Page:Popular Science Monthly Volume 61.djvu/287

Rh a hydrostatic equilibrium and spreads out in laccolites without ever reaching the surface, though normally it forces its way upward through the lightest rocks (and these are those of mountains) well toward the surface; and (6) that as the mass approaches the surface so closely as to find relief from the subterranean pressure, its volatile constituents (chiefly occluded water) flash into gas, usually with explosive violence. Now if the lava column is of exceptionally viscous material, either because exceptionally dry or because exceptionally acidic in composition, the explosively expanding steam and other gases inflate it into pumice, or even blow it into dust; while if exceptionally fluent, by reason either of wetness or of basic composition, the explosion is less violent, the steam bubbles out as from boiling liquid, and the lava flows over the crater-rim, or through some chasm rent by its own enormous weight, in streams extending perhaps for many miles—as in Kilauea in 1841, and in the New Mexican volcano near Grant shortly before the Columbian discovery. Commonly it happens that the lighter lavas first extruded are the more viscous, the later and heavier material more fluent; so that the initial manifestation is commonly more decidedly explosive, the action then running down to relatively quiescent outflow; and their other relations depending on composition of the lava, etc., too complex for ready summing.

In the light of the natural history of vulcanism, and of the mechanism traced through the phylogeny of the volcano, the Antillean eruptions may readily be placed in the general scheme of knowledge. Both Mont Pelee and La Souffriere lie in a volcanic province in which the activity culminated ages ago, so that their activity may be likened to the dying throes of a Vulcanean giant; and this fact, while by no means to be interpreted in definite prophecy, is one of some promise to future generations. Again, both volcanoes approach the Krakatoan character rather than the innocent type of Stromboli; this character is destructive in itself; moreover, in view of the normal passage from initial explosion to final outwelling of quiet lava streams, it is to be regarded as an indication either (1) that the crisis of the spasm is not yet passed, or (2) that the andesitic lavas thus far outcast are precursors of more completely differentiated matter to be erupted during coming millenniums. In either case the outlook is less roseate than the humanitarian student would wish; for the fact that the region is one in which vulcanism is decadent when it is measured by geologic ages is of far less immediate interest than the prospect measured either in days of the single vulcanean spasm, or in millenniums of the life history of particular vents.