144 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
than any of the others examined by him. The table indicates that the two of these bench 
marks, formerly determined by the Coast and Geodetic Survey leveling, subsided 75 
millimeters (3.0 inches) and that the one, determined by the city leveling, rose 43 milli- 
meters (1.7 inches). There is no apparent reason for the contradiction among the three 
bench-marks of this group. 
For the three bench-marks at the Union Iron Works, the table shows a contradiction, 
two of them having, apparently, increased in elevation and one having decreased. ‘The 
greatest change is, however, only 52 mm. (2.0 inches). The Union Iron Works is said to 
be partly on filled ground. 
The two bench-marks near the Magdalen Asylum apparently increased in elevation 
as shown by both the Coast and Geodetic Survey and city leveling. 
Of these bench-marks, the thirteen in the five groups at Fort Point, Sausalito, Fort 
-Mason, Union Iron Works, and Magdalen Asylum, showed an average apparent rise at 
the time of the earthquake of 35 millimeters (1.4 inches) as determined by the Coast 
and Geodetic Survey leveling. As the leveling simply gives relative elevations the 
question arises, Does this quantity represent an average rise of the thirteen bench- 
marks or does it represent a settlement of the zero of the tide-gage and the adjacent 
bench-marks at the Presidio Wharf? The tidal observations are not competent to 
determine this question with certainty. The general experience with determinations 
of mean sea-level, from long series of tidal observations, warrants the statement that 
the error in determination from a single year is as apt to be greater as less than 0.75 inch 
(19 millimeters) and that it may sometimes be as great as 2.5 inches (64 millimeters). 
It is possible, therefore, that the two bench-marks at the Presidio Wharf and the zero 
of the tide-gage have settled 35 millimeters or that it is, in part, a subsidence at the 
Presidio and in part a rise at the other places. 
The elevations of the group of four bench-marks in the table commencing with 40B at 
the Appraisers’ Building, were determined before the earthquake by the city engineer, — 
but not by Coast and Geodetic Survey leveling. These four, in various parts of the city, 
show no apparent change in elevation greater than 69 millimeters (2.7 inches). Two of 
them apparently rose and two subsided. 
The apparent changes in elevation of the three bench-marks in the table, commencing 
with 41 at California and Montgomery Streets, are not supposed to have much significance 
in connection with the question of whether a general change of elevation took place. 
These three bench-marks were each subject to local disturbances during the earthquake 
or were near or on filled ground. 
In ten cases the old leveling determined elevations of hydrants and the new leveling 
determined elevations on hydrants in the same locations but known, from the descrip- 
tions, to be different from the old hydrants. Similarly, in seven other cases, the old 
leveling established the elevations of points on curbstones, steps, or doors, and in each of 
these cases in the new leveling it was found to be impossible to recover the old point 
accurately. In all of these 17 cases there is, therefore, only an approximate connection 
between the old and the new leveling. The evidence from these bench-marks has all 
been examined carefully and does not lead to any different conclusion from that which 
may be drawn from the table above. 
The general conclusion from both the leveling and the tidal observations is that, within 
the region examined, there occurred no general change of elevation of sufficient magni- 
tude to be detected with certainty. 
It is an opportune time, at present, on account of local changes in elevation at various 
bench-marks, to adopt the best possible determination of mean sea-level which is avail- 
able up to date and to refer all new elevations determined since the earthquake to that 
datum. Accordingly, the reading 8.652 feet (2.7371 meters) on the tide-staff at the Pre- 
