AND VALPAEAISO EARTHQUAKES AND THEIR CAUSES. 47 



a total length of about 300 miles. The general trend of this line is 

 about X. 35° AV., but in Sonoma and ^lendoeino counties it appears 

 to have a slight concavity to the north-east, and if this curvature 

 be maintained in its path beneath the waters of the Pacific it would 

 pass very close to and possibly inside of Capes Gordo and Mendocino. 

 Along the 185 miles of this rift where movement has actually been 

 observed the displacement has been chiefly horizontal on a nearly 

 vertical plane, and the country to the south-west of the rift has 

 moved north-westerly relatively to the country on the north-east of 

 the rift. By this it is not intended to imply that the north-east side 

 was passive and the south-west side active in the movement. Most 

 probably the two sides moved in opposite directions. The evidence 

 of the rupture and of the differential movement along the line of 

 rift is very clear and imeciuivocal. The surface soil presents a 

 continuous furrow, generally several feet wide, with transverse 

 cracks, which show very plainly the effort of torsion within the 

 zone of the movement. All fences, roads, stream courses, pipe-lines, 

 dams, conduits, and property lines which cross the rift are dislocated. 

 The amount of dislocation varies. In several instances observed it 

 does not exceed 6 feet. A more common measurement is 8 to 10 

 feet. In some cases as much as 15 or 16 feet of horizontal 

 displacement has been observed, while in one case a roadway was 

 found to have been differentially moved 20 feet. Probal:>ly the mean 

 value for the amount of horizontal displacement along the rift line 

 is about 10 feet, and the variations from this are due to local causes, 

 such as drag of the mantle of soil upon the rocks or the excessive 

 movement of soft, incoherent deposits. Besides this general 

 horizontal displacement of about 10 feet there is observable in 

 Sonoma and Mendocino Counties a differential vertical movement 

 not exceeding 4 feet, so far as at present known, whereby the south- 

 west side of the rift was raised relatively to the north-east side, so 

 as to present a low scarp facing the north-east. This vertical 

 movement diminishes to the south-east along the rift line, and in 

 San Mateo county is scarcely if at all observable. Still farther 

 south there are suggestions that this movement may have been in the 

 reverse direction ; but this needs further field study. 



"As a consecjuence of the movement it is probable that the 

 latitudes and longitudes of all points in the Coast Kanges have been 

 permanently changed a few feet, and that the stations occupied by 

 the Coast and Geodetic Survey in their triangulation work have been 

 changed in position.- It is hoped that a re-occupation of some of the 

 stations by the Coast and Geodetic Survey may contribute data to 

 the final estimate of the amount of movement. 



" The great length of the rift upon which movement has occurred 

 makes this earthquake unique. Such length implies great depth of 

 rupture, and the study of the question of depth will, it is believed, 

 contribute much to current geophysical conceptions. 



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