1914] Sumner, et al.: Physical Conditions in San Francisco Bay 97 
features of interest which are not obvious to anyone who eares to 
inspect the charts. 
One feature of this sort is the greater proportion of mud in the 
upper and lower, as compared with the middle, sections of the bay. 
In the latter region mud is of frequent occurrence, but it contains 
much larger proportions of sand. A similar area was found near the 
opening of Carquinez Strait. Near the Golden Gate, mud gives way 
to fairly clean sand and gravel. On the other hand, particularly 
pure mud predominates in the southern end of the bay. 
Turning to the vertical distribution, there is in general a larger 
proportion of mud in the surface layers than in the intermediate and 
bottom ones. This law is subject, however, to many exceptions. An 
intermediate shell-bed, of considerable horizontal extent, was found 
in the lower half of the bay, throughout a region where shells are 
practically wanting at the surface. 
A conspicuous feature shown by plates 6 and 7 is the greater 
penetration of the apparatus, and consequent greater length of the 
samples, in the bottoms of fairly pure mud. 
In the nitrogen chart, no general principle can be discerned beyond 
the self-evident correlation between the amount of nitrogen and the 
amount of mud. Important correlations may be expected, however, 
between the conditions here portrayed and the distribution of various 
sedentary or burrowing organisms. The discovery of such facts, as 
well as other important ecological relations, must be left to the biolo- 
cists who are studying the various collections obtained in the course 
of this survey. 
