THE OCEAN. 665 



platform of coral islands has the same origin. The stability of sand- 

 flats in the face of the sea is owing to this cause. 



In seas of high tides and frequent storms, the platform is narrow or 

 wanting, owing to the tearing action of the heavier waves. 



2. Transportation. 



1. Transportation by Currents. — The great oceanic currents are too 

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

 too remote from coasts to receive detritus of any kind, except sparingly 

 from the very largest of rivers, like the Amazon. Whatever sinks, 

 in the main course of the Gulf Stream, is carried some distance south- 

 ward again, by the polar current beneath it. 



Sea-weeds are borne on by the Gulf Stream in great quantities, and 

 thrown off on the inner side of the current, into the great area of still 

 water about the centre of the North Atlantic, called, from the common 

 name of the plant (a species of Fucus), the Sargasso Sea. With the 

 sea-weeds, which grow as they float, there is a profusion of small life, 

 — Fishes, Crabs, Shrimps, Bryozoans, etc. 



In polar seas, where there are glaciers and icebergs, large quantities 

 of gravel, earth, and bowlders are often floated off" on the bergs. 

 From the Arctic region, they are borne south by the polar current to 

 the Banks of Newfoundland ; there the icebergs encounter the edge 

 of the Gulf Stream, and melt, dropping their freight over the bottom. 



Tidal and wind currents have the same powers of transportation as 

 rivers of equal velocity. 



2. Transportation by Waves. — As follows from the force of waves 

 against shores, stated on page 663, they have great transporting power ; 

 but their action is confined to narrow limits of depth, and is exerted 

 mainly when the plunging waters strike upon a sandy or rocky coast 

 Large rocks often have their buoyancy increased by the sea-weeds 

 attached to them. 



Stevenson reports that a block of gneiss, of 504 cubic feet (about forty -two tons), 

 lying on a beach (in Scotland), was moved five feet by the waves during one storm. and 

 was then so wedged in that its farther progress was prevented. The in-coming wave, 

 as it struck it, gave it a shove, and, pushing on, buried it from sight, making a per- 

 pendicular rise of thirty-nine or forty feet; and, in the back-run, the mass was again 

 uplifted with a jerk. 



Marine animals, or their relics, and sea-weeds are thrown abun- 

 dantly on coasts by the waves ; and, in some regions, whales that ven- 

 ture too near the land are carried up and left floundering on the sand. 

 This happens not unfrequently about the Chusan Islands, in the China 

 Seas, where the tidal currents have great force (p. 661). 



In the case of the heaviest waves, and especially earthquake-waves, 



