1921] Lawson: The Mobility of the Coast Ranges of California 435 



theory of elastic rebound in its application to the movements of the 

 region traversed by the San Andreas fault, and particularly Reid's 

 second hypothesis, in a modified form, of a single northerly subcrustal 

 flow, dragging the crust with it, and so developing in the latter a state 

 of strain, from which it is relieved from time to time by rupture, or 

 by slipping on old rupture planes. 



The geodetic data upon which my discussion rests are all to be 

 found in the paper by Hayford and Baldwin® or in a later report of 

 the Coast and Geodetic Survey,^ which supplies a convenient tabulation 

 of the positions of many points as determined by surveys of different 

 dates. 



Chronologically these surveys fall into three classes: (1) those 

 made before 1868, (2) those made between 1868 and 1906, (3) those 

 made in 1906-1907 after the earthquake ; and they may be referred to 

 as surveys I, II, and III respectively. 



The fact that there was a sudden displacement by slip on the San 

 Andreas fault in 1906, seems to have inclined Hayford and Baldwin 

 to the view that displacements which were discovered to have occurred 

 between surveys I and II were effected by a similarly sudden move- 

 ment in 1868, at the time of the earthquake of that year. This 

 assumption is made by them, however, as a matter of convenience in 

 discussion, and not because there is any evidence to establish the fact. 

 The assumption is probably justified for part of the region south of 

 the Golden Gate, as I shall point out in the sequel. But as there is 

 no evidence whatsoever for a sudden displacement of the region north 

 of the Golden Gate in 1868, and as the observed facts in that part of 

 the field can be better explained without such an assumption, I shall 

 proceed on the simpler hypothesis that there was no appreciable sudden 

 shifting of positions to the north of the Golden Gate in 1868 such as 

 occurred in 1906. 



BASIS OF HYPOTHESIS OF CAUSAL STEESS 



The reasons for adopting Reid's second hypothesis that the funda- 

 mental cause of strain throughout the region is a stress applied to 

 the lower side of the earth's crust by a single current flowing northerly, 

 are : 



1. In the region north of the Golden Gate all the stations which 

 were determined in position by surveys I and II of the Geodetic 



8 Earthquake Report, Geodetic measurements of earth movements, pp. 115-14.5. 

 7 Report for 1910, Append. 5, Triangulation in California, pt. II, by C. E. 

 Duvall and A. L. Baldwin, Washington, 1911. 



