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



It is shown in the paper on the rigidity of the heavenly bodies 

 that the strength of a planet like the earth is not appreciably de- 

 pendent upon the crust, but arises primarily from the great pressure 

 acting throughout the body, which itself in turn depends upon the 

 mass and density of the globe. The earth's crust, therefore, has 

 little importance in the theory of the earth, except in our treatment 

 of surface phenomena. 



§ 20. Is there a creeping movement of the fluid substratum be- 

 neath the crust f 



The existence of such a powerful seismic zone around the 

 Pacific Ocean, which is surrounded by unfinished mountains and 

 a ' fire girdle of volcanoes ' leads one to inquire whether there may 

 not be throughout this vast ocean, as well as in smaller seas, a 

 tendency for the explosive stresses to find relief at the margins, 

 by a slow creeping movement of the particles of the substratum 

 towards the periphery, where the chief relief is afiforded. For 

 those stresses arising under the crust in the middle of such an 

 ocean, some relief would be afforded by the rising and sink- 

 ing of certain oceanic islands ; but a greater relief would be 

 afforded around the periphery of the sea, where the great moun- 

 tain chains are in process of formation. As the crust under the sea 

 is incessantly strained by the heaving of subterranean forces, some 

 parts rising and others sinking, a slow creeping movement of the 

 fluid substratum towards the periphery seems not only possible, but 

 perhaps probable. Such a final movement would be the result of 

 the countless earthquakes which disturb the sea bottom, and in any 

 given earthquake the motion would be extremely slight. The 

 creeping fluid would tend towards the avenues of escape in islands 

 and on the margins of the sea, as well as towards areas still sub- 

 merged but rising ; and thus we recognize forces which under cer- 

 tain conditions may both elevate and depress islands in the sea ; but 

 in the long run the sinking tendency will predominate where there 

 is water, and the rising tendency where there is land. 



All along the west coast of South America Charles Darwin 

 found conspicuous evidence of elevation within recent geological 

 times ; and at Valparaiso the amount was no less than i ,300 feet. 

 The periodic subsidences indicated in certain places by beds of ma- 



