1905 



SEE— THE CAUSE OF EARTHQUAKES. 283 



and where we have no reason for supposing that bradj^-seismic movements 

 have yet ceased. 



" As examples of the flexures to which reference is here made, we may 

 take sections running at right angles to the coast lines of the various conti- 

 nents. The unit of distance over which such slopes have been measured is 

 taken at two degrees, or one hundred and twenty geographical miles. 



" The following are a few of such slopes : 



West Coast, South America, near Aconcagua i in 20.2 



The Kurils from Urup i in 22.1 1 Seismic 



Japan, east coast of Nipon i in 30.4 1 districts. 



Sandwich Islands, northwards i in 23.5 j 



Australia generally i in 91 



Scotland from Ben Nevis i in 158 Non-seismic 



South Norway i m 73 districts. 



South America, eastwards i in 243 ^ 



" The conclusion derived from this is that if we find slopes of consid- 

 erable length extending downwards beneath the ocean steeper than i in 35, 

 at such places submarine earthquakes, and their accompanying landslips 

 ma}' be expected. On the summit of these slopes, whether they terminate 

 in a plateau or as a range of mountains, volcanic action is frequent, while 

 the earthquakes originate on the lower portions of the face and base of 

 these declivities. Districts where earthquakes, often followed by submarine 

 disturbances, are most frequent are regions like the northeast portion of 

 Japan and the South American coast between Valparaiso and Iquique. 

 Here we have a double folding. The sea bed, as it approaches the shore 

 line, instead of rising gradually, sinks downward to form a trough parallel 

 to the coast, after which it rises to culminate in mountain ranges. The 

 South American trough which lies within fifty or sixty miles off the coast, 

 like the Tuscarora deep off Japan, attains depths of over four thousand 

 fathoms, and the bottoms of these double folds are well known origins of 

 earthquakes and sea waves." 



Professor Milne then goes on to show that where secular move- 

 ments are active, " the forces which have brought these mighty folds 

 (mountains) into existence have not yet ceased to act." The most 

 important question of all, however, is what are these forces ? He says 

 they appear where " mountain formation is geologically of recent 

 origin," and adds :* 



" The conclusion to which such observations lead is that wherever we 

 find in progress those secular movements which result in the building up of 

 countries or mountain ranges, there we should expect also to find a pro- 

 nounced seismic activity. Thus, while admitting a few small earthquakes 

 to be volcanic in their origin, we recognize the majority of these disturb- 

 ances as the result of the sudden fracturing of the rocky crust under the 



' " Seismology," by John ^lilne, F.R.S., p. 33- 



