Shingle on the East Coasts of Neiv Zealand. 61 



several forces at work. The real difficulties of the subject 

 lie in apportioning to the several forces at work their 

 relative value, so as to obtain a proper resultant, and, more 

 particularly, in ascertaining what force is required to move 

 materials of a given size and weight, and where such 

 materials will be deposited as the power of the water to 

 retain them in suspension is destroyed. 



(a.) Hoiv Shingle is caused to Travel along the Shore. 



22. Waves before breaking, being waves of oscillation 

 merely, have, in general, very little effect on the material 

 forming the bed of the sea. After the waves have broken 

 they become waves of translation. On a wave breaking on 

 any shore, or on a wave which has broken at a distance 

 reaching the shore, a large quantity of water is, as every- 

 one may have seen, thrown forward and flows up the slope, 

 gradually decreasing in velocity till it reaches a certain point, 

 from which it flows back down the slope. The water falling 

 from the crest of a wave, on breaking, stirs up some of the 

 material forming the beach. This will take place to a con- 

 siderable extent if the water fall directly on to the beach, 

 but to a less extent if it fall on to a cushion of water of 

 some thickness. In the former case, a portion of the material 

 disturbed is carried up the slope by the force of the water, 

 -and probably some material is carried by a back current (or 

 under-tow) down the slope under the water-line. In both 

 cases the return wave also will carry some material, when 

 not too large, down the slope with it. 



f te : 23. If the waves come directly on-shore, the material is 

 carried alternately up and down the slope in lines nearly at 

 right angles to the shore-line, so that the position of the 

 particles laterally is altered but little. If, however, the waves 

 come obliquely on to the shore, the particles are carried up the 

 slope obliquely, and, on being washed down again, are some- 

 what in advance of the position they first occupied, the 

 advance corresponding with the motion of the waves along 

 the shore. The more acute the angle, within certain limits 

 (probably about 45°), at which the waves strike the shore, 

 the greater the advance of the material at each movement. 



(b.) Wave- action on a Beach under various conditions. 



24. When a^wave breaks on a shingle beach and carries 

 stones up the slope, these stones will be gradually deposited 

 as the velocity^of the water decreases, the largest first and 



