28 Transactions. 



corrasion, and discuss briefly their bearing on the case in 

 ■question. 



The three main circumstances which affect the corrasive 

 power of a river are — (1) its gradient. (2) its volume, and (3) its 

 load. 



The Gradient. 



The following table, taken from Haast's " Geology of Canter- 

 bury," gives the grade of the rivers on that part of their course 

 between tbeir gorges and the sea. Alongside this, for pur- 

 poses of comparison, I have also put the grade of the plains 

 where the rivers cross them : — 



Slope of Slope of 



Rivers in Plains in 



Feet per Feet per 



Mile. Mile. 



Waimakariri . . . . . . 28 36 



Rakaia.. .. .. ..23* 39i 



Ashburton .. .. . . 40~ 42i 



Rangitata . . . . . . 37 45 



These figures give the average slope, but in both the grade 

 of the rivers and also in that of the plains there is a perceptible 

 flattening on approaching the sea. 



The following features are shown by this table : — 



(1.) The rivers all have a steep slope as they cross the plains — 

 in fact, they are still mountain torrents. They should therefore 

 be eroding their beds very rapidly, as their banks are com- 

 posed of incoherent material, were the erosive power given by 

 their high grade not partly counteracted by other influences. 

 Owing also to this lack of consolidation, lateral corrasion is 

 relatively great. In the upper portion of the plain track, vertical 

 corrasion is more important, so that the terraces are higher ; 

 but in the lower part, lateral corrasion becomes more important, 

 and the terraces are much broader and lower. 



(2.) The slope of the bed is dependent on the size of the 

 river. The smallest river has the most rapid fall per mile, and 

 the largest river — the Rakaia — the least fall. 



(3.) In every case the slope of the plain is greater than the 

 slope of the river, but there is no connection between the slope 

 of the plains and the size of the present river crossing them. 



Changes in the Height of the Land- 

 As the grade of the rivers will depend either directly or in- 

 directly on the height of the land above sea-level and its dis- 

 tance from the sea, it is necessary here to consider the evidence 

 for elevation and depression. 



