678 DYNAMICAL GEOLOGY. 



icebergs. Had there formerly existed a continent in the midst of the 

 present North Atlantic, America would have received from it no rock- 

 material. 



2. The great Oceanic Currents. — The great oceanic currents are 

 too feeble to transport any material coarser than the finest detritus, 

 and generally too remote from coasts to receive detritus of any kind. 

 The Gulf Stream passes the channel between Florida and Cuba at a 

 rate of five miles an hour ; but Florida feels it very feebly, and, accord- 

 ing to the dredgings under the direction of L. F. Pourtales, contributes 

 nothing. On the Cuba side the water is deep, and the current comes 

 close to the land ; but the same authority states that the debris of the 

 reefs, while found at considerable depths, even 800 fathoms, is not car- 

 ried northward more than three or four miles. 



The observations of Sir Wy ville Thomson and others go to show that 

 the bottom of the deep ocean is free from river detritus, and from such 

 detrital sedimentary deposits as make up, exclusive of limestones, the 

 strata of the land (see pages 611 and 686). But sea-weeds are borne 

 along by such currents, and the animal life which they harbor. The 

 Gulf Stream carries immense quantities of a floating Fucus, which is 

 thrown off on the inner side of the current, into the interior region of 

 the Atlantic, called, from the common name of the sea-weed, the Sar- 

 gasso Sea. With the sea-weeds, there are various species of crabs, 

 shrimps, bryozoans, fishes, and other kinds of marine life. Icebergs 

 are carried along by such currents (p. 701), and deposits of large 

 bowlders, as well as gravel and sand from the Arctic, have thus been 

 made along the Labrador coast and over the Banks of Newfoundland. 

 The cold, arctic currents which move slowly over the ocean's bottom 

 have favored the migrations of marine Arctic and Antarctic life, and 

 the passage of such species as can inhabit great depths even from one 

 polar region to the other ; and hence has come, it is supposed, a resem- 

 blance in the genera of the two distant regions. 



3. Ordinary Waves. — The transporting power of waves is illustra- 

 ted on p. 671. The great blocks torn off from submerged beds of 

 rock, or loosened from the bottom, as well as the stones and sand, are 

 thrown up the beach, and make an accumulation varying with the 

 height of the tide and the force of the waves ; and the winds add to 

 the elevation by drifts as explained on page 632. The large rocks 

 sometimes are overgrown with long sea-weeds which make them 

 more easily transportable. Marine animals, or their relics, and sea- 

 weeds, are also among the material thus accumulated ; and occasion- 

 ally large animals, as stranded whales, are added, not an infrequent 

 event about the Chusan Islands in the China seas. 



The sands and stones left below the highest reach of the breakers 



