Shingle on the East Coasts of New Zealand. 63 



due to the return waves flowing as an undercurrent down 

 the slope. The material would therefore be carried seawards, 

 The slope is here supposed to be fairly uniform from above 

 the shore-line downwards ; the case being, in fact, a beach, 

 and not a shoal or bar on which waves break. The effect 

 in the latter case would be different, there not being a return 

 wave of a similar description. 



27. The movement of the larger material down the slope 

 is doubtless soon arrested. The flow from an incoming wave 

 after breaking would cause most of it to be deposited at the 

 point of meeting the return wave ; or, on reaching the 

 water-line, it would receive a check and be deposited : 

 hence, lame shingle is often not found below the water-line. 

 Waves following at long intervals, or so as to allow the 

 return waves full play, would allow large stones to be 

 washed down to the water-line, or possibly a little below it. 

 But waves following in rapid succession must check early 

 the action of the return waves, and either prevent their 

 taking up much material, or cause the deposition above the 

 water-line of any large stones they may have moved. Small 

 shingle and sand might be carried on. as the return wave 

 would not, as before remarked, be destroyed. An incoming, 

 or on-shore, wave passing over would, by causing an oscil- 

 latory motion, temporarily arrest the seaward motion, and 

 might allow some of the material to be deposited, but the 

 finer material would be carried on. Fine sand would prob- 

 ably be carried a long distance out. These results may be 

 briefly stated as follows : — The coarser and heavier the 

 material of the beach, the less it will be drawn below the 

 water-line ; the finer and lighter the material, the more 

 readily it will be drawn below that line : the slope in each 

 case being the same. 



28. It would appear, then, from the foregoing remarks, 

 that, theoreticallv, the shingle should be arranged in regular 

 order on a beach, the largest stones at the water-line, or at 

 the level where the force of the waves is greatest, that is, 

 probably from a little above low-water level to a little 

 above high- water level, and smaller stones higher up ; the 

 stones should also decrease very rapidly in size downwards 

 under low-water level. The vertical cross section, or profile, 

 of the shore tends to become convex above high-water level 

 and concave below low-water level, and perhaps straight 

 between the two levels. Practically, these results are not 

 always arrived at, and when obtained they are seldom 



