THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 149 
movement in the firm rocks below. For the middle half of the extent of the fault-trace 
from Point Arena to Crystal Springs Lake, these maximal measurements are very com- 
monly from 15 to 16 feet, and these figures may thus be taken as a minimum expression 
for the amount of the displacement on the fault for this segment. In the southern quarter 
of the extent of the fault-trace, the maximum offset is about 8 feet, and this may simi- 
larly be taken as a general minimum expression for the displacement on this segment, 
except for the extreme south end, where it dies out. The amount of displacement at the 
northern end of the fault has not been ascertained. 
The geodetic measurements of the earth movement, as presented in the paper by 
Messrs. Hayford and Baldwin, are of extreme interest and form one of the most important 
contributions to the study of the earthquake. The evidence of displacement observed 
along the fault-trace affords measurements of the total relative movement only, while 
the geodetic work gives us an approximate measure of the absolute movement on either 
side of the fault, and the distribution of the movement away from the fault. The results 
of this geodetic work are not only set forth in detail by the paper of Messrs. Hayford and 
Baldwin, but they are also admirably summarized, so that all that seems necessary in this 
place is to discuss very briefly these results from a geological point of view. 
A notable feature of the paper is the discovery of a movement of the earth’s crust which 
antedates the earthquake of April 18, 1906, and which is referred to the earthquake of 
1868; altho it is recognized that the date and duration of the movement cannot, on the 
data available, be positively determined. Inasmuch as the time of this movement is left 
an open question, and is referred to the year 1868 largely as a matter of convenience in 
discussion, it may be of advantage to inquire briefly whether or not it may have some other 
significance than that of a sudden movement occurring in that year. 
Altho, as shown in another part of this report, the earthquake of 1868 was Pilate toa 
rupture or series of ruptures of the ground at the base of the hills on the northeast side of 
San Francisco Bay, there was no evidence of a large relative displacement such as occurred 
in 1906. It seems reasonable to suppose that if the earlier movement in question had 
occurred suddenly in the same way as that of April 18, 1906, we should have had a similar 
manifestation of faulting within the region affected. Since there was no such manifesta- 
tion the reference of the earlier movement to the earthquake of 1868 may be fairly ques- 
tioned, and another hypothesis entertained to explain it, particularly if this hypothesis 
harmonizes in some considerable measure with the results of the geodetic survey. 
This hypothesis is that the earlier movement is not immediately or exclusively asso- 
ciated with the earthquake of 1868, but is the expression of the strain in the earth’s crust 
which led to the rupture or slip of 1906 and the consequent earthquake. That rupture 
presupposes a condition of strain, and it is difficult if not impossible to conceive of such 
a sudden disruption except as a relief from strain. Such strain involves the idea of slow 
displacement; and if a series of points had been established in the territory affected at 
different dates, with reference to some base beyond it, a measure of this slow displace- 
ment or creep of the earth’s crust might have been obtained. 
The strain culminated in a slip on an old rupture plane and may fairly be suppose. 
to have been more or less symmetrically distributed with reference to that plane, so that 
when relief was effected by slip, the movement involved would be equal in amount on the 
two sides of the fault. 
This hypothesis and its implications appear to fit fairly well with the results of the 
geodetic resurvey, particularly for that portion of the territory where the earlier move- 
ment can be most satisfactorily discriminated from the displacement of 1906. For 
example in the Tomales Bay region there are ten points, viz.: Bodega Head, Tomales 
Point, Tomales Bay, Foster, and Point Reyes Hill on the west side of the fault of 1906, 
and Bodega, Smith, Mershon, Hans, and Hammond on the east, at which the two move- 
