236 VOLCANIC THEORIES. SECT. XXV. 



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 Sir Charles Lyell has 

 shown, with his usual talent, that the quantity of matter carried 

 down by rivers from the surface of the continents is compara- 

 tively 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 accumulation 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 open- 

 ing 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 diminished, 

 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 volcanoes at a distance from the 

 sea, and the exceptions that do occur are generally near lakes, or 

 they are connected with volcanoes 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 steam and lava are forced up through the fissures. 



Finally, Mr. Babbage observes that, " in consequence of 

 changes continually going on, by the destruction of forests, the 

 filling up of seas, the wearing down of elevated lands, the heat 

 radiated from the earth's surface varies considerably at different 



