THE OCEAN. 681 



gravel for wave degradation ; and, accordingly, long sand reefs stretch 

 westward from the eastern capes of all the deep harbors, across the 

 opening, sometimes nearly closing it against shipping, while depth 

 enough exists inside for the largest of vessels. There being no rivers, 

 the coast is at the mercy of the drifting tidal and current action. 



Similar facts are stated by Captain Davis, in his excellent paper on 

 the geological effects of tidal action. He mentions the cases of long 

 points thus made on the eastern extremity of Nantucket, where the 

 current on the outside of the island sets from the west to the east, and 

 from the south to the north. Vessels wrecked on the south side of 

 the island have been carried by it, by piecemeal, eastward and then 

 northward, to the beach north of Sankaty Head. The coal of a Phil- 

 adelphia vessel, lost at the west end of the island, was carried around 

 by the same route to the northern extremity. 



The formation and lengthening of Sandy Hook — the sand reef five 

 miles long forming the southern cape of New York Bay — is wholly 

 due, as shown by Bache, to the action of tidal currents. A northerly 

 current flows alike along the outer and inner sides, during both the 

 ebb and flood tides, and the conflict of the two directs the deposition 

 of the sands, and has made it in recent times elongate at the rate of a 

 mile in sixteen years. 



Where the wind-current changes semiannually, the accumulations 

 made by the current when flowing in one direction are sometimes 

 transferred to another side of an island or point, during the next half- 

 year. 



J. D. Hague states, that at Baker Island 

 (of coral), in the Pacific ((P 15' N., 176° 22' l2 S- 1098 ' 



W.), this fact is well exhibited. In Fig. 1098, J K 



I I I is the southwest point of the island, and 

 R R R, the outline of the coral-reef platform, 

 mostly a little above low-tide level; its width, 

 c d, 100 yards. In the summer season, when 

 the wind is from the southeast, the beach has 

 the outline s, s, s; during the winter months, 

 when the wind is northeast, the material is trans- 

 ferred around the point, and has the position to, 

 w, w, having a width at a b of 200 feet. A ves- 

 sel wrecked in summer, and stranded at V, 

 was transferred to V in the course of the month 

 of November. 



The flow of rivers and the movements of the ocean are, in general, 

 in direct opposition. The in-flowing tide sets back the rivers, quiets 

 the waters, and floods the adjoining tidal flats ; and, consequently, a 

 deposition of detritus takes place over the flats, and especially about the 

 mouth of the stream. The turn of the tide sets the river again in full 

 movement ; and it takes up the detritus deposited over its bed (but 



