456 Transactions. — Geology. 



when its waters are loaded with sediment that it becomes 

 such a powerful agent of erosion. 



In the course of the Wairoa from the Junction seawards 

 a number of very interesting loops occur, any one of which 

 would furnish a profitable subject for investigation, and well 

 repay the trouble spent in unravelling its liistory. Take, for 

 instance, that large loop below the junction of the Mongonui 

 Stream. In this case the river, flowing north-west, was 

 deflected at Te Pango by an elevation which, although slight, 

 was sufficient to give direction to the current. Meantime its 

 waters spread out over the low ground on what is now its left 

 bank, where, the current being checked, deposition took place, 

 gradually bringing the depressed area up to its present level. 

 This area, which must have been a large one, grew towards 

 the north, and in so doing gradually pushed the river-bed in 

 the same direction, until the process was arrested by the low 

 range of hills along the right bank, and the river was con- 

 fined to its present channel. The net result has been the 

 formation of a large flat nearly circular in shape with a 

 series of low hills )-unning across its narrow end. 



The power of the stream to erode laterally is well seen 

 in several places, notably at Dargaville. Here the line of 

 greatest velocity has impinged against the right bank, cutting 

 into it for a considerable distance. One is surprised at first 

 that it has not gone further and cut back as far as the high 

 ground behind Mangawhare. But the fact is that the process 

 of erosion here has not been long in operation. It was not 

 until the growth of the opposite bank — to be explained further 

 on — forced the current into its present course that the banks 

 on the Dargaville side began to disappear. Tlie presence 

 of the Kaihu Stream no doubt brought about at an earlier 

 time the deposits of the Dargaville and Mangawhare flats. 



Erosion and deposition are both going on ; indeed, in a 

 river of the kind wliere the fall is not great this is inevitable. 

 At Dargaville, for instance, if the right bank is disappeai-ing, 

 the left bank is gradually being pushed out into the stream. 

 The erosion on the Dargaville side is now arrested to a large 

 extent by heaps of stones placed at intervals along the bank. 

 The effect of this has been to narrow the channel, and hence 

 to increase the velocity and erosive power of the current, with 

 the result that a sandbank of considerable size, nearly 

 opposite Mangawhare, has lately disappeared. The appear- 

 ance of the banks, too, is very characteristic. A deep channel 

 runs pretty close to the right bank, where, moreover, the 

 ground is hard and presents a vertical face to the water even 

 at high tide. The left bank, on the other hand, consists of 

 a low shelving deposit of mud. 



One of the first things that strikes an observer is the 



