730 DYNAMICAL GEOLOGY. 



shores of Madeira, 8 to 10 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 23 vessels, and carried a fri- 

 gate several miles inland. Two hundred shocks were experienced 

 in 24 hours. The ocean twice retreated, to rush in a lofty wave over 

 the land. The shock to a vessel from an earthquake wave is as if 

 it had received a heavy blow or had struck a rock. 



According to Professor 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-registering tide-gauges of the Coast Survey along the coast of 

 Oregon and California ; and from the data thus afforded he was 

 enabled to calculate the mean depth of the intervening ocean, 

 stated on page 12. 



5. Causes of earthquakes. — (1.) The tension and pressure by which 

 the great oscillations and plications of the earth's crust have been produced. — 

 The effects of this tension have not yet wholly ceased. This is 

 probably the most general cause of earthquakes. 



The uplifting of the formations, moreover, must have always left 

 the interior of the crust in a state of unstable equilibrium ; and 

 any incipient slide in the progress of time along an old fracture, 

 or between tilted beds, would be attended by an earthquake-shock. 



All are familiar with the cracking sounds occurring at inter- 

 vals in a board floor of a house, and arising from change of tem- 

 perature, 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 after 

 a fire is first 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. 



(2.) Any cause of extensive fracture or movement, — as the under- 

 mining of strata, the sudden evolution of vapors, etc. 



(3.) Tidal waves in the internal igneous material of the globe. — This hy- 

 pothesis supposes the material of the interior to be sufficiently 

 liquid to have waves, and the crust to be thin. 



Some investigations by Professor Alexis Perrey, of Dijon, France, 

 seem to indicate that there is a periodicity in earthquakes synchro- 

 nous with that in the tides of the ocean, — the greatest number 

 occurring at the season of the syzygies in each lunar month. If 

 this be sustained by further research, the cause must be admitted 



