XII.] EARTHQUAKES AND VOLCANOES. 187 



probable, that the greater part of the South American coast 

 has been raised several hundred feet by a succession of such 

 small upheavals. 



When an area is thus raised by an earthquake-shock, the 

 addition suddenly made to the mass of dry land may be very 

 considerable, and will compensate for the effects of denuda- 

 tion continued through a long period. It was calculated, for 

 example, by Sir C. Lyell, that an earthquake which occurred 

 in Chile in 1822 added to the South -American continent a 

 mass of rock more than equal in weight to a hundred thou- 

 sand of the great pyramids of Egypt. If a single convulsion of 

 this kind can thus raise such an amount of solid land from 

 beneath the waters, it is obvious that earthquakes must be 

 of great service in renovating the surface of the earth, and in 

 bringing new material within reach of the ever-active agents 

 of denudation. It is proper to remark, that the actual 

 earthquake-wave seems, from theoretical considerations, to 

 be competent to produce only very slight elevation. Never- 

 theless, the wave is often accompanied by the exertion of 

 distinct elevatory forces, which produce permanent altera- 

 tions of level of considerable magnitude. 



An earthquake is just such a disturbance of the ground as 

 would result from a sudden shock, or blow, given upwards in 

 the interior of the earth, from which, as from a centre, waves 

 or tremors may be propagated in all directions through the 

 solid ground. In many cases, the shock is preceded or 

 accompanied by a rumbling noise, like that of distant 

 thunder, or by other sounds produced by the subterranean 

 disturbance. The earthquake-wave, as it travels along, 

 causes the ground to rise and fall, and frequently produces 

 irregular fissures, which may close again and thus bury 

 whatever has been engulfed, or may remain open as yawning 

 chasms, and thus modify the drainage of the country. The 



