ON ITS EXTERIOR. VOLCANOES, 227 



square miles of country with lava, which, where it met with 

 obstacles to its course, accumulated to a thickness of several 

 hundred feet. The small quantity of nitrogen emitted op- 

 poses similar difficulties to the hypothesis of the entrance of 

 atmospheric air into the crater, OL-, as it has been metapho- 

 rically expressed, to the breathing or inhaling of air by the 

 earth. An activity so general as that of volcanoes, so 

 deeply seated, and extending itself so widely in the interior, 

 oannot well have its source in chemical affinities, and in the 

 contact of certain substances found only in particular loca- 

 lities. Modern geology prefers to seek its cause in the 

 internal terrestrial heat, manifested in every latitude by the 

 increase of temperature with increasing depth, and which has 

 been ascribed to the supposed condition of the earth as a 

 body only partially cooled. If we consider volcanoes as 

 irregular intermitting springs, supplying in tranquil flow a 

 fluid mixture of oxidized metals, alkalies, and earths, which, 

 upheaved by the powerful expansive force of vapours, have 

 found a permanent outlet, we are involuntarily reminded 

 how nearly the rich imagination of Plato approached to the 

 same view, when he attributed thermal springs and all vol- 

 canic phenomena to a single cause every where present in 

 the interior of the earth, the Pyriphleyethon, or subterra- 

 nean fire. ( 225 ) 



The geographical distribution of volcanoes is wholly inde- 

 pendent of climatic relations ; they have been arranged 

 characteristically in two classes : " central volcanoes," and 

 " volcanic chains ;" the former term being applied to volca- 

 noes forming the centers of numerous orifices of eruption 

 distributed with some regularity in every direction; and 

 tjie latter to those which, placed at moderate distances 

 apart, form lines running in one direction, like chim- 



