Shingle on the East Coasts of Mew Zealand. 65 



regards shingle, by Sir John Coode in particular, who 

 admitted the disturbance by wave-action of shingle at 

 depths of 6 and 8 fathoms, but denied that the material 

 was thrown up on to the beach.* The author, without 

 pretending to decide a point on which such authorities are 

 at variance, will venture to suggest that the difference is 

 chiefly one of degree, since there are numerous instances 

 recorded of stones being thrown up from great depths. 

 Sir John Coode, therefore, could not have intended to deny 

 the action in toto ; he evidently meant that it did not take 

 place to any great extent. At great depths, unless there is 

 a projecting rock or other obstruction to the waves, there 

 is little more than an oscillatory motion of stones, the 

 size of large shingle, caused by even the heaviest waves at 

 the surface of the sea, such waves not being waves of transla- 

 tion so long as the depth of water exceeds the height of 

 the waves. Although these waves might not have power 

 to transport to a distance stones of the size alluded to, a 

 current of moderate strength could do so when the stones 

 had once commenced to move, or had been lifted from the 

 bed by wave-action. Now and then the stones might be 

 carried within the influence of waves of translation, and 

 might then be thrown up on to a beach or elsewhere. In 

 this way, the facts just mentioned can be understood, while, 

 at the same time, the results, as dependent on wave-action, 

 may be looked upon as exceptional.-f- 



32. A belt of discoloured water may often be seen along 

 the shore when nothing but shingle is to be seen on the 

 beach. This may be due to fine material carried down- 

 wards by the return waves (flowing off-shore as an under- 

 current), or to material stirred up from the bed of the sea 

 by the on-shore waves. The apparent width of this belt is 

 not necessarily the extent to which material is held in sus- 

 pension, because that on the surface is frequently in motion 

 towards the shore. The attention of the author was 

 drawn to the fact of this belt of discoloured water being 

 sometimes wider in fine weather than in bad weather, the 

 reverse of what might have been expected. The stronger 

 wind in bad weather would, if on-shore, cause particles of 

 matter floating, or in suspension near the surface, to ap- 

 proach the shore ; the waves also would break further from 



* Proceedings of the Institution of Civil Engineers, vol. xxiii., p. 241, and 

 vol. xl., p. 107. 



f See further illustration of this point in note on p. 79. 



F 



