98 IGNEOUS AGENCIES. 



The lava is often, however, in a frothy or vesicular condition. In such 

 cases the pressure necessary to produce overflow would be much less. 

 But, on the other hand, the force in most cases is not only sufficient to 

 lift lava to the top of the crater, but to project it thousands of feet in 

 the air. A rock-mass of over 2,700 cubic feet was projected from the 

 crater of Ootopaxi to a distance of nine miles (Lyell). The agent of 

 this prodigious force is evidently gas and vapors, especially steam. The 

 great quantity of steam issuing from all volcanoes, but especially from 

 those of the explosive type, is sufficient proof. Thus far theorists gen- 

 erally agree, but from this point opinions diverge into the most oppo- 

 site directions. 



The Heat. — There are many and diverse opinions as to the source of 

 the heat associated with volcanic eruptions. Two prominent views, 

 however, may be said to divide geologists. According to the one, the 

 heat is the remains of the primal heat of the once universally incandes- 

 cent earth ; according to the other, the heat is produced by chemical 

 or mechanical action. According to the former, the heat is general, and 

 only the access of water is local ; according to the latter, both the heat 

 and the access of water are local. According to the former, volcanoes 

 are openings through the comparatively thin crust, revealing the uni- 

 versal interior fluid ; according to the latter, they are openings into 

 isolated interior lakes of molten matter. The former may be called the 

 " interior fluidity " theory ; the latter divides into two branches, which 

 may be called respectively the " chemical " and the " mechanical " the- 

 ory. In all, access of water to the hot interior furnishes the force. 



Internal Fluidity Theory. — This theory supposes that the earth, from 

 its original incandescent condition, slowly cooled and formed a surface- 

 crust ; that this surface-crust, though ever thickening by additions to 

 its interior surface, is still comparatively very thin, and beneath it is 

 still the universal incandescent liquid ; that by movements of the sur- 

 face the solid crust is fissured, and water from the sea or from other 

 sources finds its way to the incandescent liquid mass, and develops 

 elastic force sufficient to produce eruption. 



By this view the focus of volcanoes is situated at the lower limit of 

 the solid crust. The theory seems clear and simple enough, but when 

 closely examined there are many difficulties in the way of its accept- 

 ance. 



Objections. — The objections to this view are : 1. That the crust, as 

 already shown, must be far thicker than this theory requires, probably 

 hundreds of miles thick, if, indeed, there be any general liquid interior 

 at all ; but volcanoes are evidently very superficial phenomena. Under 

 the pressure of this difficulty these theorists have been driven to the 

 acknowledgment of local thinnings of the solid crust in the region of 

 volcanoes. 



