MuLGAN. — The Northern Wairoa. 457 



extreme turbidity of the water, a turbidity, moreover, from 

 which it is never free. The load of mud carried is very 

 great — indeed, it is difficult to conceive of its being increased 

 to any large extent. The particles of which the mud consists 

 are extremely small, so much so that a good deal of the 

 deposit is carried far out into the Kaipara Harbour before 

 coming to rest on the sea-iioor. 



That an immense quantity of detritus has been carried 

 down by the river is abundantly proved by the deposits of 

 alluvium along its banks, and the source of supply may no 

 doubt be looked for in the higher ground of the river-basin, 

 where the low ranges consist partly of sedimentary and partly 

 •of volcanic rock. The outcrops of these are much decom- 

 posed and weathered, and the flanks of the hills are steep, 

 ■conditions favourable for the removal of a disintegrated sur- 

 face. Much of the country, however, is covered with bush, 

 which serves to bind together the surface-soil and prevent its 

 being washed away. It is not surprising, therefore, that the 

 tributary streams draining the area in question do not show 

 any noticeable turbidity— indeed, they present the appearance 

 of clear running brooks — and yet the waters of the Wairoa are 

 always heavily laden with mud. Clearly, then, the present 

 supply of detritus carried by the river is not directly derived 

 from its tributaries, but is largely the result of lateral erosion. 

 Some of it is carried down and spread out on the harbour- 

 floor, but much of it is washed backwards and forwards by the 

 tide and current before being finally deposited where the banks 

 are growing. 



So free from suspended mipurities are most of the tribu- 

 tary streams that the question arises, Would a supply of 

 material such as now finds its way into the river be sufficient 

 to furnish the deposits of alluvium found in the valley ; in 

 other words, have conditions become greatly modified since 

 the deposits were laid down ? There do not seem to be any 

 strong reasons for supposing that they have, for the drainage- 

 area is large enough to furnish all necessary supplies if only 

 time were allowed and the river-valley where the deposits 

 occur is relatively small. Still, there may have been a time 

 when deposit, owing to altered conditions, was more rapid 

 than now — vv'hen, for instance, a good deal of the country 

 at present bush-clad was comparatively bare, and when in 

 consequence the weathered surface was readily and easily 

 swept away. The country may then have stood at a much 

 higher level, which in itself would account for a more scanty 

 vegetation, and the reduction in height may have been 

 brought about partly by erosive agencies and partly by 

 depression. To reach a period when such conditions pre- 

 vailed one would have to go back a considerable time, if 



