I90.6] SEE— THE CAUSE OF EARTHQUAKES. 393 



The same reasoning applies to all the South American earth- 

 quakes, including those which have destroyed Caracas, Cumana, and 

 other places in Venezuela. This coast lies between the eastern spur 

 of the Andes and the waters of the Caribbean Sea, which is one 

 of the deepest parts of the Atlantic, and the intensity of the sub- 

 terranean forces is shown by the violence of the volcanic outbreaks 

 in the Lesser Antilles. 



§ 54. Are any earthquakes really tectonic? 



It is held that many earthquakes are tectonic because there is 

 externally only a movement of the strata, as if they were seeking 

 release from strain, and obvious volcanic forces do not appear. 

 Boussingault long ago concluded that many of the earthquakes in 

 the Andes depended on the settling of the strata in these mountains, 

 and this was the beginning of the tectonic theory that most earth- 

 quakes are due to collapse or movement for release of strain. To 

 test the validity of this theory, it is advisable to apply similar reason- 

 ing to the mountains of a country remote from the seacoast. We 

 choose for this purpose the Rocky Mountains in Colorado, which 

 were formerly near but are now remote from the sea. If what 

 Boussingault witnessed in the Andes was really the settling of the 

 mountains, the same effects ought to be going on in the Rocky 

 Mountains of Colorado. So far as the records of history go, it may 

 be safely said that not a single serious earthquake has ever visited 

 Colorado, and yet many should have been felt if Boussingault's 

 theory of the settling of the mountains is correct. As change is 

 usually ascribed to secular cooling, why should the Andes settle 

 and not the Rocky Mountains? From the absence of earthquakes 

 in Colorado, it is evident that tectonic movements have ceased in 

 that region, though secular cooling has not; and thus we see that 

 these movements after all do not depend on settlement of the moun- 

 tains due to the shrinkage of the earth. For, if so, it is incredible 

 that the Rocky Mountains can have already attained a perfectly 

 stable position. They ought to be still collapsing like the Andes, 

 since secular cooling is always going on. Thus we see that all 

 earthquakes must depend on underlying explosive forces, and not 

 on mere adjustments of strata to secure release of strain or stability 

 of position, required by the progress of secular cooling. 



