41 S HUMBOLDT'S THEORY OF VOLCANOES. 



the atmosphere which surrounds the hardened and oxy- 

 dized crust of our planet. On this theory he explained 

 the still active and the extinct craters, the direction of 

 the mountain-ridges, and the formations of the soil ; he 

 deciphered the traces of former terrestrial revolutions, 

 their relative age, and the physical powers which have 

 influenced and still influence the form of the earth's sur- 

 face. Thus the masses of lava which pour from the 

 craters were to him the petrified streams of formerly 

 gushing springs of the interior of the earth ; from the 

 connection and similaritv of effects he traced the causes 

 and conditions of the formation of rocks and superin- 

 cumbent strata, of the chemical results of volcanic erup- 

 tions, of elevations and depressions of the earth's sur- 

 face. By the strictest investigation of all occurring new 

 appearances, and by penetrating combination of analo- 

 gous, observed facts, he explained numerous physical 

 and geological problems, whose exact solution had 

 hitherto been deemed impossible. Humboldt thinks that 

 the volcanic activity of our earth, compared to former 

 ages, is considerably decreased ; it can no longer bring 

 forth new elevations or heat in the north, but can only 

 produce small craters, and an agitation of the earth's 

 surface. Before the advent of man into terrestrial nature, 

 a tropical, animal, and vegetable world flourished every- 

 where on the volcanically -heated earth ; now, on the 

 cooled planet, the petrified surface only receives warmtli 

 from the sun, the tropical luxuriance died out towards the 

 north, and only flourishes where the sun can exercise its 

 perpendicular influence over the tropics. 



In those remote ages of the boiling centre of our earth- 

 ball, the hot fluid and the gases it generated often and on 



