402 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



Darwin have shown that the earth is highly rigid, and in a recent 

 paper on the rigidity of the heavenly bodies (cf. Astron. Nachr.,. 

 No. 4104) the writer has shown that no motion of currents deep 

 down in our planet is really possible, because of the enormous 

 friction due to the pressure at great depths. Thus no currents of 

 fluids or gases exist in the earth, except just under the crust where 

 the explosive strain is terrific. And even near the surface, where 

 the lava is forced to move under the thin crust, in the building of 

 mountain chains like the Andes, the motion is usually accomplished 

 only by the dreadful paroxysm of an earthquake, which expels the 

 molten rock from under the bed of the sea. The suspicion of Capt. 

 Fitzroy and Charles Darwin that in the three months following the 

 earthquake of 1835 the Chilean coast partially subsided to its former 

 level seems not only possible, but extremely probable. Under great 

 strain the viscous mass may have yielded somewhat, and thus there 

 may have been a slow creeping tendency towards the former level 



Judging by the thickness of the sides of Aconcagua, Cotopaxi 

 and other typical volcanoes of the Andes, one would probably be 

 justified in concluding that the thickness of the crust under which 

 the lava moves when expelled is not less than five miles, and it may 

 be as much as fifteen or twenty, but ordinarily it could not well lie 

 outside of these limits. For if we suppose it to be thicker, the leak- 

 age of the water would present greater difficulty if the temperature 

 is low ; and such thickness would not be required if adequate steam 

 developed nearer the surface. On the other hand, the thickness 

 could hardly be much less than ten miles without enfeebling the 

 layers which must support great strain in the expulsion of matter 

 from the broad trough of the ocean bed. The most probable thick- 

 ness is from ten to twenty, and for most purposes we shall be safe 

 in adopting the simple mean of fifteen miles. 



There are other considerations which lead to substantially the 

 same conclusion. It is found for example by the critical investiga- 

 tion of great earthquakes that most of these disturbances proceed 

 from an average depth of something like ten to fifteen miles. Now^ 

 if the theory here developed be admissible, it will follow that these 

 disturbances usually are in or near the lower stratum of the earth's 

 crust, which thus fixes the thickness of the layer at about the same 



