280 POPULAR SCIENCE MONTHLY. 



dent — 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 metamor- 

 phosis of the lower sediments by the rise of the isogeotherms (measur- 

 ing 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 prov- 

 ince 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 Eichthofen 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 lead- 

 ing factors in determining the mechanism of volcanie 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 



