Spkight, Cockayne, Laino. — Mount Arroirxmith Dixtricf. 921 



their proiniiieut peaks removed, and tlius promotes the gradual approxi- 

 mation in height. This action l)e('omes increasingly effective as the hills 

 become loAver. 



This phenomenon is noticed in all denuded mountain regions, but it is 

 also in evidence in Canterbury, where the results of denudation have not 

 I'cached such an advanced stage. If the old surface of the Canterbury 

 plateau had been a peneplain, a generally uniform height of the principal 

 elevations would follow, because those which stood out on it above the 

 average level would be rapidly reduced to the mean height by the processes 

 described above. 



This rough plateau or peneplain has passed through a second cycle of 

 erosion, and the drainage established on it appears to have reached a mature 

 stage at the present time. 



3. Drainacje Systems. 

 (a.) Relation to the Structure of the Country. 



Mount Arrowsmith is at present the most strongly marked physical 

 feature of the Upper Rakaia and Ashburton district. Its great mass 

 dominates the whole area. From its south-eastern face flow the main 

 Ashburton River, and the Cameron River, an important tributary of the 

 Rakaia ; at the back of it rises the Lawrence ; while it is flanked on the 

 north by the valley of the Rakaia, to which it contributes numerous small 

 streams. The mountain is therefore the meeting-place of the drainage- 

 basins of the three important rivers of central Canterbury — the Rakaia, 

 Ashburton, and Rangitata. It must not be assumed that this has always 

 been the case, as the directions of the river-valleys were determined at a 

 somewhat remote date by considerations quite independent of the present 

 surface configuration. 



Its first lines seem to have been across the strike of the beds, and this 

 accounts for the general parallelism of the course of the main rivers both 

 inside and immediately outside the area under consideration. The principal 

 valleys — viz., those of the Rangitata, the Ashburton, and the Rakaia — are 

 controlled by this factor. The secondary drainage established itself in the 

 direction of the strike, but modifications ensued as the primary streams 

 cut deep down into the beds of the area, a characteristic modification 

 being that the lateral valleys trend slightly down-stream, and thus cut 

 across the strike at a small angle. 



The ternary lines of drainage appear to have reverted to the primary 

 direction, and are seen in the Upper Ashburton and the Cameron River, 

 l)ut the disturbing efi'ects of glaciation have been so marked that it is unsafe 

 to come to any definite conclusion in the matter. Even now, however, the 

 presence of weak beds dipping at high angles undoubtedly promotes the 

 formation of small tributary streams and of low saddles along the strike. 



This arrangement of the stream system I have attributed solely to the 

 normal development of drainage in a region composed of folded rocks where 

 the direction of strike is fairly constant. I am quite aware that it is also pos- 

 sible to attribvite the arrangement to lines of faulting ; but until these faults 

 can be proved on stronger evidence than the occurrence of crushed and 

 shattered bands of rock associated with steep slopes in an area where all 

 the rocks are more or less crushed owing to folding while the mountains 

 were being formed, I cannot accept the theory as sound, although subse- 

 quent more detailed work may show it to be so. 



11— Trans. 



