68 Transactions. 



led to the formation of a strait, it is difficult to believe that the feeble 

 waves of the waters of a strait, which was necessarily initially narrow and, 

 at least in parts, almost or completely landlocked, can have reduced 

 mature mountains of resistant rocks to a plain of marine erosion, over 

 a width of several miles, in a period so short that the mountainous 

 relief of the " old land " was not destroyed in neighbouring areas by 

 subaerial denudation. 



Secondly, submergence of a region of mature mountains must result 

 in the drowning of many valleys. Thus not only would a single narrow 

 strait be formed, but also many braiiching bays of considerable depth. If 

 we suppose planation of the partially drowned ridges, the peninsulas, 

 and the islands of such a region to take place, it is apparent that the 

 result cannot be a continuous denudation plain. While a more or 

 less perfect plain will have been produced, and this end will have been 

 achieved by the cutting-down of the salient features and the filling-up 

 of the drowned valleys to a common level. The sloping plateau is, 

 however, a plain of denudation throughout. Had it been otherwise the 

 plateau in its present form could not have survived, but the ancient 

 filled valleys would have been re-excavated by modern erosion, revealing 

 again the postulate-d maturely opened valley forms and the ancient 

 drainage pattern. 



Thirdly, a strait the floor of which is a platform of marine erosion 

 must have been much widened by wave-action, and if it has been so 

 widened in a region of mature mountains it must be bordered by 

 wave-cut cliffs of great height. No such cliffs, however, have been 

 pointed out by Bell. It is true that a scarp, previously referred to as 

 the Wakamarama fault-scarp, bounds the Aorere Valley on the north- 

 west side, and it is true also that steep mountain-slopes ascend from 

 the sloping plateau on the south-east side ; but recourse has never been 

 had to marine erosion to account for these, and other and more satis- 

 factory explanations are not difficult to find. 



Fourthly, the nature of the sediments of the covering strata of the 

 Aorere Valley has been previously referred to. The limestone which 

 follows the basal conglomerate is free from admixture of terrigenous 

 material, and is not the kind of deposit that might be expected to occur 

 in a strait between mountains. It has, moreover, not been shown to 

 pass into a littoral facies towards either side of the supposed strait. 



Erosional, Sedimentary , and Deformational History. 



It has been necessary to state the objections to Bell's theory of the 

 genesis of the physical features of the Aorere Valley somewhat fully, since 

 they are all arguments in favour of the hypothesis which is here offered 

 in its place. It would appear that the strong relief which the deformed 

 undermass presumably had in some earlier period had been almost com- 

 pletely destroyed prior to the deposition of the covering strata. It is 

 reasonable to suppose that this reduction of the ancient mountains was 

 effected largely by subaerial erosion, though planation was completed, 

 over at least the area of the Aorere Valley, by the advancing sea at the 

 commencement of the period of deposition of the covering strait. Next 

 followed a period of deposition over a wide area, and, later, the episode 

 of strong differential movements, which sketched out the broad outlines 

 of the land-forms of the present day, led to the formation of many con- 

 sequent rivers, and inaugurated the cycle of erosion in which the majority 

 of the details of the surface were developed. 



