140 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION, 
rena. The line from Sonoma, 79 kilometers (49 miles) long, mist the adopted position 
by 0.10 meter (0.3 foot) and all the others came closer. 
One other assumption remains to be examined. The displacements of 1868 were com- 
puted on the assumption that the line Mount Tamalpais to Mount Diablo had a certain 
length and azimuth before 1868 and a certain different length and azimuth after 1868 ; 
Mount Tamalpais being supposed to be in a new position, but Mount Diablo unmoved. 
The two positions for Mount Tamalpais were derived from certain computations based 
in turn on assumptions that certain other stations remained unmoved in 1868, or prac- 
tically so. 
The azimuth of the line Mount Tamalpais to Mount Diablo was determined by observa- 
tions upon stars in 1859, and again in 1882; the later observations made the azimuth 7.84” 
greater than earlier observations. The two adopted azimuths from the computations of 
triangulation referred to above also differ by 5.38”, the later adopted value being the 
greater. 
The fact that the two independent determinations of change of azimuth, one astro- 
nomical and one geodetic, agree within 2.46” is a strong proof that the adopted geodetic 
azimuths are correct, 2.46” being within the possible range of the various observations. 
Following the same reasoning as for Mocho and Mount Diablo, the computed displace- 
ments of 1868, as shown by red arrows on maps 24 and 25, indicate that the two azimuths 
and two lengths used for the line Mount Tamalpais to Mount Diablo, before and after 
1868, must be very close to the truth. 
CHANGES IN ELEVATION. 
The preceding portions of this Report have dealt with permanent horizontal displace- 
ments caused by the earthquake of 1906. It is important to know whether permanent 
displacements in the vertical sense also occurred. Upon this point the observations of 
the Coast and Geodetic Survey furnish evidence for a small area, involving parts of 
San Francisco, both sides of the Golden Gate, and Sausalito, 1.25 miles north of the 
rolden Gate. 
At the time of the earthquake an automatic tide-gage was in operation at the Presidio 
Wharf, in San Francisco, on the southern side and about 1.25 miles to the east of the 
narrowest part of the channel thru the Golden Gate. The gage had been in operation at 
that point continuously since July 17, 1897, and is still in operation. 
The record made by this gage on April 18, 1906, showed an oscillation, with a range of 
about six inches, in the water surface evidently produced by the earthquake, but it showed 
no evidence of a change in the relation of the gage zero to mean sea-level. In other words, 
the record for that day does not indicate that the tide-staff had been changed in elevation 
by the earthquake. 
To detect any possible small change in elevation it is, of course, necessary to examine 
much more record than that for a single day. The examination has now been extended 
by computation to include a whole year of observations since the earthquake for com- 
parison with nine years of observations before it. 
The following table shows the reading of mean sea-level on the fixt tide-staff for each of 
ten years, as determined by taking the mean of the hourly ordinates of the tidal curve. 
The annual means are taken rather than means for any other period in order to eliminate 
annual inequalities, presumably due to meteorological causes, which affect the means 
for separate months. May 1 is taken as the beginning of the complete year available 
after the earthquake. Since it is not convenient, in the computation, to separate any 
month’s observation into two parts, the year is commenced on May 1, rather than on 
April 18, the date of the earthquake. The first year, 1897-1898, is incomplete because 
the observations were not commenced until July 17, 1897. 
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