coast lines ; and when we look at the vast amount of organised exuviaz which constitute so large a portion of many of the secondary and tertiary beds—the secretions of mollusc, infusoria, and zoophytes and bearing in mind the largo proportion of continental substance which has been so formed, look to the evidence afforded by deep-sea soundings, and by coral forma tions, that the same process is going forward in open sea, far out of the reach of coast washing and river deposit (the material being takes up chemically by the river and coast waters, and chemically extracted frost them, when diffused by currents, by the processes of organic life) we shall at once perceive that any amount of pressure on the one hand and relief on the other. which the geologist can possibly require to work out his problem, and any law of distribution of that relief and that pressure, is available without calling in the aid of unknown causes." In apposition with these views of the physical geographer, taken, as it were, from an eminence based on all science (and to which we shall return). we may appropriately place the most recent view of the rela tions of earthquakes and volcanoes, regarded on a cosmical scale, that taken by Mr. Mallet in his fourth report, the following portions of which embrace the principal points of the subject, as resulting from his own seismological researches and those of M. Perrey.