954 



BULLETIN OF THE BXJKEAU OF FISHEEIES 



high in surface density throughout the summer, because of the strong tidal cuiTents 

 that constantly mix the surface stratum, as it warms, with colder and more saline 

 water from below (p. 928), and because the indraft of slope water of high saliuity is 

 directed into this side of the gulf. Consequently, the regional variation in the density 

 of the upper 40 meters is wider in summer than at any other season, with the 

 fundamental west-east gradation reappearing from year to year in essentially the 

 same spacial relationship. 



In April, and especially in May, the reader will recall, simple projection of the 

 iensitjT contours at the surface mirrors the general dynamic tendency for the whole 

 body of water in the gulf, regional distribution being essentially similar downward 

 through the whole column. This, however, is not the case in summer, because the 



Fig. 201.— Distribution of density at the surface, for the inner part of the gulf, August, 1913 



surface pool of low density in the offing of Massachusetts Bay is a superficial 

 phenomenon. In fact, the surface contour lines run almost at right angles to those 

 at 100 meters (fig. 202), which more nearly preserve the character of the preceding 

 months. The actual surface drift in tliis side of the gulf is therefore the component 

 of a rather complex screwing motion. In the northeastern part of the gulf, however, 

 the surface state more nearly mirrors the regional distribution of density for the 

 whole column. 



Unfortunately no one of our summer cruises has afforded the data needed for a 

 satisfactory mapping of density for the whole area. In the only summer (1914) 

 when the southeastern part of the area was surveyed, the coastal belt (more impor- 

 tant dynamically) was neglected. In every case, too, allowance must be made for 



