250 VOLCANIC THEORIES. SECT. XXVI. 



would raise the temperature of the strata below, and in 

 the course of ages would even reduce those at a great 

 depth to a state of fusion. Some of the substances might 

 be converted into gases ; and should the accumulation of 

 new matter take place at the bottom of the sea, as is 

 generally the case, this lava would be mixed with water 

 in a state of ignition in consequence of the enormous 

 pressure of the ocean, and of the newly superimposed 

 matter which would prevent it from expanding into 

 steam. Now Mr. Lyell has shown with his usual talent, 

 that the quantity of matter carried down by rivers from 

 the surface of the continents is comparatively trifling, 

 and that the great transfer to the bottom of the ocean is 

 produced at the coast line by the action of the sea ; 

 hence, says Sir John Herschel, " the greatest accumula- 

 tion of local pressure is in the central area of the deep 

 sea, while the greatest local relief takes place along the 

 abraded coast lines. Here then should occur the chief 

 volcanic vents." As the crust of the earth is much 

 weaker on the coasts than elsewhere, it is more easily 

 ruptured, and, as Mr. Babbage observes, immense rents 

 might be produced there by its contraction in cooling 

 down after being deprived of a portion of its original 

 thickness. The pressure on the bottom of the ocean 

 would force a column of lava mixed with ignited water 

 and gas to rise through an opening thus formed, and, 

 says Sir John Herschel, " when the column attains such 

 a height that the ignited water can become steam, the 

 joint specific gravity of the column is suddenly dimin- 

 ished, and up comes a jet of mixed steam and lava, till 

 so much has escaped that the matter deposited at the 

 bottom of the ocean takes a fresh bearing, when the 

 evacuation ceases and the crack becomes sealed up." 



This theory perfectly accords with the phenomena of 

 nature, since there are very few active volcanos at a dis- 

 tance from the sea, and the exceptions that do occur 

 are generally near lakes, or they are connected with 

 volcanos on the maritime coasts. Many break out even 

 in the bottom of the ocean, probably owing to some of the 

 supports of the superficial crust giving way, so that the 

 eteam and lava are forced up through the fissures. 



Finally, Mr. Babbage observes that " in consequence 



