1050 PRINCIPLES OF STRATIGRAPHY 



Antarctic stream of the South Pacific, together with the cold Peru- 

 vian current of the west coast of South America, acts as a barrier 

 to migration of warm water animals and plants from Australia to 

 South America, and probably did so throughout Tertiary time. 



In the Atlantic the peculiar conformation of Africa and South 

 America results in the splitting of the South Equatorial current into 

 the Brazilian current, flowing southward along the east coast of South 

 America, while the main current crosses the equator into the Carib- 

 bean Sea and the Gulf of Mexico. Here again the cold west-wind 

 drift and the cold Benguela current on the west coast of South 

 Africa prevent dispersion between the southern portions of these 

 continents of all warm water types. The presence, however, in 

 the modern fauna of gastropods on the east coast of South America, 

 which appear to be derived from species inhabiting Indo-Pacific 

 waters, suggests that migration of tropical animals up the west 

 coast of Africa is possible under certain conditions, in spite of the 

 cold Benguela current. In the North Atlantic, the Gulf Stream, 

 with its northeastward drift, favors dispersal of species from the 

 east American to the west European coast ; while the return Canary 

 and North Equatorial currents may well be effective in carrying 

 planktonic organisms from the northeast African coast and the 

 Mediterranean to the West Indian waters of tropical America. 



A factor which must be taken into consideration is the varying 

 rapidity of flow of the currents. Thus the Gulf Stream, as it issues 

 from the Florida straits, has an average velocity of 80 or 90 miles 

 a day,* while the drifts, like that crossing the middle North Atlantic, 

 may have a velocity of only from ten to fifteen miles per day. If 

 the meroplanktonic stage of a normally benthonic animal is passed 

 through very quickly it is evident that before the creature can be 

 carried very far it will end its pelagic existence and sink to the 

 bottom. In the case of such larvse carried by the transoceanic 

 currents, the depths to which they will settle after having been 

 carried for a few days is likely to be such as to prove destructive 

 to the organism. Thus organisms taken up by the Gulf Stream as 

 it leaves the Florida coast would, even if the stream retained its 

 maximum velocity, have to travel considerably over a month be- 

 fore they could reach the European coast, while with the actually 

 diminishing velocity of the drift in the North Atlantic the time 

 required will be many months. Before that time most meroplank- 

 tonic organisms will have completed their metamorphosis and have 

 perished on an uncongenial bottom. Holoplanktonic types, how- 

 ever, and even nektonic animals are widely dispersed by these cur- 



* It varies from 1.5 to 2.5 meters per second. 



