PRESENT CHANGES IX THE EAKTH. 117 



and Laguyra, in South America, were destroyed. Anoth- 

 er cause of earthquakes has been supposed to be a bend- 

 ing in the earth's crust, arising from changes of tempera- 

 ture. On this point Professor Dana gives the following 

 familiar illustrations : "All are familiar with the cracking 

 sounds occurring at intervals in a board floor of a house, 

 and arising from change of temperature, especially in a 

 room in winter that is heated during the day ; and with 

 the more common sounds of similar character from the 

 jointed metallic pipe of a stove or furnace, given out aft- 

 er a fire is just made, or during its decline. In each case 

 there is a strain or tension accumulating for a while from 

 contraction or expansion, which relieves itself finally by 

 a movement or slip at some point, though too slight a 

 one to be perceived ; and the action and effects are quite 

 analogous to those connected with the lighter kind of 

 earthquakes." Besides the vibration or wave movement 

 produced in the earth in an earthquake, there is also a 

 vastly more rapid vibration, which causes the sensation 

 of sound. The latter vibration commonly extends much 

 farther than the former. 



201. Effects of Earthquakes. Earthquakes produce va- 

 rious effects according to their violence, extent, and ac- 

 companying circumstances. They are chiefly fractures 

 of the earth's crust, sometimes very extensive; displace- 

 ments, either elevations or depressions for the most part ; 

 the draining of lakes, and the production of new lakes ; 

 the production of waves in the sea, sometimes to a great 

 distance ; and the destruction of life in fishes from the 

 mere shock that is given to them. The displacements 

 and fractures often involve a great destruction of human 

 life. In the famous earthquake at Lisbon in 1755, the 

 greater part of the city was thrown down, and 60,000 

 persons werfe killed. Some most remarkable elevations 

 and depressions have attended earthquakes. The coast 

 of Chili was in 1822 raised three feet over a space of 

 100,000 square miles, an area equal to half of France. In 



