1921] Lau'son: llie Mobility of the Coast Ranges of Calif ornia 443 



zone are sliced into short segments and moved differentially parallel 

 to themselves; there is no diminution of the width of the zone and 

 therefore no creep of the region on either side towards the fault. The 

 general result of the discussion is that it is highly improbable that 

 there is any appreciable transverse creep or shortening due to shear. 

 The reason for examining into the matter is that there is, as will 

 appear later, a suggestion of a transverse movement in the region 

 traversed by the San Andreas fault, and it seemed necessary to exhaust 

 this possibility. 



rig. 4. Shearing by slicing without transverse shortening. 



LONGITUDINAL AND TEANSVEESE STEAIN 



The subcrustal flow which, by carrying the crust with it, induces 

 strain in the latter, must be local. Beyond the region affected, in the 

 direction of the stress, there must be a stable region against which the 

 mobile region is forced so as to produce a condition of compressive 

 strain. Similarly, in the case before us, for sections transverse to the 

 direction of stress the latter must be differential, otherwise there could 

 not be induced the particular kind of strain which would be relieved 

 by a vertical shear and horizontal slip on a vertical fault. If the 

 stress, and therefore the strain, were uniformly distributed in the east- 

 west direction normal to the stress, the strained condition on reaching 

 the limit could be relieved only by slip on an inclined fault with 

 east-west strike, combined with shearing on the borders of the region 

 affected by the stress. Since, then, there are two modes of relief for 

 the strain generated under the hypothesis of a subcrustal current, it 

 will be convenient to recognize the two kinds of strain by separate 

 designations. I shall, therefore, call the strain which would be relieved 

 by slip on a low dipping transverse fault the longitudinal strain, and 

 that which would be relieved by slip on a vertical fault the transverse 

 strain. The San Andreas fault is, at least near the surface, vertical, 

 and its strike is about 144° ; that is, it intersects the northerly direc- 

 tion of strain creep or of stress at an angle of about 30°. At the 



