468 University of California Publications in Geology [Vol. 12 



probably a coincidence, for we are now in a region where the geodetic 

 data are less certain than they are to the north. The amount of dis- 

 placement of Gavilan is postulated on the immobility of Santa Ana. 

 But while Hayford and Baldwin^" appear to be justified in their 

 assumption that Santa Ana suffered no displacement in 1906, it is 

 probable that this station has participated in the strain creep of the 

 region, and it is not improbable that it moved suddenly in 1868. The 

 position of Santa x\na was determined in 1852 and its change of 

 position between that date and 1906^, whether by strain creep or 

 rebound, would seriously affect the measure obtained for the displace- 

 ment of Gavilan. ' 



Santa. Cruz and Point Pinos. — Santa Cruz and Point Pino.s are 

 somewhat anomalous, particularly in the azimuth of their direction 

 of displacement. The uncertainty as to the movements of Santa Ana, 

 however, renders futile any attempt to discuss the behavior of the 

 stations at these places. Under certain plausible assumptions as to 

 the movements of Santa Ana between 1852 and 19062?, consistent 

 with the hypothesis of strain creep and rebound, a large part of the 

 supposed displacement of Point Pinos and Santa Cruz might be unreal. 

 If, contrary to probability, Santa Ana were immobile between 1852 

 and 1906p, then the ascertained displacements of both Point Pinos 

 and Santa Cruz may be made consistent with the displacements of 

 other points nearer the San Andreas fault by assuming a larger 

 azimuth for the direction of rebound in 1868. 



The Colma Group. — The behavior, in 1906, of the group of geodetic 

 stations near Colma, comprising Flat Road, False Cattle Hill, and 

 San Pedro Rock to the west of the San Andreas fault and Black Bluff, 

 Black Ridge, and San Bruno Mountain to the east, is anomalous. The 

 anomaly, however, is not of the same kind on the two sides of the 

 fault. On the east side of the fault it appears in the smallness of the 

 amount of displacement ; while on the west side, although the amount 

 of displacement is approximately normal, the direction of that dis- 

 placement has a notable component toward the fault. Both anomalies 

 may perhaps be referred to a common cause inherent in the geological 

 structure of this part of the region. Immediatel.y to the east of the 

 San Andreas fault, between it and San Bruno Mountain, is a deep 

 wedge of late Tertiary sediments, comprising the Merced and later 

 formations, which are relatively incoherent and inelastic.-" Such 



19 Earthquake Report, vol. I, p. 130. 



20 See U. S. G. S. 15th Ann. Ept., pp. 459 et seq. ; also folio 193, pp. 14-16. 



