Speight. — Orientation of the River-valleys of Canterbury. 141 



spur of Mount Goethe stretcliing down to Mein's Knob, junctions with the 

 Ramsay, which flows south-west along the strike behind the Butler Eange, 

 and the combined stream, forming the main Rakaia, cuts across the beds at 

 right angles, past the northern end of Mein's Knob and south of Jim's Knob, 

 the southern termination of the Butler Range, and thence onward has a 

 gene.-al south-easterly trend. 



It seems certain that the direction of the folding does exert a determining 

 influence on the upper courses of the river-valleys ; but when we examine 

 the middle courses of the streams, before they leave the mountain tract 

 and debouch on to the plains, we find that their orientation has apparently 

 little relation to either. The direction of folding varies so much from 

 point to point that it is almost impossible to arrive at any general direction 

 which is in agreement with the lines of the stream-valleys. In most cases 

 they cut across the strike diagonally, with angles varying from 45 to 90 

 degrees. The Waimakariri Gorge may be an exception to this. Above 

 the narrow part, near the northern extremity of the Torlesse Range, the 

 flow of the stream is approximately parallel to the strike, as is exemplified 

 by that part of its course along the western flank of the Puketeraki Range, 

 and then it crosses the strike of the beds almost at right angles. This is 

 perhaps an illustration of the features of a young-river gorge to be noted 

 directly ; but after a consideration of all the river-valleys and the directions 

 of the fold of the greywackes I can find no agreement of the orientation 

 with even an average direction, and therefore some other cause must in all 

 probability be considered as responsible for the agreement. 



When, however, we examine the gorges of the rivers which are of recent 

 origin, we find that in numerous cases their directions either as a whole or 

 in part are directly controlled by both the directions of strike and of joint- 

 ing. The determination of the latter is at times a matter of some difficulty, 

 since its direction is by no means regular. In general a jointing at right 

 angles to the stratification predominates, but besides this there are addi- 

 tional associated joint planes which intersect the dominating ones at an 

 angle approaching 45 degrees. This is perhaps what we should expect if 

 quadrangular blocks which had already been produced by the normal causes 

 of jointing were subjected to a pressure at right angles to their faces, just 

 as when the strength of a block of cement is tested under a crushing-machine 

 it generally fails on lines which meet the faces of the block at an angle of 

 45 degrees. The persistence of the cross-joints and the relation of their 

 angle to the dominant ones can probably be explained in this way. This 

 is specially the case where the rock has shown symptoms of failure imder 

 earth-pressure ; but where it is more resistant, especially where it consists 

 of hard tough greywacke, the prevailing direction of jointing is at right 

 angles to the stratification. 



The right-angled bends at times characteristic of young-river gorges 

 probably depend in general on the strike of the beds and the directions of 

 the dominant joints, one reach corresponding to the strike and the next 

 to the joint-direction, and the length of each of these reaches will bear some 

 relation to the importance of the relative effect of the two causes. In 

 general, however, it will be found that the strike is the dominating influence. 

 If they are of equal importance, then the reaches will be approximately 

 the same length. Parallel reaches, such as occur near the heads of the 

 Rakaia and Waimakariri, are due to the river flowing first along the strike, 

 then turning at right angles in direction of the joints, and then continuing 

 the turning in the same sense, and rimning again along the strike in the 



