142 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
The zero of the tide-staff was connected by leveling with the group of bench-marks 
near the gage at various times during the interval 1898-1907, including a determination 
after the earthquake. The leveling showed no appreciable change in the relation in ele- 
vation of the bench-marks and the tide-staff. Hence, the preceding statements in regard 
to a possible subsidence of the tide-staff on two occasions and in regard to its constancy of 
elevation on April 18, 1906, also apply to this group of bench-marks. 
Before the earthquake the Coast and Geodetic Survey had done leveling which con- 
nected the gage at the Presidio Wharf with various bench-marks in San Francisco from 
Fort Point to the Union Iron Works, and with bench-marks at Sausalito. This leveling 
was not of the grade of accuracy known as precise leveling nor was it done continuously. 
There are also available for use in the present investigation certain relative elevations of 
bench-marks before the earthquake furnished to the Coast and Geodetic Survey by the 
city engineer of San Francisco. These include a bench-mark near the gage at the Presidio 
Wharf. ; 
After the earthquake Mr. B. A. Baird, Assistant, Coast and Geodetic Survey, ran a line 
of precise levels from the Presidio gage to Fort Point and Sausalito, and to the eastward 
thru San Francisco, to the Union Iron Works, connecting with various old bench-marks. 
There were 26 bench-marks connected by the leveling before the earthquake which 
were recovered with certainty by Mr. Baird and the elevations redetermined by him. 
The following table shows the elevations of these bench-marks before and after the earth- 
quake and their apparent changes in elevations. All of the elevations in the table are 
referred to the same datum, which is the reading 8.514 feet (2.5951 meters) on the fixt 
tide-staff at the Presidio Wharf, that being approximately mean sea-level. All the 
elevations are computed on the supposition that the zero of the tide-staff at the Presidio 
Wharf remained unchanged at the time of the earthquake. 
The table shows no appreciable change of elevation of the bench-marks at the Presidio 
Wharf. The maximum apparent change in elevation is 7.0 mm. (0.3 inch), a quantity 
within the possible range of error of the leveling. Mr. G. K. Gilbert, Geologist of the U.S. 
Geological Survey, at the close of an examination made soon after the earthquake and 
before the leveling had been done, exprest the opinion that if this group of bench-marks 
had not changed their relative elevations, they probably had not changed in relation to the 
tide-staff. It is probable, therefore, that these two bench-marks and the tide-staff main- 
tained their absolute elevations unchanged. 
At Fort Point, the three bench-marks near the shore show an apparent rise of 74 mm. 
(2.9 inches) on an average, and bench-mark 9, high up on Fort Point, shows a slightly 
smaller apparent rise, 59 millimeters (2.3 inches). All these are on ground supposed to 
be stable. The rise indicated by the city leveling, in the last column, is considerably 
smaller. 
The two bench-marks at Sausalito show an apparent rise of 37 millimeters (1.5 inches). 
It is not certain that this represents a real change in elevation as referred to the zero of 
the Presidio tide-staff. The errors of the old and new leveling, including the crossing of 
the Golden Gate (about 1.25 miles) in each case, may account for the apparent change. 
In the leveling before the earthquake the elevation was transferred from Presidio to 
Sausalito by water-levels and also by wye leveling with a difference of 13 millimeters 
(0.5 inch). In the precise leveling after the earthquake, the two independent crossings of 
the Golden Gate, each depending on many ‘hours of observation, differed by 30 milli- 
meters (1.2 inches). 
The three bench-marks at and near Fort-Point showed small apparent changes in 
elevation. 
From an examination made soon after the earthquake Mr. G. K. Gilbert, Geologist, 
exprest the opinion that the bench-marks at Lafayette Park were probably more stable 
