374 DYNAMICAL GEOLOGY. 



Volcanoes stand on lines of fractures in the opening of whicli their 

 existence began ; and subsequently, through geological time, slips up or 

 down may have occurred along such fractures in the earth's uneasy crust, 

 independent of local action, producing earthquakes, and, perhaps, also 

 initiating eruptions. The Mediterranean area is one of the earth's fire 

 regions, from its eastern to its western limit, and its borders are noted for 

 the relative frequency of earthquakes ; and these earthquakes, in the majority 

 of cases, are independent of action in the volcanoes of the era. This is true 

 also, according to Milne, of the greater earthquakes of Japan. 



The New Zealand Tarawera eruption of 1883, which blew out with explo- 

 sive violence for a day or two, was followed, three days after it had subsided, 

 by an outbreak in White Island, an active volcano in the Bay of Plenty, and, 

 two months later, by a violent eruption on the island of Ninafou in the Tonga 

 group. The three volcanic regions are on the same island line of the ocean, 

 the northeast or New Zealand line, which is one of the most marked in the 

 Pacific. It may be that this succession of disturbances was due to a slight 

 movement from north to south along the old fracture-plane, through the 

 opening of which the range of islands began its existence. 



The central region of an earthquake vibration, which may have con- 

 siderable breadth or length, or have the course of a long fissure, is called 

 the epic entrum. The rock-waves move off from it in all directions, but often 

 most forcibly in one. The waves are: (1) waves of com'pression, or conden- 

 sation, in which the vibrations are normal to the origin, or in the direction 

 of the movement of the wave ; and (2) waves of distortion, or transverse 

 waves. The sounds of earthquakes are attributed by Milne to preliminary 

 tremors preceding the principal shock, which have the more rapid movement 

 required to produce sound. 



The amplitude of the wave varies from less than a millimeter to possibly 

 a foot. But destructiveness depends more on rate of vibration than on am- 

 plitude. Milne observes that the greater the initial impulse, the greater the 

 speed of propagation ; and, as the propagation widens radiately, the velocity 

 of propagation decreases, the period usually becoming larger. 



C. Davison (1891) traces several earthquakes of Great Britain to slips along faults. 

 He observes that from the central portions of the slip-area will come, as a rule, the vibra- 

 tions of largest amplitude and longest period, and from its margin, and especially toward 

 the surface, minute vibrations of a period so short that they may be perceptible only as 

 sound. He thus explains the fact "that the sound-area is not concentric with the dis- 

 turbed area, and the sound-focus is nearer the surface than the rest of the seismic focus " ; 

 and also, "the fact that, in great earthquakes, the sounds are heard only within a compara- 

 tively small area immediately around the epicentrum." Liability to slips, and therefore to 

 earthquakes, diminishes with the progress of time. 



Kinds of rocks have great effect on the propagation. Milne obtained in Japan, for 

 velocities of propagation, from 200 feet per second to 630 feet ; Mallet obtained, for sand, 

 a rate of 825 feet, and for granite, of 1665 feet; Newcomb and Dutton, in the Charleston 

 earthquake, made out a rate of 17,000 feet per second, vyithout any indications of variation 

 in the speed ; H. L. Abbott in his observations on explosions at Hallet's Point in 1876, 



