222 REACTION OF THE INTERIOR OF THE EARTH 



outermost strata, the most powerful is that exhibited by 

 volcanoes proper, i.e. openings through which (besides 

 gases) solid substances of various kinds are brought in a 

 state of igneous fluidity from unknown depths to the 

 surface, where they issue forth either as streams of lava, 

 or as scorise, or in a state of the finest comminution, 

 when they are called "ashes." When, according to 

 the sense attached by ancient usage to the expressions 

 volcano and burning mountain, these are employed as 

 synonymous terms, the idea of volcanic phenomena is 

 connected, according to what was a very generally re- 

 ceived impression, with the image of an isolated conical 

 mountain, having at its summit a circular or oval open- 

 ing or crater. Such views lose, however, some of their 

 generality, when the observer has the opportunity of wan- 

 dering through volcanic regions of many thousand square 

 miles in extent ; as, for example, the entire central part 

 of the Mexican highlands between the Peak of Orizaba, 

 Jorullo, and the shores of the Pacific; or in Central 

 America ; or in the Cordilleras of New Granada and 

 Quito between the volcano of Purace near Popayan, that 

 of Pasto, and Chimborazo ; or in the Caucasus between 

 Kasbegk, Elbourz, and Ararat. In Lower Italy, between 

 the Phlegrsean Fields of the Campanian mainland, Sicily, 

 the Lipari and Ponza Isles, as well as in the Greek 

 islands, the intervening land has partly not been up- 

 heaved at the same time, and partly has been since 

 swallowed up by the sea. 



In the above-mentioned large districts in America 

 and in the Caucasus we find erupted masses (true 

 trachytes, not conglomerates of trachyte), streams of 

 obsidian, and blocks of pumice-stone obtained by 



