OF VOLOANOS. 377 



position, the relative age, and mode of origin of rocks, we must 

 compare together observations from the most varied and remote 

 regions. Problems which long perplexed the geologist in his native 

 land in these northern countries, find their solution near the Equator. 

 If, as has been already remarked, new zones do not necessarily pre- 

 sent to us new kinds of rock (i. e. unknown groupings or associations 

 of simple substances), they, on the other hand, teach us to discern 

 the great and everywhere equally prevailing laws, according to which 

 the strata of the crust of the earth are superposed upon each other, 

 penetrate each other as veins or dykes, or are upheaved or elevated 

 by elastic forces. 



If, ihen, 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 diffi- 

 cult, 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 mountains 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 respecting phenomena 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 Mediterranean, 

 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 Spo- 

 rades, trachytic rocks have been upraised from the deep bottom of 

 the sea, forming islands resembling that which, in the vicinity of the 

 Azores, appeared thrice periodically, at nearly equal intervals, in 



32* ' ; " 



