CH. Il] THE OCEANOGRAPHY OF THE NORTH-WESTERN OCEAN 37 



the southern hemisphere. This band has an area of over ten 

 millions of square nautical miles. The material composing it has 

 resulted almost entirely from the siliceous shells of diatoms which 

 live in enormous numbers at the surface of the antarctic seas, and 

 which dying, sink to the bottom, where their skeletons accumulate. 

 Then too in all these oceanic oozes other substances are found. 

 All over the deep basins of the oceans there are found the teeth 

 of sharks and ear-bones of whales, these being the only parts of 

 the skeletons of these creatures which withstand the solvent action 

 of the sea water at the great depths in which they are found. 

 In the Norwegian Sea there occur the dense calcareous otoliths or 

 ear-bones of various teleostean fishes. All over the sea bottom 

 are found the peculiar manganese nodules which were first 

 described during the famous voyage of the Challenger. Finally 

 it has been recognised that in the deepest parts of the sea there 

 are particles in the oozes which have an extra-terrestrial origin : 

 particles which result from the combustion of meteorites entering 

 the atmosphere from without. These should of course occur in 

 the deposits of all depths, but in most their presence is masked 

 by the preponderance of other substances, and it is only in the 

 deposits of the very deep basins, which are formed with extreme 

 slovv^ness, that they can easily be recognised. 



In the North- Atlantic Ocean the temperature of the water 

 varies from place to place very notably, these variations being 

 determined by a very definite circulation of the water on the 

 surface and at some depth beneath this. A glance at a chart 

 shewing the surface isotherms, such as will be found in the 

 Challenger Reports, will illustrate this variation more clearly than 

 any amount of description. It will be seen that the mean annual 

 temperature of the surface varies from about 4"" C. at the Arctic 

 circle to about 29° C. in the region of the equatorial stream (Lats. 0° 

 to 30°). Further it will be seen that the isotherms do not run in 

 a direction parallel with the lines of latitude, but are bent north 

 and south in a way which is at first very puzzling but is very 

 easily explained by a consideration of the facts of the water 

 circulation. Roughly speaking there are three wide belts or 

 regions, (1) the region of the equatorial stream where the mean 

 annual temperature of the surface is about 29° C, (2) a wide 



