50 EXPLORATIONS, WESTERN ATLANTIC, STEAMER BACHE, 1914. 
suggests that the latter also was involved, moving up the slope to 
within about 200 fathoms of the surface. All this, of course, suggests 
that upwelling from the middepths may play a réle of some importance 
in the maufacture of the zone of mixed water along the continental 
slope, though there is no evidence that oceanic upwelling ever reaches 
the continental shelf, as Petterson (1897), Clark (1914), and others 
have supposed. But while there may have been an updraught over 
the slope shortly previous to the cruise of the Bache, nothing of the sort 
was taking place at that time, because the bottom water at station 
10260 was then entirely cut off from the equally salt midlayers by 
the lower salinities at station 10258 (p. 48). 
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Fic. 50.—Temperatures, ———, and salinities, ———, off Chesapeake Bay at 20 meters, January, 1916 
(Roosevelt stations). 
A simple explanation for the fact that the descending: tongue did 
not actually follow the slope, but was separated from it by a layer of 
salter, cooler water, is that the latter is merely a contrast phenomenon, 
the water preexisting along this part of the slope cut off by the down- 
pour. The single Bache profile, unfortunately, is not-sufficient to clear 
up this question. The existence of the downpour and of upwelling 
below 1,000 meters, however, is amply demonstrated. 
The more complete survey of the shelf abreast of Chesapeake Bay 
carried out by the Roosevelt in 1916 (p. 45, 60) shows that the tempera- 
ture was as uniform vertically in January, 1916, as in the correspond- 
ing month of 1914, the greatest vertical range at any station inside 
