196 PHYSICAL GEOLOGY 





less radioactive. The composition of the salts which occur in ocean 

 water is as follows * : 



Common salt, NaCl 77-76 Potassium sulphate, K2SO4 . . . 2.46 



Magnesium chloride, MgCl . . . 10.88 Magnesium bromide, MgBr 2 . . .22 



Magnesium sulphate, MgSC>4 . . 4.74 Calcium carbonate, CaCOs ... .34 



Calcium sulphate, CaS0 4 ... 3.60 100.00 



In a discussion of the composition of sea water not only the dis- 

 solved mineral matter should be considered, but the dissolved gases 

 as well, since oxygen is essential for the existence of marine organisms 

 and for oxidizing dead matter of organic origin. In addition to oxy- 

 gen, nitrogen and carbon dioxide are present. In fact, the ocean 

 probably contains from eighteen to twenty-seven times as much 

 carbon dioxide as the atmosphere and is the great reservoir of this 

 gas. It is not, however, equally distributed, but is more abundant 

 in polar seas than in equatorial, since cold water has a greater capac- 

 ity for it than warm. 



Temperature of the Ocean. — The temperature of the surface of 

 the ocean varies with the latitude, from a mean annual temperature 

 of 8o° F. at the equator to one of 40 F. at the poles. Since the rays 

 of the sun do not penetrate the water to great depths, it is probable 

 that the seasonal changes are not felt below 50 feet. The tempera- 

 ture at the bottom of the ocean is surprisingly cold, being about 29 

 F. at the poles and 35 F. at the equator. This layer of cold water is 

 very thick ; for if we consider water above 40 F. as warm, the layer 

 of warm water is nowhere more than 4800 feet thick, and is usually 

 considerably less. The low temperature of the deep water is due to 

 the movement of the waters from the polar regions, which slowly 

 creep toward the equator along the ocean bottom, so that we find in 

 the tropics, at great depths, the low temperatures which are en- 

 countered only on the surface in the Arctic and Antarctic regions. 

 Exceptions to the rule that the temperature decreases from the sur- 

 face downward are found in such seas as the Mediterranean, the 

 Gulf of Mexico, and the Red Sea. In these seas the temperature 

 of the bottom is approximately the same as that at the bottom of the 

 strait separating them from the ocean, and the surface temperature 

 is almost constant, being practically the average temperature of the 

 surface in winter. In the Mediterranean, for example, the tem- 

 perature at a depth of 6000 feet is 55 F., while in the Atlantic at 



1 Data of Geochemistry, Bull. U. S. Geol. Surv. No. 491, p. 23. 



