62 Shingle on the East Coasts of New Zealand. 



the smallest where the force of the water is least. The 

 return wave will increase in force as it descends the slope, 

 and it will gradually remove larger and larger stones till its 

 force be checked. The steeper the slope the more rapidly 

 will the on-shore wave be destroyed, and the more rapidly 

 will the return wave increase in force ; and vice versa. 

 Whether the return wave can remove and carry down the 

 slope as large stones as had been deposited by the on-shore 

 wave, or not, will depend on the slope of the beach. On 

 a flatter slope than usual, the return wave will not acquire 

 sufficient force; but, on a slope steeper than ordinary, 

 it will doubtless acquire the force. Under certain con- 

 ditions, all the largest stones are deposited at the highest 

 level; under certain other conditions, at the lowest level. 

 It must be noted that the return wave flowing down a slope 

 is not destroyed on meeting an on-shore wave. The two waves 

 cross, and the return wave continues its course away from 

 the shore, gradually becoming less and less. 



25. It was remarked just now that a wave breaking on,, 

 or close to, the shore stirs up the material of the beach, and 

 doubtless some material is also carried by a back current clown 

 the slope under the water-line. This is a very important point 

 to consider; the more so, as it is difficult to observe clearly 

 this action on the material of which the beach is composed. 

 A floating body may be seen on the point of being thrown 

 by the waves on to the beach, when it will be suddenly 

 drawn under the surface of the water, and will in a short 

 time reappear some distance off the shore. When a wave 

 breaks, it is probable that the particles of water in the trough 

 of the wave continue the backward (off-shore) motion they 

 had immediately before the wave broke. If such be the 

 case, each wave breaking on the shore, or so close to it for 

 this backward motion to be felt on the beach, has a tendency 

 to draw under the water-line some of the material disturbed, 

 as well as to carry material up the slope, as above explained. 



26. If an on-shore wave break at a distance from the 

 shore, and on to a considerable cushion of water, the back- 

 ward motion of the water in the trough of the wave is not 

 likely to have any appreciable effect on the material dis- 

 turbed from the beach. The water falling on to a cushion 

 of water from the crest of a wave would, as before remarked, 

 stir up the material of the beach more or less accordiDg to 

 the thickness of the cushion and the size of the material. 

 The' chief horizontal movement likely to take place would be 



