Page:Popular Science Monthly Volume 61.djvu/285

Rh clear in the light of what may be called the natural history of vulcanisni; for, just as the life history of organic orders and genera is traced only by aid of fossils, so the ontogeny of the volcano may be viewed in the light of the phylogeny traced through its fossil remains—lava sheets, tuff beds, laccolites, volcanic necks like those of the Mount Taylor plateau, and other products of organic action during the ages past.

Naturally the first question concerning the volcano relates to its character; and this is answered partly by such dynamic facts as those recently observed in the Antilles, partly by the static records of the rocks. In the light of the various phenomena, it is convenient to recognize three types of eruption, viz: (1) the Stromboli type, or that of quiet outflow of highly fluent lava; (2) the type of Vesuvius, or that of explosive eruption usually followed by quieter flows of lava; and (3) the Krakatoa type, represented by violent explosions with little, if any, extravasation of lava. In some measure the types intergrade, the middle one, indeed, approaching the extremes; yet they are so connected with the character of the erupted material and other factors as to demand recognition.

The second inquiry concerning the world's volcanoes relates to geographic distribution; and this is well answered by any convenient map, such as that of Bonney (reproduced in the June number of this journal, page 187), from which it appears that nearly all of the living and recently extinct craters are arranged in lines, or zones, coinciding approximately with continental boundaries. Two of the most striking volcanic belts of the globe are those following the chain of Aleutian islands, and that of the Lesser Antilles from Porto Rico southward to the mouth of Orinoco river; several of the world's largest volcanoes occur in the interlacing zones lying off southeastern Asia; and the world's longest belt begins with the Aleutian chain, follows the coast-wise mountains of western North America, traverses Central America, takes in the great Andean volcanoes of western South America, and stretches thence to Terra del Fuego, if not across to Antarctica to end with Mounts Erebus and Terror. The volcanic belts of the globe are sometimes styled 'lines of weakness' in the earth-crust, though a sufficient number of live or recently dead craters lie apart on oceanic islands or in continental interiors to caution conservative geologists against too simple groupings; yet all the facts seem to fall into the generalization that volcanic regions coincide with zones of exceptional activity in continent-making agencies.

A third inquiry concerning volcanoes relates to their geologic distribution, or—in ultimate analysis—to their connection with other geologic agencies and processes. The observations of numberless geologists in the different countries of the globe seem to answer this inquiry in general terms, by indicating that the agency of vulcanism is