8 jvik. r. d. oldhaji on the interpretation [Feb. 1909, 



nature of compression, and could not have exceeded 1 foot at most. 

 This measure must be accepted as a maximum value ; and although 

 we may be certain that the base-line was not shortened by more 

 than a foot or so, it is by no means certain that it was altered to 

 this extent, and it is evident that the base-line was either outside, 

 or not far from, the eastern limit of the permanently distorted 

 area. 1 



To the westwards, it is impossible to say how far this extended 

 under the bed of the sea ; Farallon lighthouse, at 23 miles from the 

 fault, was certainly included, for the northerly displacement cannot 

 be explained by any shortening of the base-line, and the fact that 

 the displacements near the fault-line were greater on the west than 

 on the east suggests that the permanently distorted area extended 

 farther to the west than to the east of the fault-line. The width 

 of the area over which displacement of the ground took place to a 

 greater or less extent may be put, roughly, as not far from 30 miles 

 to the east and 50 miles to the west, or a total width of about 80 

 miles, near the parallel of San Francisco. 



§ 3. We may now turn to the interpretation of the displacements, 

 and are met at the outset by the apparently inexplicable nature of 

 the movements near the fault-line, movements which appear to 

 involve thrusts in opposite directions at the same time and the same 

 place. An explanation has been suggested to me by Prof. Perry, 

 which accounts for most of the facts. If a block of coherent 

 material is subjected to a lateral distortion as indicated by the 

 arrows outside the square in fig. 3 (p. 9), it will experience a series 

 of stresses and strains represented by the arrows inside the square ; 

 here we have a right-handed couple, indicated by the vertical 

 unbroken arrows, balanced by a left-hand couple, indicated by the 

 horizontal arrows, and as the resultant of these two couples the 

 system of compressional and tensile stresses indicated by the broken 

 arrows. These compressional and tensile stresses, in combination 

 with each other, produce a shear in the directions MM and jN"jN~ ; and, 

 if there be weakness in either of these directions, a sliding fracture 

 may arise, accompanied by movements on opposite sides in the 

 directions of the unbroken arrows, while elsewhere, if there is not 

 weakness, or the stresses are not great enough to cause fracture, 

 there will be a strain but no movement, and so we have an effect 

 produced which resembles the shifting at the San Andreas fault. 2 



1 [The distribution of isoseismals, as determined by the Califoraian Earth- 

 quake Investigation Commission, shows that this, and the statements in the 

 subsequent paragraph, are only true so far as they apply to displacements 

 directly connected with tbe movement along the San Andreas fault. There 

 was evidently another centre of disturbance in the San Joaquin Valley, about 

 40 miles east by south of Mocho and about 20 miles beyond the continuation 

 of the Mocho-Mount Diabolo line.] 



2 To prevent misunderstanding, it may be well to state that I use the words 

 strain, stress, and shear in their physical sense. Strain was defined by 

 Rankine as the change of volume and figure, constituting the deviation of a mole- 

 cule of a solid from that condition which it preserves when free from the action 



