X906 ] SEE— THE CAUSE OF EARTHQUAKES. 383 



the eruptions may be either subterranean or submarine, as in the 

 elevation of the Chilean coast noticed by Darwin and Fitzroy in 1835. 



On the theory of faults, as now held, it is difficult if not im- 

 possible to account for the observed elevations. It is perhaps true 

 that elevations apparently are rarer than subsidences, yet they 

 are much more significant and furnish a better criterion of the forces 

 at work, since many subsidences on land are due to settling of soft 

 ground which has never been consolidated under pressure. But 

 we must remember that only very few absolute levels are accurately 

 known and still fewer remeasured after an earthquake ; and therefore 

 while subsidences appear to be the more general phenomena, espe- 

 cially in regions of soft earth, one may well be very doubtful whether, 

 in the case of hard ground, elevations, though mostly unnoticed 

 because there is no easy means of measurement, do not really pre- 

 dominate. Reasons connected with the mode of formation of moun- 

 tains and the elevation of coast lines, given heretofore, point strongly 

 to elevation as the more general movement in nature. For it is this 

 movement which has uplifted both mountains and continents, and we 

 cannot suppose that it has ceased to be the dominant influence, 

 though it generally escapes notice, because w^e have no means of 

 detecting it, while local subsidences frequently are easily recognized, 

 and we naturally look for it because of the frequent shaking down of 

 " made " ground. 



In most earthquakes the heaving force is distributed over a 

 considerable area, and when the stress becomes great enough a 

 movement takes place along the nearest fault line — the path of least 

 resistance — and the observer who sees the slip says the movement 

 of the fault caused the earthquake. 



The fact that most earthquakes are found to originate at a depth 

 of from ten to twenty miles shows that the epicentrum is below the 

 depth at which the strata have any opportunity of moving; and the 

 proof that the shock usually comes from an area and not from a 

 point or from a line, shows that the shock depends on an explosive 

 stress spread over a considerable region, and in no way depends on 

 dislocational or fault movements, which are always quite superficial. 



§ 51. Explanation of the rotatory motion observed during an 

 earthquake. 



