CURRENTS AND MIGRATION 243 



iterraneans and epicontinental seas of the northern hemisphere, 

 when not a part of the main oceanic circulation, as in the American 

 mediterraneans, is normally a singly cyclonic movement in counter- 

 clockwise direction, while the movement of the main oceanic circu- 

 lation of this hemisphere is clockwise. Only in the case of the seas 

 lying on the Equator does a reversal of conditions occur when the 

 sun crosses to the north of the Equator during the northern summer 

 or the period of southwest monsoons. 



The type of currents found in funnel seas of the Californian type 

 is illustrated by that water body. During the cooler months the 

 prevailing northwest winds drive the surface waters southward, 

 while in summer the monsoon-like southeast winds drive the surface 

 w^aters into the gulf. At a depth of 50 meters the stream flows 

 again southward. 



Marine Currents in Relation to Migration and Dispersal, 

 Past and Present. As will be more fully set forth in Chapter 

 XXIX, ocean currents are among the important factors in influenc- 

 ing migration of organisms, and they are the chief cause of the dis- 

 persal of the plankton or floating matter, organic and inorganic, of 

 the sea. The numerous records of the wide dispersal of floating 

 matter, such as sealed bottles, purposely thrown into the sea, wrecks 

 of known date, tree trunks brought by tropical rivers to the sea, 

 and carried by the ocean currents to the arctic regions, and others 

 have indeed been one of the chief sources of our knowledge of the 

 direction of these currents. Taking our cue from these dispersals 

 in the modern sea, we may look for similar evidence of currents 

 in the past. In such determination the dispersal of the holo- 

 planktonic organisms serves perhaps as the best guide. In the early 

 Palaeozoic, the graptolites seem to furnish reliable indications of the 

 general course of the currents, and they have been so used in the 

 construction of Pal?eogeographic charts. ( Ruedemann-60 : ^5*5* ; 

 Grabau-29; Map figs. I, 2, 7, 5.) Sometimes the direction of the 

 current can be found by the position which the rhabdosomes of the 

 graptolites assumed in the strata, as in the case cited by Ruedemann 

 from the Utica shales of Dolgeville, New York, where not only the 

 rhabdosomes of the graptolites, but also the spicules of sponges, 

 fragments of byozoans and shells of Endoceras protciforme have a 

 parallel arrangement, indicating an east-northeast by south-south- 

 west direction of the currents. "That the flow came from N. 78° 

 E. and ran toward S. 78° W., can be inferred from the aj)pcarance 

 of the mud-flow structure, the drift ridges behind the fossils (Eji- 

 doceras), the eastward pointing of the apices of the Endoceras 

 shells, and often also of the sicular ends of the graptolites. . . . 



