156 Transactions. 



Fig. 5 shows the Porirua shore-lme (at high tide) at a later stage of 

 development than that represented in fig. i : the coast-line has been 

 straightened by cliff-cutting and by the closing of the bays bv shingle- 

 banks ; delta deposits are rapidly filling the bays thus cut off ; and the 

 hilltop islands near the entrance of the harbour have been reduced to 

 island remnants. The shore-line at this period appears to have reached 

 a submature stage. 



Before the interruption referred to below, the above processes were 

 carried to an even more advanced stage than that depicted in fig. 5. The 

 lagoons in the former embayments were completely filled with deltaic 

 deposits, and converted into salt and, later, into fresh-water marsh. The 

 island remnants also disappeared, having been cut down to sea-level by 

 continued marine abrasion ; at low tide their former sites were marked 

 by rock reefs. 



The progress of the shore-line towards complete maturity was inter- 

 rupted by the earthquake of 1855. The narrow shore-bench or incipient 

 plain of marine abrasion, to the between-tides level of which the hill-slopes 

 of the initial shore-line had been cut down in the process of cliff-cutting 

 prior to 1855, was raised on that date to a height at Porirua Harbour of 3 ft. 

 above high-water level (see Plate XXXV, and fig. 2, a). 



The above considerations clearly indicate that the Port Nicholson and 

 the Porirua areas belong to one and the same physiographic district, since 

 it has now been demonstrated that even the last diastrophic event — viz., 

 the small uplift of 1855 — was common to both.* The principal divergence 

 in the parallelism of the life-histories of the two areas is the difference in 

 form and origin between their respective harbours : one of these is of 

 tectonic origin — a complicated graben-like depression subsequently modified 

 by the various mechanical agents of change ; the other is the result of the 

 partial drowning of a normal valley-system modified since the submergence 

 by the several phases of marine abrasion and deposition. 



List of Papers cited. 



Adkin, G. L., 1919. Further Notes on the Horowhenua Coastal Plain and the Asso- 

 ciated Physiographic Features, Trans. N.Z. Inst., vol. 51, pp. 108-18. 



1920. Examples of Readjustment of Drainage on the Tararua Western Foothills, 



Trans. N.Z. Inst., vol. 52, pp. 183-91. 

 Aston, B. C, 1912. The Raised Beaches of Cape Turakirae, Trans. N.Z. Inst., vol. 44, 



pp. 208-13. 

 Bell, J. M., 1910. The Physiography of Wellington Harhour, Trans. N.Z. Inst., 



vol. 42, pp. 534-40. 

 Cotton, C. A., 1912. Notes on Wellington Physiography, Trans. N.Z. Inst., vol. 44, 



pp. 245-65. 



1918. The Geomorphology of the Coastal District of South-western Wellington, 



Trans. N.Z. Inst., vol. 50, pp. 212-22. 



Crawford, J. C, 1869. Essay on the Geologv of the North Island of New Zealand, 

 Trans. N.Z. Inst., vol. 1, 2nd ed., pp. 305-28. 



Field, H. C, 1892. On Earthquakes in the Vicinity of Wanganui, Trans. N.Z. Inst., 

 vol. 24, pp. 569-73. 



Henderson, J., 1911. On the Genesis of the Surface Forms and Present Drainage- 

 systems of West Nelson, Trans. N.Z. Inst., vol. 43, pp. 306-15. 



* Contrary views have been held by some — e.g., Sir Charles Lyell, quoted by C. A. 

 Cotton (1912, p. 257). 



