Vol. G] Beid: The Elastic-Rebound Theory of Earthquakes. 4l*5 



suddenly by a sudden displacement of the block; the movements 

 at the time of the slip would have been exactly the same in the 

 two cases. 



We must turn to other considerations to determine which 

 of the above cases represents the earth movements leading to the 

 California earthquake. 



A very important chapter in the history of the earth is con- 

 cerned with the record of the various movements which have 

 taken place in the crust. The vertical movements are thor- 

 oughly attested by the various heights to which strata, deposited 

 under the sea, have been raised; and by the unconformities which 

 exist in the geological column. The horizontal movements are 

 shown by the compression of the strata into folds, and by great 

 overthrust faults. That these movements have continued until 

 recent time is shown by the existence of raised beaches, and other 

 similar evidences; and that they have not been simply due to a 

 general rising, or sinking, of the surface of the ocean follows 

 from the fact that the elevations vary at different places. For 

 instance, the Cretaceous strata of Maryland are now near the 

 sea-level, whereas in the great plateaux of Utah and Arizona, 

 they have an elevation of about 8000 feet. Innumerable in- 

 stances of the various elevations of strata once at the same level 

 could be mentioned. Moreover, many of the raised beaches, which 

 must have been horizontal when they were formed, are now 

 tilted. We have excellent examples of this tilting in the account 

 given by Baron de Geer of the raised beaches in Scandinavia ;. 

 and in the tilted shore-lines of the old glacial lakes in the region 

 of the Great Lakes. Mr. Gilbert's discussion of the tide-gauge 

 readings at various points on the Great Lakes, and of the topo- 

 graphic changes taking place on their shores, shows, without 

 reasonable doubt, that the tilting is going on at the present time; 

 and that the difference of level between two points 100 miles 

 apart and lying on a NNE and SSW line is changing by prob- 

 ably more than half a foot per century. 



It is firmly established that since the beginning of geological 

 history the crust of the earth has been in continual movement, 

 rising in one place, sinking in another, here squeezed into folds, 

 there somewhat stretched. It is generally assumed that these 



