284 C. A. Cotton — Block Mountains in New Zealand. 



by the Shag Eiver. Portions of these streams are now super- 

 posed on the undermass of the slophig plateau as a result of 

 shifting far over to the side of the gniding depression and fail- 

 ing later to slip off the gently inclined surface while cutting 

 vigorously downward in response to rejuvenation. Each such 

 portion is now imprisoned in a narrow rock-walled gorge, while 

 reduction of the general level of the adjacent portions of the 

 depressions is being carried on by minor streams. The case 

 is closely analogous, though not strictl}^ homologous with that 

 of the horseshoe bend of Hoxie Creek, described by Gilbert as 

 an exception to the general rule of monoclinal shifting (Henry 

 Mountains, 1877, pp. 137-138). 



For some portions of the course of the Manuherikia thus 

 imprisoned in schist gorges Park has given a different expla- 

 nation (1906, p. 12). He says, "The river leaves its old course 

 and plunges suddenly into a deep, narrow rift" which is formed 

 by " a number of intersecting faultlike fractures, along which 

 the river runs, passing from one to the other." 



Similar superposition has been described in the Clarence 

 Yalley fault-angle depression in Marlborough (Cotton, 1913, 

 pp. 233-234:). The Aorere Yalley fault-angle depression in 

 northern Nelson apparently affords another example, for which, 

 however, Bell (1907, p. 27) has proposed a theory of capture 

 similar to that advanced by Park for the Manuherikia. 



The Ckitha River System. — Beyond the area with which we 

 are immediately concerned are the largest rivers of the region 

 — the Waitaki to the north and the Clutha (Molyneux) to the 

 south. The latter crosses the Manuherikia depression at its 

 southern end, where the Clutha is joined by the Manuherikia 

 Piver. The upper course of the Clutha is guided by a smaller 

 similar chain of depressions which may be termed the Upper 

 Clutha chain. A large tributary, the Kawarau, also receives 

 the drainage from a number of narrow fault angles. Inter- 

 mediate portions of its course may be in great part consequent 

 on the deformation, being guided by the lower sags or fault 

 angles in the highland plateau, considerable areas of which, as 

 Park has shown (1908), still survive in the various flat topped 

 ranges. The Kawarau is now the outlet for Lake Wakatipu, 

 which formerly overflowed at its southern end by way of the 

 air gap leading to the upper Mataura. The upper Kawarau, 

 now deepened into a gorge throughout its length, is thus prob- 

 ably of recent origin and perhaps subsequent. 



The combined waters of the Kawarau and the Clutha form 

 a river of great volume and enormous energy. The portion of 

 this stream which cut the deep Dunstan Grorge (fig. 23), lead- 

 ing from the tectonic depression of the Upper Clutha to the 

 Manuherikia, seems to have been guided by a sag in the high- 



