362 SEE— THE CAUSE OF EARTHQUAKES. [October 19. 



large islands also, and not merely the islands, but the continents which can 

 be lifted up together with the sea ; and both large and small tracts may sub- 

 side, for habitations and cities, like Bure, Bizona, and many others, have 

 been engulfed by earthquakes." 



VIII. On the Great Sea Waves which frequently accompany 



Violent Earth:quakes. 



§ 40. Great sea zvaves caused chiefly by the subsidence of the sea 

 bottom after the lava has been expelled by the throes of earthquakes. 

 We have already shown how the sea bottom may subside over a 

 large area after a violent earthquake which has forced out from be- 

 neath the bed a large amount of steam-saturated lava. This lava 

 is usually forced up under adjacent mountain ranges, mountain 

 peaks and volcanoes, or under the intervening coast plains. If 

 relief is afforded by forcing the column of molten matter along so 

 that the remotest part raises in the mountains, which are at the top 

 of the arched portion of the neighboring region of crust, we should 

 have no means of discovering the resulting slight elevation of the 

 peaks. For the heights of mountains are always uncertain by several 

 feet, because extremely exact levels are difficult to establish so far 

 above the sea. We are thus unable to say whether the various 

 peaks and ranges are rising in height, or to tell what fluctuation of 

 altitude they may undergo from time to time, by the heaving of 

 earthquakes. In the case of plains near the sea it is easier to detect 

 changes of level, but in many places characterized by violent earth- 

 quakes unfortunately no such observations are taken. Tidal obser- 

 vatories are best adapted to keeping record of any changes of level 

 that may occur, because it is only by analyzing the tides carefully 

 that we can detect changes of the sea level. 



As already remarked, the sudden upheaval noticed in the sea- 

 coast of Chili after certain earthquakes was especially remarked by 

 Charles Darwin and Capt. Fitzroy, who experienced the severe 

 earthquake at Conception, February 20, 1835, and noticed the result- 

 ing elevations of the coast line. 



Let us consider briefly the upheavals of the Chilian coast wit- 

 nessed by Darwin and Fitzroy. The only reasonable explanation 

 of this fact is that lava had been forced under the coast, and it is 

 obvious that the matter must have been expelled from under the 



