338 PHYSICAL GEOLOGY 



blocks of the earth, when the earth's crust is yielding to strains; 

 (3) some, that it is chiefly or entirely primal, i.e., derived from a sub- 

 stratum of unknown thickness. Of these, the last (3) seems to be 

 more in accord with the known facts (p. 337) than the others. 



The activity of a given volcano is usually independent of all others, 

 as is shown in the history of Mauna Loa and Kilauea (p. 308), which, 

 though forming one great mound of lava, erupt independently. On 

 the other hand, eruptions of Pelee and Soufriere on the West 

 Indian islands of Martinique and St. Vincent have been almost 

 simultaneous. 



Origin of Volcanic Gases. — The problem of the origin of gases 

 and water vapor is to a large extent identical with that of the origin 

 of lava. It has been proved by experiment that all rocks, even the 

 most dense and most crystalline, contain large quantities of gas, so 

 that a comparatively small volume of rock would be sufficient to 

 furnish practically all of the gases and all of the water vapor given off 

 during an eruption even of the first magnitude. It has been held, 

 however, that water vapor, which constitutes the greater part of the 

 emanations of dormant volcanoes, as has been stated, is derived, to 

 a large extent at least, from either sea water or from meteoric water 

 which has percolated down to the molten lava and been absorbed 

 by it. 



Cause of the Ascension of Lava. — Every theory of volcanism 

 must account for the force which raises the lava to the surface of 

 the earth and often throws it as fine dust thousands of feet into the 

 air. There is general agreement that this force is to be found (1) 

 in the tidal and other strains to which the earth is subjected ; (2) in 

 hydrostatic pressure resulting from the weight of the overlying rock; 



(3) in the enormous expansive force of the gases dissolved in the 

 molten magma, whether original or derived from other sources; and 



(4) to some degree in the expansional energy of the injected 

 mass. 



Cause of Periodicity. — The lava which cools in the throat of a 

 volcano is characteristically tough. Since the cones of explosive 

 volcanoes are built of loose ash deposits of little strength, it is evi- 

 dent that if renewed activity were to result from an explosion alone, 

 ;iii opening would usually be made through the side of the mountain 

 Instead of through the crater. The latter is, however, usually the 

 case. There must therefore be some preliminary weakening of 

 the plug, and apparently the only cause for such weakening is to be 



