i88 



PRINCIPLES OF STRATIGRAPHY 



not fall below 3,000 m., for the temperature of 2.4° C. is that of the 

 2,950 m. depth (Fig. 2^^. The mid- Atlantic rise is broken in the 

 equatorial region just west of the great Romanche deep by cross 

 channels descending to 5,000 m. Through these channels the cold 

 waters of the Brazil basin (derived from the Antarctic) find access 



Sea Level 



SOUTH AFRICAN TROUGH 



2.4°G 



CAPE TROUGH 

 2.+°c. 



Fig. 2"]. Diagrammatic section of the Whale Ridge in the South Atlantic, 

 showing the differences of temperature on opposite sides. 



to the North African basin, but this trans-passage is a mild one, ow- 

 ing, probably, to the narrow character of the cross channel, for its 

 effects are no longer noticeable beyond 2° or 3° N. lat.. where the 

 normal bottom temperatures of 2.2° to 2.6° characteristic of the 

 North African basin are found. 



so. 



N.E. 



Fig. 28. Diagrammatic transverse section of the Wyville-Thomson Ridge 

 in the North Atlantic, showing its effect as a thermal barrier. 

 (After ProuYot and Haug.) 



In the North Atlantic the bottom temperatures of 2.0° to 2.6° 

 are preserved through the barring of the cold waters of the Arctic 

 Ocean by the Wyville-Thomson and Fa roe -Iceland and Denmark 

 Straits ridges, which completely divide the two oceans below a depth 

 of 550 to 580 meters. To the north of these ridges the temperature 

 sinks as low as" — 1.2° C. in 2,222 meters (Fig. 28). 



