224 Transactions. — Geology. 



It is a geological certainty that on the bottom of the 

 greater part of the inner harbour — perhaps the whole of it — 

 there are rocks standing practicall}'^ on end. How far this 

 arrangement of rocks passes beyond the Boulder Bank it is 

 difficult to say, but that they do pass beyond must also be 

 considered a certainty. Now-, if the purely drift theory is to 

 be accepted, we must suppose that these rocks were planed 

 down by the action of the sea for at least 20 ft. below low 

 water before any boulders were deposited. This, of course, 

 is not an impossible thing to happen. Geological sections 

 often reveal such an arrangement of rocks on their edges 

 covered by horizontal strata. But in this case there is a 

 difficulty. If these rocks were planed down, say, 20 ft. 

 below the level of the sea, why were the Arrow Eock and 

 the reefs near it not also planed down to the same depth? 

 How would this denuding action reach from Mackay's Bluff 

 to the Arrow Eock and then stop short ? The thing is well- 

 nigh impossible ; when the sea planes it usually planes pretty 

 smoothly. Then, supposing the planing-down process to the 

 depth indicated did actually take place, what became of' the 

 boulders that were being formed at Mackay's Bluff at the 

 same time? Did they stay there till the adjacent rocks had 

 been planed down 20 ft. below sea-level and then start drift- 

 ing south-west? Not a likely thing to happen. Another dif- 

 ficulty against the acceptance of this theory is the largeness 

 of some of the stones at the extreme south of the bank. The 

 advocates of the drift theory realise the difficulty, and sug- 

 gest that heavier seas must at one time have prevailed in 

 the bay. Now, how could those heavier seas have been pro- 

 duced — seas mighty enough to roll huge boulders ten miles 

 along a level sea-bottom at a depth of 20 ft. from the surface ? 

 Only by so altering the coast-line that Tasman Bay would be 

 more exposed to the ocean than it is at present. To effect 

 this change of coast-line in the direction indicated we should 

 have to give up some of our fundamental ideas of New Zea- 

 land geology. It would involve first the submergence and 

 then the reappearance, in comparatively recent geological 

 time, of some of the land by which the bay is at present 

 sheltered. Then, too, the absence of gradation in the size 

 of the stones on the bank, when viewed lengthwise, is an- 

 other difficulty against the acceptance of the drift theory. It 

 is a fact that some of the stones at the south end of the 

 bank are as large as any to be found at the north end of the 

 bank. If all these stones had been drifted from one place, 

 one would expect to find large stones near the source, and 

 a gradual diminution in size down to gravel or even sand as 

 the distance from the source was increased. But such is not 

 the case on the Boulder Bank. 



