EARTHQUAKES AND OTHER SEISMIC MOVEMENTS. 305 



EAKTHQUAKES AND OTHER SEISMIC MOVEMENTS.* 



WE are accustomed to think of the land of the earth as something 

 solid and fixed ; and, as a testimonial of this impression, the 

 Latin phrase terra firma, firm land or solid ground, has been natural- 

 ized in the languages of nearly all civilized peoples. On the other 

 hand, we speak of water as unstable. But the geological history of 

 the earth and the more careful observations of modern times have 

 taught us that these ideas do not correctly represent the qualities of 

 the land-masses and water-masses of the globe as compared with one 

 another. The ancient shore-marks on the continents and the phenom- 

 ena of elevation and subsidence that have been observed in historic 

 times, confirming their evidence, show that the land and the ocean are 

 continually changing their level as to one another ; and it has further 

 been made evident, by experiment, as well as by a priori reasoning, 

 that it is not the ocean that changes, but the land which undergoes 

 alternate movements of elevation and depression. An earthquake- 

 shock is a phenomenon well adapted to destroy the faith of any person 

 who feels one in the fixedness of the earth ; and such, by the evidence, 

 is the effect for the time on all who experience these shocks. Even the 

 light pulsations which sometimes pass over parts of the United States 

 occasion panic and excite a momentary impression that everything is 

 falling over or sinking away ; while the more violent shocks that are 

 felt in earthquake-infested countries produce indescribable terror ; 

 and such catastrophes as those historical earthquakes of Lisbon and 

 Caracas, and the more recent ones of Ischia and the Strait of Sunda 

 amount to a demonstration that the reason for such terrors is real, and 

 that the continents also can not escape the general law of change and 

 perishability. 



Earth-movements the name by which these phenomena may be 

 most conveniently described are various, and comprise, so far as they 

 are now considered, earthquakes, or sudden violent movements of the 

 ground ; earth-tremors, or minute movements which usually escape 

 attention by the smallness of their amplitude ; earth pulsations, or 

 movements which are overlooked on account of the length of their 

 period ; and earth oscillations or movements of long period and large 

 amplitude like the shifting of levels of land-masses which attract 

 attention from their geological importance. Some of these movements 

 have only recently begun to attract attention. They are all intimate- 

 ly associated in their occurrence and their origin. 



The study of earthquakes is of interest to the geologist in many 



* Earthquakes and other Earth Movements. By John Milne, Professor of Mining 

 and Geology in the Imperial College of Tokio, Japan. International Scientific Series. 

 No. LV. New York : D. Appleton & Co., pp. 348. 

 VOL. xxix. 20 



