Volcanoes, active or extinct, are found in those portions of the earth which have been subject to extensive disturb ances of the strata. It has been custom ary to speak of their distribution as occurring in chains. This is but a partial truth, and, as usually stated, involves a rather strained use of the idea of linear arrangements; and it is believed that the notion is in some important re spects a misleading one. The general law seems to be that volcanoes occur in regions which have been subject to that mysterious subterranean action which elevates the land, builds moun tains and plateaus, and deforms the strata; and there is reason to infer that they constitute one of several classes of phenomena which ultimately originate in the same category of causation. It was suggested by Charles Darwin that volcanoes occur in regions which are undergoing elevation. So far as it has been possible to put this suggestion to test it seems to have been sustained by observation. The determining cause of volcanic action has been subject to much speculation, but no satisfactory theory of it has yet been proposed. The view which has received the most considera• tion is that surface waters penetrate the earth to depths where the tempera ture is sufficient to melt the rocks, thus furnishing the elastic agent under con ditions of temperature and pressure which are adequate to the result.
The principal theaters of modern vol canic action are the Chilean Andes, the Andes of Ecuador and Peru, Central America, the Dutch East Indies, and the Philippines, Iceland, and the Mediter ranean. To these must be added the remarkable Hawaiian volcanoes, and perhaps also those of New Zealand. We have reason also to infer that many ex tensive outbreaks occur beneath the ocean, but we know very little about them. The number of active volcanoes is usually reckoned as about 350; but it is difficult in many of these cases to de cide whether the volcano is active or extinct. But the number now active is small indeed in comparison with those which have become extinct in compara tively recent geological times. Many
regions abound in volcanic piles which in the course of human history have given no sign of activity. This is espe cially the case in the W. part of the United States, and most especially in the N. W. States and Territories. Here may be found great regions tens of i thousands of square miles in extent which have been overflowed with lava, and the successive sheets are thousands of feet in thickness. In many portions of the West the indications point to a very recent epoch for the eruptions, and it is by no means improbable that some of them have occurred since the Span ish conquest. In all ages of the geological history of the planet, and as far back as we are able to discriminate the rela tive antiquity of the strata, volcanic eruptions have been abundant and of the same general character as those which now prevail. Nor is it possible to sPy whether they were generally more extensive or less so than at present. Two of the most remarkable of active vol canoes are Mauna Loa and Kilauea, in Hawaii; the former 13,760 feet in height and the latter, though only about 4,000 feet high, 8 miles in circumfer ence. On the summit of Mauna Loa is a circular crater 8,000 feet in diameter, its walls nearly vertical and many hun dreds of feet deep. The eruptions of this volcano have been of great volume and at an average interval of eight years; each outflow representing more lava than Vesuvius has sent out since the de struction of Pompeii. The Hot Lakes district of North Island, New Zealand, was the scene of remarkable volcanic disturbances in 1886. The effect ex tended over 60 square miles, burying whole villages and causing much loss of life. The eruption was accompanied by violent earthquakes and hundreds of new geysers broke out. The fine vol canic dust that settled over the province of Auckland proved to be beneficial to vegetation. The most destructive of modern volcanic eruptions was that of Mont Pelde in the island of Martinique in May, 1902.