Vol. 65.] OF THE CALIFORNIA!* EARTHQUAKE F 1906. 7 



base-line, because it reduced the apparent displacements to a 

 minimum, had very probably been shortened 1 ; and as this shortening 

 would have the effect of increasing the scale of the survey, and of 

 making the more distant stations appear to be farther removed 

 from each other and from the base-line, it became evident that the 

 calculated displacements were in all probability made up of two 

 elements, (1) the real shift due to the earthquake, and (2) an 

 apparent shift due to an alteration in length of the base-line. It 

 was not, however, possible to determine the separate amounts due 

 to each of these causes. 



On examining the chart of displacements in California, the same 

 feature is evident as that which was observed in Assam; that is 

 to say, there is, in spite of irregularities and exceptions, a general 

 tendency to displacement outwards, or away from the base-line, which 

 suggests that this had been shortened. For instance, Point Pinos, at 

 the southern limit of the area, has been shifted about 16 or 19 feet to 

 south-east by east; Bodega Head, near the northern limit, has been 

 shifted about 17 feet to north by west, as the combined result of the 

 earth-movements in 1868 and 1906. A large part of these apparent 

 displacements could be explained, on the supposition that the base- 

 line has been shortened by about 8 feet ; but it is not suggested that 

 shortening to this extent has taken place, for a consideration of the 

 apparent displacements of other stations shows that the alteration, 

 if any, must have been considerably less than this. 



For the 1906 earthquake the data are, unfortunately, scanty near 

 the limits of the area, and the only real check which can be applied 

 is the displacement of Farallon lighthouse by 5-8 feet to north 62° 

 west, or at an angle of about 27° with the general direction of the 

 fault-line. A shortening of the assumed unaltered base-line would 

 give rise to an apparent westerly displacement of Farallon by about 

 double as much as the shortening of the base-line : if, then, the 

 whole of the westerly displacement of Farallon is apparent, it involves 

 a shortening of the base-line by about 1-3 feet ; and, if the real 

 displacement of Farallon was parallel to the fault-line, it leaves an 

 apparent westerly displacement of 1*8 feet to be explained by a 

 contraction of '9 foot in the length of the base-line. We may, 

 therefore, conclude that any alteration which took place in the 

 distance between Mocho and Mount Diabolo must have been of the 



1 Obsessed by the knowledge that the earth is losing heat by radiation into 

 space, and the deduction, by no means inevitable, that it is therefore contracting, 

 geologists have been too prone to regard all strains in the crust of the earth as 

 necessarily congressional, and to ignore the possibility that large areas may 

 possibly be subject to tensile strains. In the case cf the Assam earthquake the 

 hypothesis of origin, which seemed most probable to me, almost necessitated a 

 shortening of the line taken by the Trigonometrical Survey as an unaltered base ; 

 but, if Col. Harboe's suggestion ('Beitragez. Geophysik' vol.v, 1903, pp. 206-36) 

 be accepted, this necessity disappears, and the expansion of the area resurveyed 

 may be real. In California, there seems no escape from the conclusion that there 

 has been a lengthening of the coast-line ; the width of the opening of Monterey 

 Bay has increased 10 feet and the length of the Bay of San Francisco by about 

 the same amount, as the combined result of the movements connected with the 

 earthquakes of 1868 and 1906. 



