412 SEE— THE NEW THEORY OF EARTHQUAKES [November 15 



Addendum. 



§ 23. On the Sinking of Deep Trenches in the Sea Bottom. — 

 Since the above paper was finished it has occurred to the writer to 

 point out a Httle more clearly the circumstances which may lead to 

 the sinking of the sea bottom, when lava is expelled from beneath it. 

 In the accompanying figure, the process of mountain formation is 

 supposed to be going on. The expulsions of lava naturally are 

 accompanied by a sudden relief of crustal stress, and a mighty hori- 

 zontal thrust in the direction of the movement, as the lava is pushed 

 under the land. 



This terrific lateral pressure is powerful enough to push the lava 

 along under the crust, in spite of the great weight of the latter. By 

 this enforced movement the ridge is elevated, as at h, and at the 



same time the crustal block moved a good many feet in the direc- 

 tion of the land ; this stretches the sagging crust, and pulls it apart 

 along certain fault lines under the sea, as at a; so that when some 

 of the support from beneath is withdrawn by the expulsion of part 

 of the supporting lava, and the fault opened, by the horizontal move- 

 ment, the crust naturally gives down. The shaking accompanying 

 the horizontal thrust of the crust opens the fault under the sea, thus 

 temporarily removing the friction of one crustal block against 

 another and permitting the walls to slide without great resistance. 

 And as the support is weakened by the simultaneous expulsion of 

 part of the underlying lava, sinking necessarily follows. 



From these considerations we see that sinking is most easily 

 brought about where the ridge is sharply elevated so as to permit 

 some lateral movement. It is true that the inclination of the crustal 

 block throws a little more weight on the side under the sea and this 

 would increase the resistance ; yet when the fault is opened during 

 the vibrations of an earthquake sinking the more easily takes place. 

 This explanation enables us to understand why sinking of the sea 



