288 HOBBS— THE EVOLUTION AND THE [April 24. 



(that is to say, Chili and Peru), as it was very likely that the seismic activity 

 would extend to either end along the great zone in question, and as the 

 coasts of the countries above named are often visited by strong earth 

 convulsions. 



About two months after the prediction was made occurred the 



Valparaiso earthquake, but at the same hour an earthquake of the 



same order of magnitude visited an area in the Aleutian Islands 



within the same seismic belt, though nearer and in the opposite 



direction from the one predicted. On the same grounds Lawson 



in a lecture read in March, 1907, said of the stretches between 



southern California and Central America, and between northern 



California and southern Alaska: 



These strips, I believe, will be visited before long, and then the long line 

 of this earthquake will be complete from Chili to Alaska. 



The Guerrero earthquake in Mexico occurred only a few weeks 

 later and bore out the geologist's faith in the soundness of his 

 hypothesis. 



The method upon which such predictions are based is already 

 indicated in the quotations given. Briefly expressed it is the prin- 

 ciple of immunity from shock for a considerable period after heavy 

 earthquakes, combined with the conception of relief secured through- 

 out an extended zone in sections by alternation. An extended zone 

 on the earth's surface is recognized to be what might be called an 

 orographic unit; that is to say, it is all undergoing progressive 

 though interrupted elevation. Stresses tending to produce uplift 

 are presumably cumulative and may be of varying amounts in dif- 

 ferent sections of the zone. The resistance to movement under the 

 strain — whether due to the rigidity, to the vice-like compression, to 

 the absence of suitable fissure planes on which the movement might 

 occur, to the healing of such fissures by mineral matter, or to any 

 other causes — may be assumed to be different in different parts of 

 the zone. Relief of stress through sudden uplift should, therefore, 

 occur first within some one section of the zone where stresses are 

 greatest, resistance least, or both. The earthquakes furnish abun- 

 dant proof of the general correctness of this view. Now it is sim- 

 pler to assume that relief having been secured in one section of the 

 belt, a certain lowering of the potential energy of the system of 



