286 SEE— THE CAUSE OF EARTHQUAKES. [October 19 



distortion through all the rocks between the two oceans, and disturb 

 the whole earth, are not produced by so small a cause as the slipping 

 and bending of ledges of rock. 



Humboldt and Charles Darwin long ago associated earthquakes 

 with secular elevations and depressions, and it is noticeable that 

 Professor Milne likewise thinks these disturbances occur with in- 

 creased frequency in regions where such changes are still in progress. 



Montessus de Ballore concluded from his elaborate study of sta- 

 tistical data that in adjacent seismic regions, instability of the earth 

 is increased by differences of topographic relief ; and that the 

 unstable regions are associated with the greatest lines of corrugation 

 of the earth's crust. Like Professor Milne, he observes that rapidly 

 deepening shores which slope gently, especially if they are the con- 

 tinuations of flat or moderately falling coast plains, are stable. His 

 results are illustrated by steep regions of the seashore in South 

 America, Japan, and other parts of the world, and by other regions 

 where the slope into the sea is more gradual. 



These views and others of similar tenor bv several investis^ators 

 have led some geologists and seismologists to conclude that many 

 of the earthquakes noticed along shores which are steep are due to 

 the sliding of unstable deposits of sediment settling on the rock 

 slopes. But if we recall, as above, the smallness of these slopes, even 

 where the descent is most rapid — it never exceeds that of our 

 mountains upon the land, and is seldom as steep, — and observe that 

 the surrounding sea water is quiescent and would both greatly buoy up 

 and resist the motion of any supposed sliding deposit, so that it is 

 doubtful if appreciable sliding really takes place, and certain that 

 if it does occur the effect in disturbing the earth would be very 

 slight, we shall find it diflicult to believe that the theory is well 

 founded. It appears that such a deposit, resisted by the surrounding 

 water, would slide with extreme slowness, and settle gently without 

 any appreciable jar, and consequently no earthquake of importance 

 could be produced in this way. 



