124 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
There is about 1 chance in 7 that the apparent displacement of Bodega, shown in table 
3, is due to errors of observation. It is reasonably certain that this is a real displacement. 
For the remaining six points in group 1, Sierra Morena, Mount Tamalpais, Farallon 
Light-house, Point Reyes Hill, Tomales Bay, and Sonoma Mountain, each of the appar- 
ent displacements given in the tables as observed is real, being in each case clearly 
beyond the maximum which could be accounted for as errors of observation. 
Prof. George Davidson has believed for many years that Mount Tamalpais moved dur- 
ing the earthquake of 1868 and that the triangulations made before and after that date 
showed such a displacement. Accordingly in 1905, at his request, a reéxamination was 
made at the Coast and Geodetic Survey office of the evidence furnished by the triangula- 
tions, and the conclusion was reached that a real displacement of Mount Tamalpais 
occurred in 1868. At that time, however, convincing evidence was not discovered that 
any other triangulation station moved in 1868. In the more extensive studies made in 
connection with the present investigation, and with the additional skill acquired in 
recognizing the effects of earthquakes upon triangulation, it became evident, as shown 
in table 2, not only that Mount Tamalpais moved in 1868, but also that the Farallon 
Light-house and Ross Mountain moved at that time, the three apparent displacements 
being clearly beyond the range of possible errors of triangulation. The displacements 
for these three stations are similar. The amount of the displacement is least at Faral- 
lon Light-house, 1.389 meters (4.6 feet), and greatest at Ross Mountain, 1.70 meters 
(5.6 feet). The azimuth of the displacement is least at the Farallon Light-house, 153° 
(27° W. of N.), and is greatest at Ross Mountain, 182° (2° E. of N.). (See map 24.) The 
apparent differences in direction and amount of the three displacements may or may 
not be real. It is certain therefore that in 1868 the large part of the earth’s surface 
included between these three stations, at least 700 square miles, moved about 1.5 meters 
(4.9 feet), in about azimuth 168° (12° W. of N.). 
Within the triangle defined by the three stations, Mount Tamalpais, Farallon Light-house, 
and Ross Mountain, which certainly were displaced in 1868, are the three stations, Point 
Reyes Hill, Tomales Bay, and Bodega, of group 1. It is therefore believed to be reasonably 
certain that these stations were displaced at that time. The probable displacements were 
interpolated from the three displacements observed at the first three stations, taking into 
account the relative positions of the stations. The resulting interpolated displacements 
are shown in table 2. Other evidence, tending to show that these interpolated values of 
the displacements are real, will be brought forward later. 
For the three stations, Point Reyes Hill, Tomales Bay, and Bodega, the positions were 
determined before 1868 and after the earthquake of 1906, but not during the interval 
1868-1906 ; hence the computation of the positions determined by triangulation for these 
stations furnishes simply the combined displacements of 1868 and 1906 as shown in table 
3. As noted in the preceding paragraph, the displacement of 1868 has, for these three 
stations, been interpolated from surrounding stations and entered in table 2. The 
differences * between these inferred displacements in table 2 and the observed combined 
displacements in table 3 were then taken and are shown in table 1, as inferred displace- 
ments in 1906. As indicated in the marked column of table 1, these inferred displace- 
ments are believed to be certain for two of these points and somewhat doubtful for the 
third, Bodega. 
The doubtful apparent displacements at Rocky Mound and Red Hill in 1868 (see table 
2) agree with other displacements which are certain, in having a decided northward 
component. 
In table 1, showing the displacements of 1906, there are three stations, Sierra Morena, 
Mount Tamalpais, and Farallon Light-house, at which observed displacement is certain, 


1 The differences were taken separately for the meridian components and the prime vertical com- 
ponents and then combined to secure the direction and amount of the resultant. 
