128 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
No one of these ten points was determined between 1868 and 1906, hence the observa- 
tions served to determine the combined displacements of 1868 and 1906, as shown in 
table 3, but not the separate displacements. ‘The separate displacements have been de- 
termined by interpolation from surrounding stations for the three points, Point Reyes 
Hill, Tomales Bay, and Bodega, as indicated in the discussion of group 1. The same 
process has also been applied to the seven points of group 4. 
Starting with the interpolated displacements of 1868 for the three points, Point Reyes 
Hill, Tomales Bay, and Bodega, as shown in table 2, and with map 25 before one, it was a 
sithpis matter to interpolate separately the meridian components and the prime vertical 
components of the displacements of 1868 for the seven stations of group 4. This amounts 
practically to interpolating the displacements for these points from the three observed 
displacements of 1868 at Mount Tamalpais, Farallon Light-house, and Ross Mountain. 
The resulting interpolated displacements of 1868 are shown in table 2. Each of these 
being subtracted, component by component, from the corresponding combined displace- 
ment of 1868 and 1906, as shown in table 3, leaves the displacement of 1906 as shown 
in table 1. 
A study of the possible accumulated errors in the triangulations shows that all of the 
seven displacements of 1906 in group 2 are certain except for Hans and Hammond. 
There is about one chance in five that the apparent displacements of 1906 for these two 
points are simply due to errors of observation. 
The ten displacements of 1906 in this group show clearly the four laws already sug- 
gested in regard to such displacements. All points to the eastward of the fault moved 
southerly and those of the western side, northerly. Four of the five points to the west- 
ward of the fault moved in azimuths between 141° and 143° with a mean of 142° (88° W 
of N.). The azimuth of this part of the fault is about 145° (35° W. of N.). The azi- 
muth of the fifth displacement on the west side, at Bodega Head, is 172° (8° W. of N.). 
The azimuths of the three reasonably certain displacements of points to the eastward of 
the fault vary from 323° to 348° with a mean of 334° (26° E. of 8.), which is within 9° of 
being parallel to the fault. Of the five points to the westward of the fault, the one nearest 
to the fault, Foster, has the greatest displacement. The other four, all between 2.0 
and 2.7 kilometers from the fault, have nearly equal displacements. The five displace- 
ments for points to the eastward of the fault show a slight tendency to stand in inverse 
order from the distances from the fault. But one only of these displacements differs by 
more than 0.42 meter (1.4 feet) from the mean of the five, and the estimated distances 
from the fault vary only from 0.5 to 2.6 kilometers. When the uncertainty of the posi- 
tion of the fault beneath Tomales Bay is considered, as well as the small variation 
in distance of these ten points from the fault, difficulties are to be expected in 
detecting the relation between displacement and distance from the fault in this group. 
The mean displacement of the points to the eastward of the fault is 1.86 meters 
(6.1 feet) and of the five points to the westward 2.1 times as much, namely, 3.88 meters 
(12.7 feet). 
Group 5. Vicinity of Fort Ross. — There are twelve points in this group, all de- 
termined by secondary triangulation in 1875-1876 and again in 1906, the scheme of 
triangulation being in each case substantially the same as that shown on map 25. The 
base from which these positions are determined is not independent of observations made 
before 1868, but is gotten by making the observations preceding that date conform to 
those made between 1868 and 1906. From the small size of the necessary corrections to 
the observed angles, and from the fact that the position of Ross Mountain, which pre- 
dominates the group, is determined by observations made entirely after 1868, the error of 
assuming that these twelve points belong to the period between 1868 and 1906 is deemed 
negligible. 
