742 DYNAMICAL GEOLOGY. 



over several thousand square miles in the Caraccas, on the plains of 

 Calaboso, and on the banks of the Rio Apure. At the Lima earth- 

 quake, in 1746, a subterranean noise, like a thunder-clap, was heard at 

 Truxillo, where the earthquake did not reach. The rate of progress 

 will vary with the elasticity of the rock, and somewhat, also, with the 

 elevations over the surface. 



Regular progression may be a usual fact, although not generally 

 observed. Henry D. Rogers has shown that an earthquake, on the 

 4th of January, 1843, traversed the United States from its north- 

 western military posts, beyond the Mississippi, to Georgia and South 

 Carolina, along an east-southeast course, Natchez lying on the south- 

 ern border, and Iowa about the northern. The rate of travel ascer- 

 tained was thirty-two to thirty-four miles a minute. 



Phenomena attending Earthquakes. — (1) Fractures of the earth, 

 sometimes of great extent ; (2) subsidences or elevations of extended 

 regions, and draining of lakes ; (3) displacements of loose rocks, and, 

 where a mass overlies another, and is not attached to it by its precise 

 centre, a partial revolution, resulting from an onward impulse ; (4) 

 destruction of life in the sea, on the same principle that a blow on the 

 ice of a pond will stun or kill the fish in the waters beneath ; (5 ) 

 production of forced waves in the ocean ; (6) destruction of life on 

 the land. Destructions of cities and of human life have been too 

 often recounted to need special illustration in this place. 



The elevations that take place are sometimes spoken of as effects 

 of an earthquake, although not properly so. Vibration may be at- 

 tended by fractures and uplifts ; but these effects result from the cause 

 that produces the shaking. 



Some of the elevations and subsidences that have attended earth- 

 quakes are mentioned on page 585. 



Earthquake oceanic waves have been alluded to on page 662. One 

 or two additional examples of their effects may here be added. In 

 1755, accompanying the Lisbon earthquake, the sea came in, in a wave 

 forty feet high along the Tagus, sixty at Cadiz, eighteen on the shores 

 of Madeira, eight to ten on the coast of Cornwall. One in 1746, on 

 the coast of Peru, deluged the sea-port Callao, and the city of Lima 

 seven miles from the coast, sunk twenty-three vessels, and carried a 

 frigate several miles inland. Two hundred shocks were experienced 

 in twenty-four hours. The ocean twice retreated, to rush in a lofty 

 wave over the land. The shock to a vessel from an earthquake wave 

 is like that from a heavy blow or from striking a rock. 



As announced by A. D. Bache, the oceanic waves, produced by. the 

 great earthquake at Simoda (Japan) in 1854, crossed the Pacific, and 

 were registered, as to their number, intervals, and forms, on the self- 



