STRUCTURE AND ACTION OF VOLCANOS. 355 



formations. It is thus that the inorganic world may be said 

 to reflect itself, more or less distinctly, in every mountain of 

 any great extent. It is necessary, however, in order perfectly 

 to understand the most important phenomena of the com- 

 position, relative age, and origin of formations, to compare 

 together the observations made in regions of the earth most 

 widely remote from each other. Problems which have long 

 baffled the geologist in his own northern region, find their 

 solution in the vicinity of the equator. If, as we have 

 already observed, remote regions do not present us with 

 new formations, that is to say, with unknown groupings of 

 simple substances, they at least help us to unravel the great 

 and universal laws of nature, by showing how different strata 

 of the crust of the earth are mutually superimposed on, .and 

 intersect, each other in the form of veins, or rise to different 

 elevations in obedience to elastic forces. 



Although our geological knowledge may be thus exten- 

 sively augmented by researches over vast regions, it can hardly 

 be a matter of surprise that the class of phenomena constitut- 

 ing the principal subject of this address should have been so 

 long examined in an imperfect manner, since the means of 

 comparison were of difficult, and almost, it may be said, 

 of laborious access. 



Until towards the close of the eighteenth century all that 

 was known of the form of volcanos and of the action of their 

 subterranean forces was derived from observations made on 

 two volcanic mountains of Southern Italy, Vesuvius and 

 Etna. As the former of these was the more accessible, and 

 (like all volcanos of slight elevation) had frequent eruptions, 

 a hill became to a certain degree the type according to 

 which a whole world the mighty volcanos of Mexico, South 

 America, and the Asiatic Islands was supposed to be formed. 

 Such a mode of reasoning involuntarily calls to mind Virgil's 

 shepherd, who believed that in his own humble cot he saw 

 the image of the eternal city, Imperial Rome. 



2 A2 



