Gilbert — The California Earthquake. 49 



points near the fault line. For points at greater distance the 

 changes were less. The discussion of the data, bj J. F. Hay- 

 ford and A. L. Baldwin, led to the conclusion that the absolute 

 movement was greater west of the fault than east of it, and in 

 both directions diminished with distance from the fault, the dimi- 

 nution being most rapid in the immediate vicinity of the fault. 



This earthquake is practically unique, among the small 

 group that have been broadly studied, in that the stress couple 

 to which the fault may be referred lay in the horizontal plane. 

 The main associated distortions were distortions in ground 

 plan, with little vertical complication. They were, therefore, 

 exceptionally adapted for measurement by the method of 

 triangulation, and the results actually obtained are more syste- 

 matic than any previous results of the same character. It is, 

 therefore, peculiarly unfortunate that they were qualified by 

 a lack of chronologic unity in the trigonometric surveys 

 preceding the fault, which were strung along through several 

 decades. This fact made it impossible to discriminate between 

 deformation at the time of rupture and progressive deforma- 

 tion during accumulation of strain before rupture ; and if 

 progressive deformation took place before rupture, the pre- 

 cision of the adjusted triangulation was thereby impaired. 

 Nevertheless the results invite the careful attention of geophys- 

 icists. To the reviewer the distribution of dislocation, and 

 especially the existence close to the fault, on each side, of a 

 belt of maximum distortion, seems clearly not that which 

 would obtain if the fault passed completely through a solid 

 crust to a liquid substratum. And it appears also that, on the 

 assumption of continuous solidity from the surface downward, 

 the geodetic results might yield to adequate analytic treatment 

 a conception of the order of magnitude of the vertical distance 

 to which the fault penetrated. 



The surface outcrop of the fault was definitely traced from 

 San Juan to Point Arena, a distance of 190 miles. At Point 

 Arena it passes under the sea, and there is doubt as to its 

 further course. A fault made at the same time on a more 

 northerly part of the coast may be its continuation, after 

 inflection, or may be on an independent line; but in either 

 case the total length of dislocation was about 270 miles. 



At all points the fault follows a peculiar topographic feature 

 to which the name San Andreas rift was given ; but the rift 

 is more extensive than the fault of 1906, having been traced to 

 the Salton basin, several hundred miles southeast of San Juan. 

 In its larger expression the rift is a trough, a trough coin- 

 ciding in general trend with the Coast Eanges, but crossing 

 various mountain ridges obliquely, or even following their crests. 



Am. Jour. Sci. — Fourth Series, Vol. XXVII, No. 157. — January, 1909. 

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