§20. SUMMARY OF VOLCANIC PHENOMENA. 841 



tions of the land, and at length escape either through the 

 former vents or by opening new channels. Hence, in the 

 language of Humboldt, the volcanic force must be contem- 

 plated as formative of new rocks, and transformative of 

 those which were pre-existing. But the volcanic operations 

 now going on, are only a faintly reflected image of that 

 energy which took place in the earliest geological epochs, 

 under very different terrestrial and atmospheric conditions. 

 The vast chasms and fissures which it is probable existed in 

 the solid portions of the earth's crust in the ancient periods, 

 from the contractions which must have taken place before 

 refrigeration had proceeded so far as to admit of accumula- 

 tions of aqueous sediments on the primary mineral masses, 

 have since been closed by the protrusion of mountain chains 

 through them, or filled up by dikes of granite, porphyry, 

 and basalt.* 



Many ingenious theories have been proposed to account 

 for the immediate cause of volcanic action. Of these, 

 the oxydation of the metallic bases of certain earths and 

 alkalies by percolations of water into deep-seated beds of 

 these substances, suggested by Sir Humphry Davy, is 

 still powerfully advocated by a high authority, — Dr. Dau- 

 beny ; but proof that such bases do exist in a metallic state 

 in the interior of the earth is required, before the hypothesis, 

 however ingenious, can be admitted as a vera causa. t The 

 intense heat and chemical changes which evolve gaseous 

 vapours and fluids, are the first elements in volcanic action j 

 and the volume of gas is often so great as to uplift the 

 molten lava, and at length to burst through and escape in 

 violent explosions. Without being able to determine the 

 precise nature of the first link in the chain of volcanic 



* Cosmos, p. 237. 



f The phenomena attendant on the combination of oxygen with 



potassium, sodium, &c the rapid and violent evolution of heat, light, 



and expansive force — mus be familiar to the intelligent reader. 



