AND OTHER WATER WAVES 161 



chiefly in the English Channel, and almost daily 

 during some years on the beach near Branksome 

 Chine, between Bournemouth and Poole Haven. 

 They do not apply to lakes or tideless seas. 



Usually the floor of our seas is covered with 

 clean sand, from the lower parts of the beach which 

 are uncovered daily by the recession of the tide, 

 to depths as great as enable the bottom to be seen 

 from a boat. In greater depths the sand dredged 

 up is less clean and homogeneous. In addition 

 to this covering of sand upon the floor of the 

 shallow sea there are here and there great banks of 

 sand rising nearly to the surface from water of some 

 depth. Such are the Skerries Bank, east of Start 

 Point (Devon) and the Shambles Shoal, east of 

 Portland Bill (Dorset). Also, where shingle is not 

 too abundant, the whole of the surface of the beach 

 is often composed of sand (which buries the occa- 

 sional stones). In such localities one may often 

 see the breakers piling up a ridge of sand on the 

 top of the beach in the manner already described 

 for the shingle Fulls. The water does not, however, 

 drain out of the sand-ridge so completely between 

 waves, and the influence of percolation upon accu- 

 mulation is, therefore, not so great as in the case of 

 shingle, although it is still a main factor in the accu- 

 mulation of the ridge at the summit of the beach. 



