Geology and Mineralogy. 83 



ment of that date took place is at the time of writing not fully 

 known. It is, however, known from direct field observations 

 that it extends certainly from the mouth of Alder Creek near 

 Point Arena to the vicinity of San Juan in San Benito County, 

 a distance of about 185 miles. The destruction at Petrolia and 

 Ferndale in Humboldt County indicates that the movement on 

 the rift extended at least as far as Cape Mendocino, though 

 whether the rift lies inland or offshore remains 'as yet a matter 

 of inquiry. Adding the inferred extension of the movement to 

 its observed extent gives us a total length of about three hundred 

 miles. The general trend of this line is about N. 35° W., but in 

 Sonoma and Mendocino counties it appears to have a slight con- 

 cavity to the northeast, 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 southwest of the rift has 

 moved northwesterly relatively to the country on the northeast 

 of the rift. By this it is not intended to imply that the northeast 

 side was passive and the southwest 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 unequivocal. 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 six feet. A more 

 common measurement is eight to ten feet. In some cases as much 

 as fifteen or sixteen feet of horizontal displacement has been 

 observed, while in one case a roadway was found to have been 

 differentially moved twenty feet. Probably the mean value for 

 the amount of horizontal displacement along the rift line is about 

 ten feet and the variations from this are due to local causes such 

 as drag of the mantle of soil upon the rooks, or the excessive 

 movement of soft incoherent deposits. Besides this general hori- 

 zontal displacement of about ten feet, there is observable in 

 Sonoma and Mendocino counties a differential vertical movement 

 not exceeding four feet^ so far as at present known, whereby the 

 southwest side of the rift was raised relatively to the northeast 

 side, so as to present a low scarp facing the northeast. This 

 vertical movement diminishes to the southeast 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 consequence of the movement it is probable that the lati- 

 tudes and longitudes of all points in the Coast Ranges have been 

 permanently changed a few feet, and that the stations occupied 



