■ 1921] Lawson : The Mobility of the Coast Ranges of California 451 



Of the five stations on the west side of the fault four are very con- 

 sistent as to direction of displacement, while one, Bodega Head, departs 

 notably from the mean of these. The four consistent stations are 

 therefore taken as the best expression of the displacement on the west 

 side. Point Reyes Hill is reduced to the same interval of strain creep 

 as the other three stations, by adding the creep for the three years from 

 1856 to 1859, at the rate of .055 meters per year in the direction 173°, 

 making the displacement for that station 5.30 meters instead of 5.15 

 meters. 



The mean distance of the four stations. Point Rej^es Hill, Tomales 

 Bay, Tomales Point, and Foster, from the fault is 2.17 km., their mean 

 displacement is 5.41 meters, and the mean direction of the displace- 

 ment is 150°. 



On the east side of the faiilt the results in regard to two of the 

 stations, Hans and Hammond, are doubtful and anomalous. The 

 other three, Mershon, Smith, and Bodega, yielded figures which are 

 certain and consistent. These latter are taken, therefore, as the best 

 expression of the movement of the ground on the east side of the fault. 

 Their mean distance from the fault is 1.9 km. ; their mean displacement 

 is .64 meters; and the mean direction of their displacement is 279°. 

 With these mean values we may construct a diagram, figure 8, which 

 will make clear the movements of the ground on both sides of the fault. 

 Let A be the position in 1856 of a small circle bisected by the trace 

 of the San Andreas fault, aa'. From A, the circle migrated northerly 

 due to the accumulation of strain for the whole period from 1856 to 

 1906, or fifty years. The rate of this strain creep was found to be 

 .058 meters per year at Tamalpais and .052 meters at Chaparral. The 

 mean of the two rates is .055 meters per year. The direction of the 

 creep is 168° at Tamalpais, 173° at Chaparral, and 182° at Ross Moun- 

 tain. The mean of the azimuths at Chaparral and Ross Mountain, 

 which are near each other is 177°, and the mean of this and the azimuth 

 at Tamalpais is 172°. Since the Tomales group of stations is about 

 midway between Tamalpais and Chaparral I shall adopt 172° as the 

 direction and .055 meters as the rate of strain creep for the group. 

 Using this rate and direction the small circle would have arrived at 

 B in 1906a, and the fault trace would have moved with it to the 

 position by. By the slip on the fault in 1906 the small circle was 

 severed. The semicircle on the west side moved northwesterly, and 

 the one on the east side moved southeasterly. Both semicircles were 



