86 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
For November 25, 1906, the predicted height of low-water at the San Francisco entrance 
was 2.1 feet; for October 17, 1907, 2.2 feet. The comparison of the clam patch with the 
view made eleven months earlier showed a marked difference, a much larger area being 
submerged at the later date. Four days afterward, when the predicted height of low- - 
water was 0.3 foot, I again compared the appearance of the clam patch with the photo- 
graph, finding the water-stage somewhat lower than when the photograph was made. 
As the tide rose its stage was found to coincide with that shown by the photograph 
one hour after low-water, and the calculated allowance for the corresponding change in 
water-level at the San Francisco entrance is about 0.3 foot, so that the predicted tide- 
stage for that moment was 0.6 foot. As the predicted stage at the time represented 
by the photograph was 2.1 feet, there was an apparent discrepancy of 1.5 feet. If this 
was occasioned by a change in the height of the ground at the clam patch, then there 
was a subsidence of 1.5 feet between November 25, 1906, and October 21, 1907. Before 
accepting so important a conclusion it should be cheekt i in every practicable way, and 
especially by comparing the water-stages at the clam patch with simultaneous water- 
stages as recorded at the tide-gage station of the U.S. Coast Survey in the entrance to 
San Francisco Bay. The distance of that station from the clam patch is about 15 miles, 
of which one mile is inside the narrowest constriction of the Golden Gate. On the days of 
observation at the clam patch the sea was calm, except for a moderate groundswell, so 
that the normal equilibrium of the water-surface between that point and the tide-gage 
was presumably not impaired by meteorologic influences. Off the clam patch the 
groundswell broke at a distance from the shore, leaving the water so quiet at its actual 
margin that its level could be observed with little error. The general and local conditions 
were thus favorable for a comparison of water-stages at the two points; and the numer- 
ous details of the photograph of November 25, 1906, made it possible to identify, with 
close approximation, the arrival of the tide at the same plane on October 21,1907. The 
observations at the tidal station, for which I am indebted to Capt. Aug. F. Rodgers, 
Assistant U.S. Coast Survey, were made with the tide-staff at low-water, and are referred 
to the arbitrary zero of the tidal station. 
FErr 
At low-water on the afternoon of Nov. 25, 1906, the tide-staff reading was . . . . . 6.10 
At low-water on the afternoon of Oct. 21, 1907, ‘the tide-staff reading was . « Ot 
In the hour following low-water the computed rise of the tide was 0.3 ft.; ‘and this 
gives as the height of water at the time of the observation at the clam patch . 
Difference se em ne em ea en 
Thus the discrepancy of 1.5 feet, deduced from a consideration of the predicted tides, 
is reduced by a comparison of the observed tides to about 0.2 foot, a quantity so small as 
to be referable to errors of observation. 
Before tide-gage records were obtained I had revisited Pepper Island and Limantour 
Bay. On Pepper Island the vertical dislocation was remeasured and found to be un- 
changed. At Limantour Bay the subject of examination was the condition of the new 
growth of Salicornia. If the land had subsided since the preceding June, the colonies 
of Salicornia which had invaded the mud flat (plate 56p) would have been subjected to 
unfavorable conditions, and might be expected to show the influence of those conditions. 
All the colonies that had previously been observed were reéxamined and they were 
found, without exception, to have deteriorated. The green heads, which had formerly 
testified to their lusty growth, had become much less numerous and were modified in 
color; their stems were blackened on the surface and had become somewhat curled, 
and in general they appeared less healthy than the plants of the same species growing 
at higher levels. Where the slope was continuous, there was a fairly sharp line of separa- 
tion between the healthy and unhealthy plants, and two measurements indicated the 
zone of impairment to have a vertical range of 10 inches. 
