216 STRUCTURE AND MODE OF ACTION 



the earth are superposed upon each other, penetrate each 

 other as veins or dykes, or are upheaved or elevated by 

 elastic forces. 



If, then, our geological knowledge is thus promoted by 

 researches embracing extensive parts of the earth's surface, 

 it is not surprising that the particular class of phenomena 

 which form the subject of the present discussion should long 

 have been regarded from a point of view the more restricted 

 as 'the points of comparison were of difficult, I might almost 

 say arduous and painful, attainment and access. Until the 

 close of the last century all real or supposed knowledge 

 of the structure or form of volcanos, and of the mode of 

 operation of subterranean forces, was taken from two moun- 

 tains of the South of Europe, Vesuvius and Etna. The 

 former of these being the easiest of access, and its eruptions, 

 as is generally the case in volcanos of small elevation, being 

 most frequent in their occurrence, a hill of minor elevation 

 became the type which regulated all the ideas formed re- 

 specting phsenomena exhibited on a far larger scale in many 

 vast and distant regions, as in the mighty volcanos arranged 

 in linear series in Mexico, South America, and the Asiatic 

 Islands. Such a proceeding might not unnaturally recall 

 Virgil's shepherd, who thought he beheld in his humble 

 cottage the type of the eternal City, Imperial Rome. 



A more careful examination of the whole of the Mediter- 

 ranean, and especially of those islands and coasts where men. 

 awoke to the noblest intellectual culture, might, however, 

 have dispelled views formed from so limited a consideration 

 of nature. Among the Sporades, trachytic rocks have been 



