HiLGENDORF. — Ou Influence of the Earth's Rotation. 211 



why the river should have corroded this higher bank rather than 

 run down beside tliis bank. 



The Rakaia when it first became independent was running 

 through its present gorge in a direction that would carry it well 

 into Lake EUesmere, along the line where the Ashburton fan 

 overlay the general seaward slope of the plains. This would give 

 the river a course from gorge to sea of about forty-five miles, 

 while its present course from gorge to sea is thirty-eight miles, 

 and the shortest possible course is thirty-five. Now, the shorter 

 the course is, the greater is the fall per mile, the greater the 

 velocity, and the greater — by the sixth power — the corrasion. 

 Suppose the river formed a delta at its mouth : one arm would 

 flow southward into the sea, and the other north ; but the one 

 flowing south would have a shorter distance to go for the same 

 fall, would corrade more rapidly, and would become the master 

 stream. Thus the continual tendency of the stream's mouth 

 would be to move southward so as to reach the position in which 

 it would have the least distance to go to pass through the fixed 

 amount of fall from the gorge to the sea. I believe this con- 

 clusion is valid, and my belief is strengthened by Von Haast's 

 reference to an apparently well-known law — namely, " that 

 rivers that unite tend to do so by the shortest line." If this 

 statement is a true one it should also apply to rivers that reach 

 the sea : they endeavour to do so by the shortest line. At the 

 same time, I must confess that in my working models of this 

 system of fans and rivers the stream that represented the 

 Rakaia corraded the fan and shifted its mouth southwards in 

 a less marked degree than I had anticipated. 



This consideration, if accurate, then, explains the only ex- 

 ception to the general truth — that the rivers flowing through the 

 plains have eaten into their left bank more than their right 

 bank. The Rangitata was in the same position as the Rakaia 

 in regard to the Ashburton fan — namely, it ran down the side 

 of it ; so that we may neglect the presence of this fan altogether, 

 for its influence on one bank of one river is counterbalanced 

 by its influence on the other bank of the second. I have 

 therefore prepared an average section of the larger rivers of 

 Canterbury, including all but the Waimakariri (fig. 8). If 

 it is considered that the fan of the Ashburton has too great 

 an effect to be negligible on the courses of the Rangitata and 

 Rakaia, we will reject those rivers, and take an average section 

 of those remaining — viz., the Ashley, Opihi, Ashburton, and 

 Waitaki (fig. 7). I have omitted the Waihao, Makikihi, and 

 Otaio owing to their small size ; but these streams show the 

 same peculiarities in a very marked degree. In either case, it 

 will be seen that the average left bank is steeper than the average 



