WATER. 661 



current, and especially the shoreward side, towards which the waves 

 set the floating material ; also where capes make a lateral eddy, 

 and where any obstruction tends to retard the waters. A vessel sunk 

 in the passage may divert the waters a little to one side, where they 

 may have an easier flow, and become itself the basis of an accumu- 

 lating sand-bank. 



The increase and shaping of a sand-spit depend usually on the 

 action of the in-flowing tidal current and waves on the outer side, 

 and that of the out-flowing on the inner, the latter being the deeper 

 and often the more effectual. 



This point is well illustrated 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, in 

 piecemeal, eastward, and then northward to the beach north of Sankaty Head. 

 The coal of a Philadelphia vessel, lost at the west end of the island, was carried 

 around by the same route to the northern extremity. 



Where the wind-current changes semi-annually, the accumula- 

 tions made by the current when flowing in one direction are some- 

 times transferred to another side of an island or point during the 

 next half-year. 



A. Hague states, in a recent article, that at Baker Island (of coral), in the 

 Pacific (0° 15' N., 176° 22' W.), this fact 

 is well exhibited. In fig. 946, III is 

 the southwest point of the island, and 

 REE, the outline of the coral-reef plat- 

 form, 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, 8 ; during the winter months, when 

 the wind is northeast, the material is 

 transferred around the point, and has 

 the position w, w, w, having a width at 

 a b of 200 feet. A vessel wrecked in 

 summer, and stranded at V, was trans- 

 ferred to V in the course of the month of November. 



(3.) The combination of wave-action and marine currents with the cur- 

 rents of rivers produces results analogous to those proceeding from 

 marine currents and waves alone, but with greater complication, 

 and, in the present age, of far greater extent, because rivers add so 

 vastly to the material of deposits by their detritus. 



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



