ON ITS EXTERIOR. VOLCANOES. 235 



This general description of volcanoes, one of the most 

 important manifestations of the internal activity of our 

 planet,, has been based in part on my own observations, but 

 still more,, and in its more comprehensive outlines, on the 

 labours of my friend, Leopold von Buch, the greatest geolo- 

 gist of our age, who was the first to recognise the intimate 

 connection of the several volcanic phsenomena, and their mu- 

 tual dependence. Yolcanic action, or the reaction of the in- 

 terior of a planet on its external crust and surface, was long 

 regarded as an isolated and purely local phsenomenon, and 

 wa considered solely in respect to its destructive agency ; 

 it is only in modern times that, greatly to the advantage of 

 geological science founded on physical analogies, volcanic 

 forces have been contemplated as formative of new 

 rocks, and transformative of those which were pre- 

 existing. We here arrive at the point I previously indi- 

 cated, at which a well-grounded study of volcanic ac- 

 tivity in its various manifestations, branches into, and con- 

 nects itself with the mineralogical portion of geology 

 (the science of the structure and succession of terrestrial 

 strata), and with the configuration of continents and islands 

 which have been elevated above the level of the sea. The 

 enlarged view presented by this connection of phenomena is 

 a result of the philosophical direction, which the more 

 earnest and serious study of geology has now so generally 

 assumed. The prosecution and improvement of the sciences 

 has the same tendency as political and social improvements, 

 to bring together and unite that which had long been 

 divided. 



If instead of arranging rocks, according to their differences 



