1921] Lawson: The Molilittj of the Coast Ranges of California 469 



sediments are incapable of accepting a condition of elastic compression. 

 Any stress applied to them would not generate a condition of strain, 

 but would be dissipated in plastic deformation. It is, therefore, highly- 

 probable that although the firm rock below the Merced may have been 

 in a condition of strain, this strain could not have been communicated 

 to this body of sediments. They, therefore, lacked in large measure 

 the ability to rebound when the strain of the region was relieved by 

 the slip on the San Andreas fault. It is true that they might have 

 been carried south by the elastic rebound of the underlying firm rocks, 

 but, owing to the suddenness of this rebound, the principle of inertia 

 would operate to restrain movement at the surface, and the tendency 

 would have been largely quenched in plastic deformation. It is thus 

 comprehensible that in the Merced terrane the condition of transverse 

 strain prerequisite to rebound was lacking, and that, therefore, in the 

 upper mile of the crust the transverse strain could not be transmitted 

 to the firm rocks of San Bruno Mountain and Black Ridge, so that 

 their tendency to rebound was also feeble. 



The eastward component of motion of the ground on the west of 

 the fault may be explained by a deformation of the fault plane itself, 

 due to a slight east-west condensation of the wedge of Merced sedi- 

 ments. At the time of the slip in 1906 the large east-west component 

 of the free elastic force on the west side of the fault may have been 

 sufficient to stove in the fault plane towards the feebly resistant wedge 

 of inelastic Merced strata. 



APPARENT DISTENSION 



Certain phenomena of apparent distension call perhaps for a 

 special discussion. Hayford and Baldwin-^ first directed attention to 

 the fact that in the interval between surveys I and III there was an 

 increase of 3 meters in the distance between Taiualpais and Black 

 Mountain, of 2 meters between Black Mountain and Loma Prieta, of 

 1.2 meters between Loma Prieta and Gavilan and of 3 meters between 

 Santa Cruz and Point Pinos. The last mentioned increase is somewhat 

 doubtful owing to the assumption of immobility of Santa Ana, the 

 controlling station, but there seems to be no reason for doubting the 

 reality of the others. This increase of the distances between geodetic 

 stations was taken by both Rothpletz and Wood as significant of a 

 deep-seated distension of the region. The phenomena are, however, 



21 Op. cit., pp. 1.32-133. 



