Page:Popular Science Monthly Volume 61.djvu/286

280 —that it culminated before the beginning of that definite world—growth recorded in the stratified rocks, revived locally during various periods down to the later Tertiary, and is probably less vigorous to-day than during any earlier eon of geologic time. Going further into detail, Powell has defined what may be called the normal sequence of vulcanism in any particular geologic province: The first stage in this sequence is that of loading, or accumulation of sediments in areas of deposition; the second is that of baking, compression, and metamorphosis of the lower sediments by the rise of the isogeotherms (measuring proper terrestrial heat); the third stage is that of uplift, partly by reason of the expansion and crumpling consequent on the heating from below; the fourth stage is that of unloading, or degradation by rain and rivers; and the final stage is that of vulcanism, supervening as the degradation proceeds and sometimes continuing until the province is once more submerged.

The distribution of volcanoes, both on the present earth-face and throughout the periods of earth-growth, covers essential phases of the natural history of vulcanism; yet the mechanism of the volcano remains to be traced through specific interpretation of both processes and products, so far as these lie within reach of observation. An epoch-marking step towards the interpretation of volcanic products was made by Baron von Richthofen a third of a century ago, when he recognized a natural system of volcanic rocks; and the goal was attained when Dutton, in a flash of genius, saw 'the double function of density and fusibility' ('Geology of the High Plateaus of Utah' 1880, p. 137) which conditions the extrusion of molten rock-matter. Diller, Iddings, Lawson and others have extended the interpretation; yet the later researches have but established the inference that density, or specific gravity, and fusibility (itself affected by wetness) are leading factors in determining the mechanism of volcanic action. Briefly, it may be said (1) that the seat of normal volcanic action is deep in the earth-crust, so deep that the vertical column of denser sea-bottom rocks is heavier than the longer column of mountain rocks rising thence to the cratered crest; (2) that here the rock-matter is subjected to the enormous pressure of superincumbent miles of rock, yet so highly heated as to become mobile with any relief from pressure; (3) that by reason of the variable strains due to unloading or other causes, some portion of this confined rock-matter is sufficiently relieved from pressure to become mobile, whereupon it seeks the level determined by its density, and forces itself upward through any overlying strata of greater density in channels or vents enlarged by continuous flow; (4) that the molten rock arranges itself in the vent in the order of specific gravity, the lighter above, the heavier (and generally wetter) below; (5) that sometimes the upwelling stream of molten rock reaches