148 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
evidence of differential vertical movement. For many segments of the fault-trace in this 
region, there is no suggestion of displacement of this kind. In other portions, notably 
in the vicinity of Black Mountain and southward, the movement appears to have been 
distributed over a considerable zone, with the formation of many auxiliary cracks. 
Upon the latter scarps were formed, but these in some cases faced the northeast and in 
others the southwest, and the resultant effect is not known. Judging from the localities 
where the movement was not so distributed, but was confined to a narrow zone, the 
differential vertical displacement was nil. 
Similarly, the evidence of vertical displacement, based on a comparison of the relative 
position of land and sea-levels before and after the earthquake, is limited to the region 
north of the Golden Gate. The Point Reyes Peninsula appears, from this class of evi- 
dence, to have been probably upraised slightly by the fault movement; but the evidence 
is not entirely conclusive. 
Observations conducted by the Coast and Geodetic Survey thruout the year suc- 
ceeding the earthquake, at the tide-gage station near Fort Point in the Golden Gate, show 
that the relative level of land and sea at that point is the same as it was before the earth- 
quake. Since this station lies on the northeast side of the fault, the observation would 
appear to indicate that any upward movement of the crustal block on the southwest side 
was an absolute one. 
The horizontal displacement on the fault, as measured on fences, roads, and various 
structures which crost the fault-trace, is also apparently quite variable, ranging from a 
foot or less up to 20 or 21 feet. This variation is probably due to a number of causes. 
The principal one of these is the fact that the displacement was not always confined to the 
sharp line upon which an offset was observed at any locality. Auxiliary cracks, dis- 
tributed over a zone not uncommonly a few hundred feet wide, took up portions of the 
displacement; and these auxiliary cracks doubtless escaped observation in many cases. 
Indeed, owing to the yielding character of the superficial mantle of soil and regolith, it is 
probable that many of these auxiliary cracks did not appear as ruptures at the surface. 
Besides this distribution of the displacement on auxiliary cracks satellitic to the main 
rupture, the deformation of the ground along the latter, both superficially and in its 
deeper portions, was probably variable. The extent of this drag is shown in a few in- 
stances that have been susceptible of measurement; notably the fence at Fort Ross, sur- 
veyed by Mr. E. 8. Larsen, on which a displacement of 12 feet was distributed over a 
distance of 415 feet on the southwest side of the fault-trace; the roadway near Point 
Reyes Station, where a displacement of 20 or 21 feet was distributed over 60 feet; the 
fence south of Mussel Rock, surveyed by Mr. H. O. Wood, in which a displacement of 13 
feet was distributed over 250 feet on the southwest side of the fault-trace and 40 feet on 
the northeast side; the 3 fences surveyed by Mr. R. B. Symington near San Andreas 
Lake, one showing a displacement of 16.9 feet, distributed over more than 1,100 feet, the 
second a displacement of 10.4 feet distributed over more than 300 feet, and the third a 
displacement of 12.7 feet distributed over more than 2,200 feet; and the tunnel at Wright, 
surveyed by the engineers of the Southern Pacific Company, showing a displacement of 
5 feet distributed over nearly a mile on the southwest side of the fault-trace. These 
instances are doubtless indicative of the general character of the deformation of the ground 
in the immediate vicinity of the fault, and aid in understanding the variable expression of 
the amount of offset at the main fault-trace. The recognition of the distribution of the 
movement on auxiliary cracks, some of which may not have appeared at the surface, and 
the deformation of the ground along the zone of rupture, justifies the conclusion that, 
except under peculiar conditions — such, for example, as in the marsh at the head of 
Tomales Bay — the maximum figures obtained for the displacement by the measure- 
ment of offsets at the surface must be a minimum expression for the true extent of the 

