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list of the earlier earthquakes within the province, beginning with 1784. 

 Seven pages are devoted to a list of data from instruments at foreign stations, 

 and fifteen pages to a discussion (largely mathematical) of the depth of 

 the earthquake centrum. 



Seismologists will welcome the elaborate catalogue of Mexican earth- 

 quakes which is published in the last number of the series, with its earnest 

 of further work along the same line. 



Harry Fielding Reid. " Seismological Notes," Proc. Am. P kilos. 

 Soc, Vol. XL VIII, No. 192, 1909, pp. 303-12. 



Under this somewhat unimpressive title Professor Reid has put forward 

 an entirely new theory of the cause of earthquakes. In his own summary 

 this theory is thus stated: 



Tectonic earthquakes are caused by the gradual relative displacement of 

 neighboring regions which sets up elastic strains so great that the rock is ruptured: 

 and that at the same time of the rupture no displacements of large areas take 

 place, but there occurs merely an elastic rebound, to an unstrained position, of 

 the lips of the fault extending but a few miles on each side of it. 



This theory is visualized for the reader by diagrams representing two 

 short wooden blocks joined by a thick layer of stiff jelly which has been 

 divided by a sharp knife into two equal layers. The blocks being held 

 together under slight pressure, they are given a shearing motion. The 

 jelly is thereby deformed much as would be a rubber layer, and the friction 

 between the jelly surfaces is reduced by a release of the pressure upon the 

 blocks. The two jelly layers now suddenly resume their former unstrained 

 attitudes with the production of a fault of lateral displacement at their plane 

 of junction. This fault is supposed to simulate in its manner of formation 

 the recent displacement along the California rift, and the theory will com- 

 mand attention, particularly, since Professor Reid, as a member of the 

 California State Earthquake Commission, has been intrusted with the prob- 

 lems of mechanics involved in the recent earthquake displacements, and 

 has in preparation the second vokime of the report of the commission. 



The value of the theory will be adjudged differently by different workers, 

 but it seems safe to say that its assumptions are far too sweeping and that 

 the theory in its present form would never have been devised had the 

 study of any save the California earthquake led to its framing. Of all 

 known earthquakes which have been accompanied by visible displace- 

 ments in the surface of the ground, this one is unique by reason of the large 



