Speight. — Orientation of the River-valleys of Canterbury. 139 



as far as their main portion is concerned, as we approach the main divide 

 there appears to be a marked departure from the arrangement. This is 

 specially noticeable in the case of the Upper Waimakariri and the Rangi- 

 tata, and the direction of the line of fracture accounting for the Rakaia 

 is made by Dobson to follow the line of the Mathias River, and not the 

 main Rakaia, from the jimction of the former with the latter, although 

 the tributary is of comparatively small size when compared with the main 

 stream, whose upper valley lies right athwart the direction of the suggested 

 fracture in that region. Therefore, if we grant that the dominating in- 

 fluence which determined the course of the main valleys was a system of 

 earth-fractures, some other cause has determined the direction of the head- 

 waters. 



Dobson was no doubt conscious of other determining causes, since he 

 has noted very fully the direction of fold and joints of the rocks and their 

 important effect on stream-directions. I think that if full note be taken 

 of the direction of the joints and folds it will explain most of the anomalies 

 in the directions of the passes through the main divide and the departure 

 from the system of earth-fissures- — -if, indeed, they exist — which is admitted 

 by both Dobson and Haast. 



First of all, the general direction of folding in the neighbourhood of 

 Arthur's Pass and farther north is N. 22° E. We have therefore the peculiar 

 circumstance that the direction of folds crosses the range at an angle, and 

 is not parallel with its general trend. This is also the case in the Kaikoura 

 Mountains, as I have been informed by Drs. Allan Thomson and C. A. 

 Cotton. The angle cited by Dobson is the average angle, and it will be 

 found that there are marked divergencies from this value in particular 

 places — for example, in Otira Gorge the line of strike is almost in a line 

 with the direction of the tunnel, being practically due north. In some 

 places the same bed can be seen, owing to slight local variations, following 

 along the top of the inside of the tunnel for several chains— that is, in a 

 direction N. 8° W. It is thus apparent that the line of the Otira and Bealey 

 Rivers follows almost exactly the line of strike of the beds ; and, as the dip 

 is in contrary directions on opposite sides of the Otira Valley, so that the 

 valleys of these rivers have been eroded along the axis of an anticline, the 

 formation of the pass between the heads of the valleys is in all probability 

 due to the capture of the eastward-flowing streams originally running 

 through the pass by streams discharging west — a result to be attributed 

 very largely to glacier erosion and the sapping-back of heads of glaciers 

 in the manner first described by Matthes in the report on the " Glacial 

 Sculpture of the Bighorn Mountains " (21st Annual Report U.S. Geol. 

 Surv., 1899-1900, pp. 167-90). This idea has been amplified and ex- 

 tended by W. H. Hobbs in a paper entitled " The Cycle of Mountain 

 Glaciation," published in the Geographical Journal, vol. 35, 1910, p. 146. 

 This piracy has no doubt been hastened by the steeper grade of the rivers 

 and also of the ice on the western side of the main divide. 



On considerixig the directions of the valleys at the headwaters of the 

 Waimakariri the influence of the general direction of folding is quite 

 apparent, especially in the upper waters of the White River, the main 

 southern tributary of the Waimakariri. This is one of the cases noted by 

 Dobson as difficult to explain on the supposition that the valley-directions 

 were determined by a series of earth-fractures, since the strongly defined 

 mass of Mount Greenlaw completely blocks the end of the valley of the 

 jnain river. 



