74 Shingle on the East Coasts of New Zealand. 



would get smaller, and at length it would close up alto- 

 gether. Several of the smaller rivers on the Ninety-mile 

 Beach appear from the chart to be so circumstanced (see 

 par. 10). The water of the rivers then would either filter 

 through the shingle, or it would accumulate till it was able 

 to force its way through now and then. 



51. The mouth of the River Tuki Tuki, two miles south 

 of the River Ngaruroro, is at present more constant than 

 that of the latter river. The author attributes this to the 

 very large mass of shingle which is lying in front of the 

 former river. The tendency is for the mouth to shift north- 

 wards, but the large mass of shingle is not quickly cut 

 away, and, before any great change can be made, a flood 

 coming down the river brings the opening in the shingle 

 opposite the mouth of the river once more. It is easier for 

 the smaller river, Ngaruroro, to cut away the earthen bank, 

 than for the larger river, Tuki Tuki, to cut away the shingle. 

 The author was informed that several rivers of New 

 Zealand are subject to frequent and heavy floods for a series 

 of years, and then the reverse is the case for another series 

 of years. This is expressed, in other words, by there being a 

 cycle of wet seasons at intervals. When the time comes for 

 the floods of the rivers to be less for a few years in succession, 

 the shingle will be less frequently broken through, and then 

 the mouth of the River Tuki Tuki may shift like that of 

 the River Ngaruroro ; probably both will have one and the 

 same mouth for a time. 



V. — Movement of Shingle Below the Water-line. 



52. This is a matter upon which the author is not pre- 

 pared to say much. Although there may not ordinarily be 

 any tendency for the shingle to descend below the water- 

 line, so long as the beach is continuous and has its normal 

 slope, and the movement of the shingle is unobstructed, this 

 will happen when a sudden change in the trend of the coast- 

 line occurs, as mentioned above (in par. 48), or when a break 

 occurs in the beach at the mouth of a river or other channel 

 which is kept open by a strong current of water. In such 

 a case a spur is formed in the direction the shingle is 

 moving. This extends a certain distance, depending on 

 the strength of the current and the force of the waves ; the 

 current has to force its way on the one side, and the waves of 

 the sea on the other side either driving the shingle towards 



