264 Transactions. 



fault-line until reaching the Long Valley. It has reversed the drainage of the 

 Long Valley for a mile and a half. It follows a winding course, but the taper- 

 ing shape of the spurs on the concave sides of the meander- curves indicates 

 that the winding character is due, at least in part, to lateral cutting that has 

 accompanied the deepening of the gorge. At one point a narrowed and 

 almost cut-off spur is a conspicuous feature in the Ngahauranga Valley. 

 Plate XXI, fig. 1, is a view looking north-east across this spur and up the 

 valley. The height of the narrowed neck above the stream on the down- 

 stream side is 200 ft., and on the up-stream side 90 ft. Its breadth is about 

 100 yards, while the distance roundabout by the course of the stream is 

 three-quarters of a mile. Beyond the divide, 500 ft. above sea-level, at 

 the head of the obsequent Ngahauranga, is the head of the Porirua Stream, 

 which, robbed of two-thirds of its ancient length, still occupies the northern 

 end of the Long Valley. Probably this was the outlet at the close of the 

 Tongue Point cycle. 



The cause of most of the captures in the Long Valley is, as has already 

 been indicated, the subsidence of the Port Nicholson block, particularly 

 along the line of the Wellington fault, giving a short descent to sea-level. 

 With regard to the Silver Stream, it seems remarkable that its capture 

 had not taken place earlier and in a less roundabout way than the pre- 

 sent outlet to the Karori Stream. A reasonable explanation seems to be 

 that in earlier times, when streams followed the Long Valley and the other 

 main longitudinal valleys of the Wellington Peninsula, the peninsula formed 

 part of a land-area extending to the north-west and to the south far beyond 

 its present limits. Reasons have already been given for the writer's belief 

 that the present coast was determined by fractures after the main lines 

 of the present drainage were established. 



Type of Topography. 



A consideration of the courses of streams and the elongation of ridges 

 of the Wellington district leads to the conclusion that, apart from local 

 complications due to unequal vertical movement, the topography of the 

 south-western end of the North Island mountain-chain is of the Appa- 

 lachian type — namely, an old, folded range subjected for a sufficient time 

 to denudation to bring about longitudinal drainage by subsequent streams 

 adjusted to structure, not following original synclinal folds, and afterwards 

 elevated sufficiently to allow dissection by revived streams to produce a 

 surface of strong relief. The analogy with the Appalachian Mountains 

 must not be pushed too far. For example, planation in the earliest cycle 

 seems to have been far from complete, and the absence of transverse streams 

 following antecedent courses is especially noticeable. Their unfortunate 

 absence accounts for the difficulty of railway-construction between Wel- 

 lington and the western coast. In spite, however, of the obvious differences 

 the remarkable similarity of our range to the Appalachians is brought out 

 by a comparison with Lesley's map of Pennsylvanian topography, repeated 

 by de Lapparent,* or with the detailed maps of smaller areas given by 

 Salisbury and Attwood.| It may be noted that the " great Cook Strait 

 river" of Crawford, J it if existed, must have been transverse for part of its 

 course ; but reasons have been given above for believing that Cook Strait 

 is not a drowned river-valley. 



* " Lemons de Geographie physique," 1907, p. 613. 



t U.S. Geol. Survey, Prof. Paper GO, 1908, especially pi. 5 and 56. 



% Trans. N.Z. Inst., vol. 7, 1875. p. 448. 



