450 Transactions. — Geology. 



unreasonable to suppose that this may have passed to the southern hemi- 

 sphere, and drowned out some of the southern lands. It is conceivable that 



hemisph 



northern 



at the same period, the conditions may have been reversed. Possibly this 



theory may give a clue to the date at which the great Cook Strait River 

 existed. 



But, apart from such a speculation, let us consider the effect of local 

 causes. The west coast of the North Island, broken through by the lavas of 

 Mount Egmont and other volcanos, has evidently been an area of depression. 

 This involves the converse proposition, viz., that at one time it was an area of 

 elevation, and we may suppose that the elevation extended across the strait 

 and joined the two islands. 



We may suppose that before the deposition of the tertiary rocks there was 

 a period of elevation of the older rocks, followed by periods of depression, 

 during which the tertiaries were deposited. This was probably succeeded by 

 a period of elevation, during which the tertiaries emerged to a greater extent 

 than at present ; followed again by a period of depression during the time 

 when the volcanos of Mount Egmont and Ruapehu were in a state of activity. 



It may be fair to infer that it was during the latter period of elevation 

 that the Cook Strait River existed. We may picture to ourselves at that period 

 a similar condition to that of the rivers of Canterbury— an upper valley 

 system within the present limits of the strait, the emergence of the river from 

 a gorge in the narrows between Wellington Head (Tory Channel) and 

 Terawiti, and a further course through the plains to the eastward. Whether 

 these plains extended beyond the present sixty or seventy fathom plateau, or 

 were continued for a long way to the eastward, is a point for future research . 

 It is possible that the investigations of H.M.S. Challenger may tend to throw 

 the amount of light on the subject which may be gathered from a study of 

 soundings and of the configuration of the sea bottom. 



The arguments for the former existence of a Cook Strait River may be 

 concisely recapitulated as follows : 



1. The appearance of the land in the neighbourhood of Wellin 

 peculiar denudation, lead to the inference that it was formerly the summit of 

 a mountain range of considerable elevation. 



2. It is to be inferred that formerly the structural axis of the country lay 

 to the westward, because the volcanic district of Taranaki may be considered 

 to be an area of depression, and, as a converse proposition, formerly an area of 

 elevation, because the sounds of Queen Charlotte, Pelorus, etc., must have 

 been formerly excavated by running water, whereas now they show deep 

 soundings ; and because thfi tiwi<l«f foil n f a„ i.„u * ^ , «. .. ,., 



