EAKTHQUAKES AND VOLCANOES. 



earth. Not a century passes that these terrible phenomena are- 

 not occasionally developed with such an energy and extent that 

 vast tracts of country are laid waste, cities and towns destroyed, 

 and thousands of human beings buried beneath their ruins.. 

 Volcanic eruptions are permanent indications of subterranean 

 agencies, modifying more or less the surface. Torrents of lava 

 and clouds of ashes ejected from them cover surrounding regions,, 

 and sometimes entomb entire cities. The solid bottom of the 

 ocean is occasionally heaved upwards by a force from below, so as 

 to form new islands, which sometimes subsiding are again sub- 

 merged. These and countless other phenomena show that the 

 crust of the globe is not so solid and unchangeable as it is generally 

 assumed to be. 



10. But if, instead of confining our view to that comparatively 

 brief interval in the life of the earth limited by the existence of 

 the human race, and the other organised tribes animal and 

 vegetable, which now prevail upon it, we extend our enquiries 

 over those periods of the past, far more vast, with which the dis- 

 coveries of astronomers and geologists have made us to some 

 extent familiar, we shall find monuments and records of physical 

 changes produced by eruptive forces emanating from the central 

 parts, on a scale so prodigious, that compared with them, the most 

 devastating earthquakes and volcanic eruptions are utterly insigni- 

 ficant. The central fluid matter pressing unequally on its con- 

 fining shell, has at various times cracked it in different directions, 

 and the molten mineral matter, issuing from the fissures, and 

 gradually cooling, taking first a pasty and semifluid consistency, 

 and afterwards solidifying, has formed mountain chains over the 

 fissures. Such has been the operation by which the vast ridges 

 of the Andes, which traverse the new continent from north to 

 south, and those of the Alps and Himalaya, which traverse the 

 old continents from east to west, have been formed. 



11. Postponing, however, for the present all notice of those 

 physical revolutions of which the date is prior to the creation of 

 the present inhabitants of the earth, we shall limit our observa- 

 tions to the circumstances which produce and attend the principal 

 convulsions of nature which have manifested themselves in 

 historical times, and which must be traced to the reaction of the 

 interior fiery fluid upon the thin solid shell of the earth. 



That fluid, like the waters of the ocean, is subject to undula- 

 tion. If its undulations be so limited in their play that the 

 materials of which the terrestrial shell is formed have sufficient 

 elasticity to yield to their pressure without being fractured, they 

 will produce on the exterior surface of that shell corresponding 

 undulations by which all bodies placed upon its surface must be 

 150 



