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. AU this, of course, suggests 

 that upwelling from the middepths may play a role of some importance 

 in the maufacture of the zone of mixed water along the continental 

 slope, though there is no evidence that oceanic upwelUng 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 BacJie, 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). 



•■'ZOOM, 



Fig. 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 



