96 bulletin: museum of comparative zoology. 



the cold wall on the American side of the Atlantic to be able to trace 

 with security the origin of its waters." Quite a different explanation 

 for the cold wall is proposed by Tizard, (1907, p. 343) who believes 

 that the chief factor in forming the cold coast water is the discharge 

 of fresh water from the rivers along the American coast, by which 

 means large quantities of cold fresh water and fresh ice are emptied 

 over the coastal slope. And he argues that neither upwelling of 

 oceanic bottom water, nor the Labrador Current, has anything to do 

 with the formation of the cold wall. 



The partial isolation of the Gulf of Maine from oceanic waters by 

 the sill formed by George's and Brown's Banks, makes it possible that 

 its cold waters need a different explanation from those of the "cold 

 wall" west of Cape Cod; and the discussion of the latter is best post- 

 poned until we have a better knowledge of their salinity. But so far 

 as the Gulf is concerned, we can safely say that the low salinities in 

 July and August certainly show that its waters are not predominantly 

 Atlantic abyssal water welling up over the continental slope, because 

 the salinity of the bottom water over most of the North Atlantic is 

 about 34.9 (Murray and Hjort, 1912). 



The same index, salinity, shows that Tizard has suggested a factor 

 of real importance, for besides the fresh water emptied into the Gulf 

 of Maine annually by its rivers (p. 90) there is also the annual rainfall 

 of about 40 inches, a total annual increment of fresh water, which 

 would make a layer more than a fathom thick over the entire Gulf. 

 To offset this, there is the annual evaporation; and while this is not 

 exactly known for any off-shore station in the Gulf, conditions on the 

 neighboring coasts indicate that it is probably less than the rainfall. 

 Rainfall and inflow from rivers combined are likewise considerably 

 in excess of the annual evaporation all along the coast of Nova Scotia 

 where the salinity, according to both Dickson, (1901) and Schott, 

 (1902) is 32% or less. 



The Gulf of St. Lawrence, has, of course, been mentioned by pre- 

 vious authors as a source of fresh water, but its importance must be 

 greater than has been usually recognized, because of the enormous 

 ■extent of its watershed, including the St. Maurice, Saguenay, Humber, 

 and other large rivers, besides the St. Lawrence itself. Its rainfall, 

 too, exceeds evaporation. The little that is known about the currents 

 in its two mouths (Dawson, 1910) shows that its main outlet must be 

 through Cabot Straits, as Schott represents it in his chart of ocean 

 currents, (1902, pi. 39) not through the Straits of Belle Isle. The 

 ■comparatively fresh St. Lawrence water is continuous with the water 



