622 
BULLETIN OF THE BUREAU OF FISHERIES 
The upper layers of the gulf thus present much the same picture from summer 
to summer when studied in west-east cross section, with isotherms closely crowded 
in the western side but spreading over the eastern coastal bank, and the uppermost 
stratum cooling from west to east as already described (p. 588). Invariably, too, the 
gulf has proved at least as cool at 100 meters as at any level in July and August, and 
usually coolest there in the form of a definite layer of minimum temperature spread- 
ing seaward, centripetally, from the western and northern shores. However, the 
spacial distribution of temperature at depths greater than 150 to 175 meters varies 
from summer to summer, depending on the volume and velocity of the bottom cur- 
rent drifting in through the Eastern Channel at the time or shortly previous (p. 613), 
as well as on the precise route followed by this water within the gulf. When this 
current has been in large volume shortly previous, it tends northward and westward 
around the eastern and northern slopes of the basin, so that the conditions described 
for 1914 and 1915 prevail (fig. 62). Following a long slack period, a reproduc- 
tion of the temperatures of 1912 or of 1913 may be expected. 
A composite profile (fig. 64) , based on observations taken in the summers of 
1913, 1914, and 1915, illustrates the relationship which the western extension of the 
warm bottom current bears to the shoaler water along the coast of Maine, on the one 
hand, and to the central part of the basin, on the other. When this drift is active, it 
hugs the northern slope of the basin as it eddies around to the westward, a state- 
ment supported by the evidence of salinity as well as of temperature. 
The much lower surface temperature (12°) at the inshore end of this profile than 
over the basin offshore (16°) is simply the result of active vertical circulation 
along the coast; so, too, is the reverse relationship prevailing at the 60 to 100 meter 
level. I may also point out that this profile, like those already discussed, shows 
the cold mid-layer (of 5.3° to 6.04° at 100 to 150 meters) characteristic of the inner 
parts of the gulf in most summers, and which is reminiscent of the low temperature 
to which the whole mass of water shoaler than this had been chilled during the pre- 
ceding winter (p. 689). 
The maintenance of comparatively high temperatures down the slope, at depths 
greater than 30 meters, which is probably characteristic of the summer season in 
this part of the gulf, may have some biologic importance by making an especially 
favorable environment for such bottom animals as prefer a moderate temperature 
within narrow limits where they would find no sudden thermal bar to vertical 
migration. 
Profiles crossing the mouth of Massachusetts Bay fron Cape Ann to Cape Cod, 
for the cold July of 1916 (fig. 65) and for August 22 of the warm summer of 1922 
(fig. 66), are introduced for graphic demonstration of the thermal stratification that 
develops there by the end of the summer. It is surely worth emphasis that the bottom 
temperature should be only between 4° and 5° in water as shoal as 75 meters in as 
low a latitude as 42° N. at the end of August, with a surface temperature as high 
as 18°, as was the case in 1922 — and this in a warm year. 
The presence of a surface stratum of homogeneous water (18.6° to 18.7°) nearly 
10 meters thick, blanketing the northern part of the August profile (station 10633), 
is rather contrary to our previous experience in this part of Massachusetts Bay, 
where low surface temperature usually has been recorded, reflecting upwellings or 
