1921] Lawson: The Mobility of the Coast Ranges of California 461 



(4) An expansive rebound which carried the San Andreas fault 

 parallel to itself to the southwest, if we accept the assumption of the 

 immobility of the Mocho-Diablo base. 



Rocky Mound. — The geodetic station Rocky Mound, situated 32 km. 

 from the fault to the eastward, is on the border between the region to 

 the south which participated in the sudden shift of 1868 and the region 

 to the north, which did not. The border is naturally vague and in- 

 determinate so that it is uncertain whether the station was displaced 

 in 1868 or not. I shall therefore discuss both possibilities. The 

 position of the station was determined in 1854, 1885, and in 1906p. 

 Assuming first that the station was not displaced in 1868 the displace- 

 ment of .5 meters in the direction 188° between 1854 and 1885 gives 

 us a rate of strain creep of .016 meters per year. In figure 11, I, let 



Fio-. 11, I. The path of Rocky Mound under the assumption of no displace- 

 ment in 18G8. 



Fig. 11, II. The path of the same station if displaced in 1868. 



A represent the position of the station in 1854 and B its position in 

 1885. At the rate deduced the point woiild have been at C in 1906ffl. 

 But after the earthquake it was found at D by the Geodetic Survey. 

 If the station moved in 1906 by reason of the slip on the San Andreas 

 fault, the direction of movement due to that cause must have been 

 parallel to the fault, or in the direction 306°. There is thus an 

 apparent transverse shift of about .25 meters, which may l)e real or 

 may be explained as before by the migration of the Mocho-Diablo base. 



Secondly, we may assume that Eocky Mound was suddenly dis- 

 placed to the south in 1868. In this case let A in figure 11, II, 

 represent its position in 1854, B in 1885, and C in 1906p. Under this 

 assumption there is no basis in the data for deducing the rate and 

 direction of creep, and I shall therefore adopt the rate found at 

 Tamalpais of .058 meters per year in the direction 168° for the pur- 

 pose of illustrating the general path of the station under the hypothesis 



