THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 125 
and two others, Point Reyes Hill and Tomales Bay, in group 1, at which the displacement 
inferred from indirect evidence is considered certain. Of these five stations, the four 
which are to the westward of the fault of 1906 moved northwestward and the one which 
is to the eastward of the fault, Mount Tamalpais, moved southeastward (see map 24). 
The displacements of four of the five points were nearly parallel, their azimuths being 
for Sierra Morena, Point Reyes Hill, and Tomales Bay, 136°, 143°, and 142° respectively, 
with a mean of 140° (40° W. of N.), while that of Mount Tamalpais was 324° (36° E. of S.). 
The azimuth of the displacement at the fifth, Farallon Light-house, is 118° (62° W. of N.) 
at an angle of about 22° with the other four. The portion of the fault near these points 
has an azimuth of about 145° (35° W. of N.), hence the displacement of four of the five 
points was practically parallel to the fault, the departure being in each case within the 
range of possible error of the determination of the displacement. For the four points 
to the westward of the fault, the amounts of the displacement are in the inverse order of 
their distances from the fault, with the exception of Sierra Morena. For Tomales Bay, 
which is only 2.1 kilometers (1.3 miles) from the fault, the displacement is greatest, 3.89 
meters (12.8 feet), and for the Farallon Light-house, which is 37 kilometers (23 miles) 
from the fault, the displacement is much less, 1.78 meters (5.8 feet). 
From these five stations, one may deduce four laws governing the distribution of the 
earth movement which occurred on April 18, 1906. First, points on opposite sides of the 
fault moved in opposite directions, those to the eastward of the fault in a southerly direc- 
tion and those to the westward in a northerly direction. Second, the displacements of all 
points were approximately parallel to the fault. Third, the displacements on each side 
of the fault were less, the greater the distance of the displaced points from the fault. 
Fourth, for points on opposite sides of the fault and the same distance from it, those 
on the western side were displaced on an average about twice as much as those on the 
eastern side. 
If the proof of these four deduced laws rested upon the evidence of these five stations 
only, it would be insufficient to convince one. Much other evidence in proof of these four 
deduced laws will be shown in this report. The laws are here stated in order that they 
may be kept in mind and tested by the evidence as presented. 
The apparent displacements of the remaining five points of group 1 may now be com- 
pared with the stated laws. 
The displacement of Point Reyes Light-house, believed to be determined with reasonable 
certainty, is apparently about 1.6 meters (5 feet) greater than and differs about 32° in 
direction from the displacement which might be inferred from the above laws and com- 
parison with the surrounding stations. 
The displacement of Bodega, of which the determination is somewhat doubtful, is Just 
what would be inferred from the deduced laws, as its amount is greater than for Mount 
Tamalpais, corresponding to the fact that it is closer to the fault, and its azimuth agrees 
within 2° with that of the fault. 
The displacement of Ross Mountain, of which the determination is doubtful, agrees 
very closely in amount with that at Mount Tamalpais and differs only 15° in direction. 
Ross Mountain is on the same side of the fault as Mount Tamalpais and at practically 
the same distance from it. 
The apparent displacements of Rocky Mound and Red Hill, 32 and 19 kilometers 
(20 and 12 miles) from the fault and to the eastward of it, of which the determinations are 
doubtful, agree with the laws in being small but are contradictory as to direction. 
For Sonoma Mountain the triangulation serves to determine the combined displacements 
of 1868 and 1906 as shown in table 3, but not the separate displacements, as this station 
was not involved in triangulation done between 1868 and 1906. ‘The combined displace- 
ments at Sonoma Mountain are of about the same amount and are in approximately the 
same azimuth as displacements of 1868 at Mount Tamalpais, Point Reyes Hill, Tomales 
