1909.] OUTLOOK OF SEISMIC GEOLOGY. 289 



stresses is to be expected in the near-lying sections on either side, 

 particularly since the shock tends to discharge the system of strain 

 as would a fulminate. On the theory of probabilities the area next 

 to be relieved should be the most distant, providing stress has there 

 been accumulated for an equally long period. The third and fourth 

 steps in the cycle of release of strain should in position be inter- 

 mediate between the first and second on one side or the other. Later 

 steps in the " letting down " process should affect especially the still 

 intermediate unrelieved sections of the zone. 



This method, simple as it is in theory, permits of only the broad- 

 est generalization and, as already stated, has been tested in but one 

 zone and for one cycle of relief. This zone is the great circle belt 

 which surrounds the Pacific Ocean, and the cycle of relief seems to 

 have begun with the Colombian earthquake of January, 1906. Only 

 two months after this disturbance came the Formosa earthquake, in 

 a province between one third and one half the distance around the 

 planet. The area of the California earthquake, which occurred a 

 month after that in Formosa, is intermediate between the first two, 

 tliough nearer the first than the second. By examination of Fig. 5, 

 which is drawn to scale, it will be noted that the distances separating 

 the approximate centers of these and the later disturbances in the 

 series, generally bear out the hypothesis that each later earthquake 

 afifects an area farthest removed from those sections of the zone 

 which have already found relief. 



The rapidity with which the steps in the process of securing 

 relief have here succeeded to one another, lends strong support to 

 the view that the zone in question should be regarded as a definite 

 orographic unit, and that the stress-strain conditions within all 

 except the southernmost portions were before relief began, remark- 

 ably uniform. The planetary order of magnitude of the movements 

 would thus seem to be clearly indicated. The section of the zone 

 last to be relieved was, it is interesting to note, one which had been 

 partly relieved of stress during two earthquakes six years and four 

 years before the main cycle of relief was inaugurated. The section 

 which separates the district of the Aleutian from that of the Cali- 

 fornian earthquake had also been visited by earthquakes seven years 

 and six years previous to the main cycle of relief. The portions 



PROC. AMER. PHIL. SOC, XLVIII. I92 T, PRINTED SEPTEMBER 7, I909. 



