402 T. C. CBAMBERLIN 



than the foregoing, and are mainly secondary to movements in other 

 parts of the ocean body. In their natures these waters belong to 

 the deeper layers from which they come. 



The middle Atlantic layer. — The origin of the middle layer of the 

 Atlantic column is most clearly illustrated by the transformation 

 that takes place in the isolated basin of the Mediterranean. As 

 previously noted, a side current from the southward-flowing branch 

 of the Gulf Stream west of Spain enters the Straits of Gibralter and 

 forms the upper layer of the Mediterranean in its western portion. 

 As it creeps eastward, its salt content is concentrated by evaporation 

 in spite of the fresh waters that flow into it from the adjacent lands. 

 By the time it has reached the east end of the Mediterranean, it 

 has attained a saHnity of 39 parts per thousand. In spite of its 

 warmth this salinity gives it a density sufficient to make it sink and 

 creep back westward as the bottom layer. This at length flows out 

 into the Atlantic as a bottom current in the lower part of the Straits 

 of Gibraltar. On entering the Atlantic, it sinks by reason of its 

 saHne density through the upper oceanic waters, and spreads out 

 in delta fashion, until it reaches a depth of about 2,000 meters, 

 where its base finds a horizon of equilibrium. On its left hand, it 

 has been traced about 10° southwestward. On its right hand, it 

 pushes northwestward and has been detected as far as 52° to 53° 

 N. Lat., or half way to the mouth of the BafSn Bay inlet. Beyond 

 this* point, data are so scant as to leave its further extension obscure. 

 It seems probable that it merges on the west with the similar waters 

 rendered saline by evaporation in the Sargasso area and that the two 

 together make up a true middle layer distinct from that below by 

 reason of its warmth, and fairly distinct from that above by its 

 higher salinity, though, of course, it is constantly grading into the 

 contact waters above and below, by various forms of comminghng 

 and diffusion. 



A portion of the middle layer, in this large and general sense, 

 appears to creep southward to compensate, in part, for the surficial 

 equatorial waters shunted into the North Atlantic by Cape St. 

 Ro.que, while another part creeps northward to maintain, by inter- 

 change according to the laws of equihbrium, the saHnity of the north- 

 em waters. Waters of adequate sahnity must obviously flow into 



