68 Shingle on the East Coasts of Neiv Zealand. 



slope becomes flat enough to withstand the wave-action. 

 Increased force of the waves in a heavy sea probably has 

 less effect on the slope than is usually supposed, if that slope 

 be the one best suited to the material forming the beach ; the 

 increased force is destroyed by the waves breaking further 

 out. More important points, in the opinion of the author, 

 are the rapidity with which the waves follow one another 

 and, more especially, the direction from which they come on 

 to the beach. On the seashore, where the action of the 

 waves is perpetually varying, the slope may often change ; 

 though the author is of opinion that other causes are usually 

 at work when the changes are considerable. 



38. If the above reasoning be correct, as the shingle 

 becomes reduced in size, as will be presently explained, in 

 moving along the coast, the slope of the beach should 

 gradually become flatter.' Further, so long as shingle 

 remains on the beach any sand formed by attrition of the 

 pebbles against one another is liable to be drawn under the 

 water-line, and thus to be lost to view ordinarily. The 

 observations of the author lead him to believe that these two 

 propositions will generally prove to be correct. Where such 

 is not the case, or where the slope of any beach differs much 

 from that of a beach exposed to similar wave-action else- 

 where, it is probable that some other agency is at work. 



39. If the author may refer to the Ghesil Bank in illus- 

 tration of this point, he would suggest that the steep slope 

 on the southern side of that bank is due to the strong 

 current which flows round Portland Island. This slope at 

 the south-east end of the bank is given at 1 in 5 J, some- 

 times increasing to 1 in 3J ; at the other end of the bank 

 the slope is flatter. After a gale, the slope sometimes 

 decreases to 1 in 9. The slopes observed by the author 

 in New Zealand, when the beach had its normal slope 

 — that is, when it was not being denuded of shingle — 

 was about 1 in 10, decreasing when the material became 

 finer to probably 1 in 15, or thereabouts. When the 

 shingle was being carried forward rapidly, a portion of the 

 slope might become as steep as 1 in 4, but this was only 

 temporary. More definite information regarding the normal 

 slope of shingle is required. 



(e.) Illustrations of the Foregoing Remarks. 



40. It may serve to make clearer some of the foregoing^ 

 remarks to state here the result of an examination, by the 



