Adkin. — Porirua Harbour. 147 



The main drainage-lines leading to Porirua Harbour have, with one 

 notable exception, the same parallelism, trend, and longitudinal elonga- 

 tion that distinguish the intervening ridges. The one exception is the 

 Pahautanui Stream, which, together with its drowned lower valley — viz., 

 the broad eastern arm of Porirua Harbour — lies transverse to all the other 

 topographical features of the district. The present Pahautanui Valley and 

 its drowned lower portion is too widely-opened and ancient a feature to have 

 had so recent an origin as to belong to a young stream consequent on a 

 bounding flexure of the Port Nicholson - Porirua Harbour tilt-block, and 

 must be regarded as antecedent to the adjacent longitudinal ridges and 

 drainage-lines. 



From what is now known of the morphology of other portions of the 

 Wellington district, it is evident that the ridge-tops to the north-east of 

 Porirua Harbour are the residuals of a peneplaned surface, in all probability 

 belonging to the Kaukau cycle of erosion. In the Wellington Peninsula, 

 however., the peneplaned surface of the Kaukau cycle has been uplifted 

 uniformly, whereas the corresponding surface to the north-east of Porirua 

 Harbour has a decided westerly or south-westerly tilt. This is clear 

 evidence of the existence either (1) of one large block having a warped 

 surface which changes from a uniform level in the south-west to a decided 

 upward slope in the north-east, or (2) of two distinct earth-blocks differ- 

 entially uplifted with respect to each other. The two blocks, or two parts 

 of a single original block, as the case may be, are now divided by the 

 subsided Port Nicholson - Porirua tilt-block already referred to. 



(2.) Influence of Deformation on the Relief. 



The rocks of the Porirua district are the well-known closely-folded 

 greywackes and argillites usually referred to the Maitai formation of Trias- 

 Jurassic age. The strike of the strata in this vicinity appears to have a 

 general N.E. by E. to E.N.E. direction (N. 55° to 70° E. true = N. 38° to 

 53° E. magnetic), and the dip is for the most part vertical, indicating an 

 isoclinal structure. 



As stated above, the trend of the series of subparallel hill-ridges which 

 form the moderately elevated country north-east of Porirua Harbour is 

 usually N.E. by N., while the average strike of the rocks forming them 

 is N. 62° E. (true). With ridges and strike intersecting each other at so 

 large an angle (about 27°) it is difficult to understand the genetic relation- 

 ship of the hill-ridges to the geological structure so far as the ancient folding- 

 is concerned. 



Two theories have been advanced to account for the longitudinal 

 features of the orography of the Maitai rocks of southern Wellington as 

 exemplified by the Tararua Range and the lesser hills to the south-west. 

 By one theory the longitudinal ridges and drainage-lines are regarded 

 as being respectively dependent on, and in adjustment to, the original 

 geological structure (Cotton, 1918, pp. 213-14) ; in the other, secondary 

 deformation is held to be the determining factor in the production of 

 these features (Adkin, 1920, p. 184). 



It is extremely likely that both secondary deformation and adjustment 

 to structure were of prime importance, each exercising a predominant 

 influence, but in different localities. In the Wellington Peninsula where 

 the secondary corrugation of the highest peneplaned surface was com- 

 paratively weak, and also where its axes appear to have coincided with 

 the strike of the ancient folding, adjustment to structure was probably the 

 factor that determined the trend of the ridges and valleys ; but on the more 



