150 Transactions. 



marine abrasion acting under very favourable conditions, points to the 

 extremely recent date of the uplift of the rock platform and of the 

 initiation of the present cycle of marine denudation. 



As is well known, the earth-movement of 1855 was of the nature of a 

 tilt to the west or north-west affecting both sides of Cook Strait. The 

 hinge -line of the tilt was evidently situated along the bed of the strait, since 

 its western shore was depressed and its eastern uplifted. The locality of 

 maximum uplift was Cape Turakirae (see fig. 1). the amount being 9 ft. 

 At Port Nicholson the uplift amounted to 5 ft., and at Porirua Harbour 

 entrance 3 ft. Even at Wanganui there was a slight uplift (Field, 1892, 

 p. 573), indicating that the hinge-line of the tilting block did not intersect 

 the coast of the mainland south of that place. 



As already indicated, the raised shore-platform is being rapidly de- 

 molished by wave-action, so that only a portion of the original surface 

 remains. This rapid demolition is due to several causes, the principal 

 of which are the low altitude of the platform, the thin-bedded character 

 of the rocks forming it, the presence of numerous faults and closely-spaced 

 joints, and— perhaps most potent of all — the very effective tools at the 

 disposal of the waves — viz., a plentiful supply of exceedingly hard grey- 

 wacke boulders. The raised shore-platform has, naturally, suffered greatest 

 destruction along its outer margin (see Plate XXXV), but in some places, 

 owing to the presence of a broad band or a series of narrower bands of the 

 weaker argillite, it has been entirely removed to within perhaps a few yards 

 of the old sea-cliff, and replaced by a tiny bay. This effect of the weaker 

 strata is often very striking, as also is the influence of bedding, joint, 

 and fault planes which determine the position of deep grooves across the 

 platform. 



The raised wave-cut shore-platforms of south-western Wellington are 

 especially suggestive and instructive in connection with the subject of 

 intermediate incipient or partial erosion-cycles which go to make up the 

 composite topography characteristic of many New Zealand landscapes. At 

 Porirua Harbour the raised shore-platform, which, as elsewhere, represents 

 a small interruption in the geographical cycle, is being rapidly destroyed. 

 In many cases a pronounced interruption in the geographical cycle may 

 well be the sum of a series of small interruptions each of which has been 

 obliterated in turn, thereby giving the whole the false appearance of a single 

 great interruption. This is probably the key to the origin of occasional 

 isolated hillside or shore-line benches and other similar indications of inter- 

 mediate erosion-cycles — fragments preserved in exceptionally favourable 

 localities long after all other traces of the cycles to which they belong have 

 been obliterated. 



(b.) The Reef. 



The Reef is the name given to a pair of interesting rock shoals situated 

 in mid-channel at the entrance of Porirua Harbour. At high tide the 

 higher rocks reach uniformly to 3 ft. above sea-level. This uniform level 

 corresponds in every respect to the surface of the raised shore-platform 

 that fringes the mainland. The Reef marks the former site of an island, 

 or pair of islands, which were completely planed off by marine abrasion 

 prior to the uplift of 1855. At low tide the two groups of higher rocks 

 are surrounded b) 1- a much more extensive area of low rocks just awash. 

 This lower surface is the present plane of marine denudation, and is the 

 level to which the 3 ft. surface has been cut down since 1855. The 

 evidence furnished by the Reef is supplementary to and confirmatory of that 

 afforded by the raised shore-platform on the mainland. 



