154 MARINE ANIMALS 



land. This coincides with the usual lower limit of the continental shelf. 

 The tidal currents in many places appear to extend to great depths; 

 whereas in 10 years at least 25 m. of globigerina ooze is deposited in the 

 open ocean, a cable near the Canary Islands in depths of 1800 to 

 2000 m. was found to be entirely free. The same was true of rocks on 

 the sea bottom off the south coast of Ireland, at a depth of 1800 m. 



A compensatory movement of the deep water must equalize the 

 effect of the surface currents. On the lee coasts within the influence of 

 the trades, the warm surface water is continually blown away and 

 driven against the windward coasts, where it accumulates. It is re- 

 placed by the upward flow of cold water on the lee coasts, especially 

 in bays. The warmth-limited reef corals accordingly thrive especially 

 in the tropical east coasts of the continents, and are less abundant on 

 the west coasts, with their colder water from the depths, which reaches 

 the surface at 15° or even 14°. In the Bay of Kiel, which is open to the 

 east, continued west winds blow the surface water away, and the rise 

 of the water from the depths is noticeable on account of the higher 

 salt content, which is characteristic of the deeper water of the Baltic 

 (cf. Chapter XV). The colder and hence denser waters of the polar 

 seas sink, and flow slowly along the ocean bottom toward the equator. 

 The rate of flow at great depths has been estimated at about 1.5 milea 

 per day. 



The vertical currents, both up and down, are of especial importance 

 in the mixing of the sea w r ater ; they will be discussed at greater length 

 in a subsequent chapter. They have been observed with certainty in 

 some places; in parts of the Bay of Naples the heavy dredge may be 

 lifted and supported for some distance by such currents. 16 The currents 

 both up and down and the whirlpools of the Straits of Messina are 

 famous; 43 species of deep-sea fishes have been collected on the beach 

 at the lighthouse, 17 and other deep-sea forms are found at the surface 

 there. These currents appear to be due to the difference in tidal phases 

 in the Ionian and Tyrrhenian Seas, so that counter currents meet in 

 the Straits of Messina. The rate of up welling near southern California 

 is apparently about one meter per day. 18 The so-called "Tongue of 

 Cold" in the south equatorial current north of Ascension Island is sup- 

 posed to be produced by the rise of sea water from the bottom. De- 

 scending currents seem to be present in the Sargasso Sea and perhaps 

 in similar centers of oceanic drift. 



The rate and direction of all these currents are subject to continual 

 fluctuation, in part with a kind of periodic pulsation, whose nature and 

 origin are as yet obscure. The combination of all these motions pro- 

 duces a phenomenon of extraordinary complexity. 19 



