622 BULLETIN OF THE BUKEATJ OF FISHEEIES 



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 coohng 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 1.50 to 175 meters varies 

 from summer to summer, depending on the volume and velocity of the bottom cur- 

 rent drifting in tlirough 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 sahnity 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 



