668 DYNAMICAL GEOLOGY. 



of the waves are often accumulated into higher ridges, and make the 

 wind-drifts and dunes described on page 631. 



(2.) The tidal and wind currents give directions to the material 

 taken up by the waters. This material may be the sands, pebbles, 

 etc., of a beach, or the finer material from the bottom, or the mud 

 stirred up from greater depths, down even to one hundred fathoms, by 

 the heavy waves of storms. The currents, their general course being 

 otherwise determined, flow where they find the freest and deepest 

 passage, and drop their detritus wherever there is a diminution of 

 velocity. This precipitation takes place in the waters thrown off either 

 side of the current, and especially the shoreward side, toward 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 accu- 

 mulating sand-bank. The flow of the tidal wave or current along any 

 coast, while aiding in fixing the limit of the barrier, often transfers 

 detritus up or down the coast, according to the direction of the move- 

 ment ; and it thus tends to make the barrier follow, for long distances, 

 a nearly even line ; so that, however indented such a coast may be 

 after a change of level, it will become straightened, if the waters out- 

 side are shallow, through the forming barrier, while the waters shut in 

 by the barrier may still have an irregular inner shore-line. The same 

 action assists the ebbing tide in giving form and length to sandspits, 

 like Sandy Hook. The Hook, according to A. D. Bache, has been 

 elongating at the rate of " one sixteenth of a mile in twelve years," 

 since the first precise observations were made. 



This point is well illustrated by Captain Davis, in his excellent paper on the geolog- 

 ical 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 



Fig. 1098. Jt, by piecemeal, eastward and then northward, 



/K I to the beach north of Sankaty Head. The 



/ // coal of a Philadelphia vessel, lost at the west 



S . ■■ \?y end of the island, was carried around by the 



//V .■■'// same route to the northern extremity. 



/ ,.--'' // Where the wind-current changes 



/ i // semi-annually, the accumulations made 



s s'\ / Ug b J by the current when flowing in one 



/ "X j--j y>+ ••;;.?--w direction are sometimes transferred to 



r\ w '"'•- ••...].... [ "" another side of an island or point, dur- 



' -^c ■*-- -- R i n g the next half-year. 



