THE SAN FRANCISCO EARTHQUAKE 113 



feet pertains to every point near the fault line, but it is not supposable 

 that the same dislocation affects points at a great distance from the 

 fault. At some remote point, for example Z , in the direction B'C, 

 there was no displacement. If B'C and FG were both produced in that 

 direction they would be found not precisely parallel, but would eventu- 

 ally coalesce. How far the undisturbed region Z may be from the 

 fault line is a matter of pure conjecture, but we may plausibly assume 

 that the transverse dimension of the 

 area affected by the displacement 

 is of the same order of magnitude as 

 the length of the fault line and is 

 measured by hundreds of miles. If 

 this assumption is correct, then 

 throughout a great region in central 

 and northern California all points 

 have experienced a change in geo- 

 graphic position, the change in the 

 vicinity of the fault being of about 



b 



Y 



five feet and the amount diminish- fig. 14. diagrammatic plan of a 



-, ,1 ,-, t n Portion of the Earthquake Fault, illus- 



ing toward the northeast and south- trating changes in geographic position . 



west. If the only determinations 



of latitude and longitude within this area were of the ordinary approxi- 

 mate character, it would be impossible to measure the changes in geo- 

 graphic position theoretically accomplished by the fault; but it fortu- 

 nately happens that the region is traversed by two belts of the tri- 

 angulation of the United States Coast and Geodetic Survey, one being 

 a system of triangles for the control of the coastal map work, and the 

 other the elaborately measured transcontinental belt. The region thus 

 contains several scores of points whose coordinates have been deter- 

 mined with a high degree of precision, and it is possible by the rede- 

 termination of these positions to measure the dislocations which have 

 taken place in connection with the earthquake. As all topographic and 

 hydrographic maps of California are dependent for their latitudes and 

 longitudes upon the positions given by this triangulation, and as there 

 is reason to believe that many of these positions have been disturbed 

 by a measurable amount, the superintendent of the Coast Survey has 

 determined to repeat so much of the work of triangulation as may be 

 necessary in order to redetermine the geographic positions. And it is 

 proposed to carry this work far enough eastward to connect the rede- 

 termined points with stations that may safely be regarded as quite 

 beyond the effect of the recent fault. When this has been accomplished 

 much light will be thrown on the nature and distribution of the strains 

 which were relieved by the dislocation along the fault line, and it will 



VOL. LXIX. — 8. 



