,9o6] SEE— THE CAUSE OF EARTHQUAKES. 303 



Krakatoa.^ It has so often been observed that the earthquakes 

 ceased on the eruption of a neighboring volcano, that one cannot 

 doubt that direct rehef was afforded by the eruption. 



Viewing the relation of earthquakes and volcanoes in this light, 

 we can easily understand why many of the so-called tectonic earth- 

 quakes are doubly severe — much more violent than those closely 

 connected with volcanoes — because where no volcanic outlet has 

 been available, the explosive strain increases to an enormous extent 

 before it can obtain any relief whatever ; and when the yielding does 

 occur the shock is one of appalling violence, and does great damage 

 causing the slipping of rocks, faults and subsidences, and is felt 

 over a very large area, because the explosive strain has become deep- 

 seated and intense. 



The great depth at which many of the so-called tectonic earth- 

 quakes have been proved to occur, is at once an argument against 

 the dislocational or faulting theory, and a convincing proof that 

 shocks of this type are due to the explosive power of superheated 

 steam. These shocks are obviously too deep-seated to be accounted 

 for by subsidences, and moreover the resulting vibrations are too 

 complex to be due to mere slipping of a ledge of rock, as will be 

 more fully explained hereafter. It may be shown that no possible 

 subsidence of rock faults could produce a conspicuously rotatory 

 earthquake like that which destroyed San Francisco. 



IV. The General Cause of the Formation of Mountains and 

 THEIR Geographical Distribution. 



§ 14. On the formation of mountains and cordilleras, as illus- 

 trated by the Andes. 



If we consider the deep trough running for a great distance 

 parallel to the coast line of the western shore of South America^ 

 and recall that other deep troughs of the same kind exist parallel 



^ In chapter XVI of his valuable work on " Earthquakes," edition of 

 1903, Professor Milne cites several other eruptions of fearful violence accom- 

 panied in each case by terrible earthquakes. 



^The trough is not of uniform depth throughout its course, but the 

 depression is always conspicuous, so that everywhere the earth's crust is 

 arched downward. 



