I go 



PRINCIPLES OF STRATIGRAPHY 



from 40.5 permille to 40.7 permille. The elevation of the marginal 

 rim in the Straits of Bab-el-Mandeb, which rises to within 185 m. 

 of the surface, prevents the cooler and less saline waters of the 

 Indian Ocean from entering. These waters in the Arabian Sea and 

 Gulf of Aden have a temperature, at 800 m., of 11° to 13°. This 

 is 4° above the normal and is due to the outflow of the warm waters 

 of the Red Sea. At 2,000 m. the temperature of the Arabian Sea is 

 5° to 6°, whereas that of the Red Sea at this depth remains at 21.5° 

 (Fig. 29). 



The three Central American mediterraneans, the Mexican, Yuca- 

 tan, and Caribbean, show homothermal conditions below 1,700 m., 

 from which depth the temperature of 4.2° C. continues to the great- 

 est depth at 6,269 m. In the corresponding depths of the Atlantic 

 the temperature ranges from 4° to 2°. 



RED SEA 



GULF OF ADEN 



Fig. 29. Diagrammatic cross-section of the ridge dividing the Red Sea and 

 the Gulf of Aden, to show temperature differences. 



Among the epicontinental seas the Baltic may serve as an ex- 

 ample of a special type of low salinity. The surface temperature 

 varies, of course, greatly with the season, the minimal temperature 

 ranging in the Danzig Bay region from 1.6° to 2.8° C, according 

 to the rigor of the winter. This temperature extends through the 

 upper 40 to 60 m., which are also homohaline. The maximum 

 density of water of the salinity of the Baltic (73^2 permille) is 

 reached at 2.4° C. Hence cooler waters will remain at the surface. 

 The freezing point of this water is, moreover, at — 0.4° C. Down- 

 ward the temperature increases, so that at the bottom of the Danzig 

 Bay temperatures of 5.68° and 5.88° C. have been found in Febru- 

 ary, while those of August range from 3.82° to 4.90°. In the Alands 

 deep, near the mouth of the Bothnian Gulf, surface temperatures 

 of — 0.2° C. extending to a depth of 20 meters were found in Feb- 

 ruary, 1903, and below this an increase to 3.07° at 273 meters. In 

 the summer months the temperature of the upper layers, varying 

 in different years from 20 to 45 m., ranges from 15° to 18° or over 



