1914] Sumner, et al.: Physical Conditions in San Francisco Bay 67 
further evidence of a rise in salinity at the lower end of the bay (see 
p. 85). 
It might have been expected that the difference between the sur- 
face and bottom salinity would be greatest at the point where the 
larger part of the fresh water enters the bay, and that this difference 
would be found to decrease gradually as we passed to the southward, 
owing to the mixing of the various strata amid the swift tidal cur- 
rents. On the other hand, the divergence of the surface and bottom 
curves from the extreme south towards the neighborhood of San 
Francisco points to the conclusion that little fresh water is discharged 
directly into the southern half of the bay. It seems likely that the 
ereater part of such fresh water as it receives is brought thither 
through the ageney of the tides and comes originally from the rivers 
entering at the north. 
Figure M represents the regional distribution of salinities during 
each of the six periods covered by the observations. It will be noted 
that the period of lowest salinity was the second one (April 23- 
May 6), when the mean figure for the bay as a whole was 25.23, while 
the period of highest salinity was the fourth one (October 7-12), 
when the mean for the bay was 30.51. These facts accord well with 
what we know of the rainfall and the discharge of the rivers, as will 
be pointed out below. 
It will be noted that, with a single exception, all of the curves 
are indented at station H-4974, that is, either an absolute decrease 
of salinity, or a diminished rate of increase, was found in passing 
from station H—4981 to that just mentioned. (In the exceptional 
case, the curve is indented at an adjacent station). This phenomenon 
has just been discussed and no further comment is necessary. 
In figure N we have portrayed the annual range of salinity for each 
station. Each of the ordinates represents the difference, at that point, 
between the highest and lowest salinity recorded for the year. While 
the distribution of a species with relation to salinity seems more 
likely to be determined by the maximum or minimum salt concentra- 
tion, it is not impossible that in some cases it may be due to the total 
range of salinity to which the organism is exposed in the course of 
a year. 
This range, as will be readily noted, is highest in San Pablo Bay, 
where an annual range of nearly 12 per mille is found at one point. 
Here the mean salinity during the October period was more than 
twice that found during the April-May period. 
