450 University of California Publications in Geology [Vol. 12 



Tomales Bay Group. — The geodetic investigation proved, as Hay- 

 ford and Baldwin pointed out, that a large section of the Coast Ranges, 

 embracing at least the region between Tamalpais, Chaparral, and 

 Farallon, moved northerly about 1.5 meters in the interval between 

 surveys I and II. This movement was assumed by these writers to 

 have been sudden, and to have occurred in 1868. "It is certain 

 therefore that in 1868 the large part of the earth's surface included 

 between these three stations, at least 700 square miles, moved about 

 1.5 meters in about azimuth 168°."^^ AVithin this territory there are 

 ten stations near Tomales Bay which were properly assumed to have 

 participated in this general movement. Inasmuch as the points in this 

 group were located by surveys I and III but not by survey II, the 

 displacement of 1906, both as to amount and direction, was found by 

 a process of interpolation, which practically amounted to deducting 

 the 1.5 meters of northerly movement from the total displacement 

 found for each of the ten stations of the group for the interval between 

 surveys I and III. The difference was taken to be the measure of the 

 sudden displacement of 1906. The hypothesis that the northerly 

 movement is the expression of slow strain creep which continued 

 throughout the entire interval between surveys I and III, and of 

 which the 1.5 meters is a partial measure, modifies the results of this 

 interpolation, since the figures thus obtained include not only the 

 sudden displacement of 1906 but also the strain creep between surveys 

 II and III. It is therefore desirable to reconsider the data concerning 

 the Tomales Bay group of stations. 



Of the ten stations five are on the east and five on the west side 

 of the San Andreas fault. 



The data necessary for a discussion are tabulated below : 



Displacement 



Date Distance between surveys Direction 



of from fault I and III of 



Station Survey I Km. Meters displacement 



Point Eeyes Hill 1859 2.7 W 5.15 150° 



Bodega Head 1856 2.2 W 5.22 173° 



Tomales Bay 1856 2.1 W 5.32 150° 



Tomales Point 1856 2.0 W 5.01 151° 



Foster 1856 1.9 W 6.01 149° 



Mershon 1856 1.1 E .39 330° 



Smith 1856 2.6 E .64 25'9° 



Bodega 1860 2.0 E .90 239° 



Hans 1856 .5 E .65 52° 



Hammond 1856 1.2 E 1.75 81° 



11 Earthquake Report, vol. I, p. 124. 



