84 Transactions. 



those of lighter colour are- — (1) Dehydration of iron oxides under great 

 pressure and moderate temperature ; (2) diffusion operating under con- 

 ditions of warmth and moisture. Oxidation of the ferrous iron might be 

 accompanied by hydration ; if so, the conditions postulated by Barrell for 

 its dehydration would obtain in the neighbourhood of a fault-zone. That 

 some diffusion of the dehydrated iron oxide has taken place is evident 

 from the appearance of the reddened argillite, and is also shown by the fact 

 that in most cases the silica of the associated quartz veins is also slightly 

 reddened. The necessary conditions for this diffusion would also be found 

 as an accompaniment of the faulting. 



The changes mentioned above have no doubt been induced as the result 

 of more than one movement of faulting. The folded quartz vein at Sinclair 

 Head points to movement after its formation. In this connection it is 

 interesting to recall the position of the Tertiary conglomerate at 150 ft. 

 above sea level. That there has been revival of faulting along old fault- 

 lines has been pointed out by C. A. Cotton (13, p. 295). 



Origin of the Green Argillites. 



It seems clear that the land from the waste of which the Maitai series 

 was derived supported a considerable vegetation, as evidenced bj' the 

 plant-remains and the general dark colouring of the argillites. The absence 

 of organic matter from some argillites, and its general absence from or less 

 proportion in the greywackes, are points to be explained. 



The theory of Joseph Barrell (31, p. 428) to account for the sharp de- 

 marcation of shales and sands seems the best so far put forward. He 

 assumes that the waste from a land area is laid down in shallow w^ater, 

 the coarser material in general nearer the shore. A violent storm will be 

 effective in churning up the material and carrying the sand-product to a 

 greater depth, where it is deposited. The silt of this depth is also stirred 

 by the storm, and is in part worked farther seaward, in part settles back 

 in place. This latter portion, being lighter, is held longer in suspension, 

 and is deposited after the coarser sands. Repetition of this process results 

 in the development of such finely demarcated series of sands and clays as 

 are common the world over. With the lighter material held in suspension 

 will be the vegetable matter contained in the rock-waste. 



Lack of organic matter in quantity sufficient to impart a dark colour to 

 the argillites may have been due to an actual scarcity of vegetation on the 

 parent land-mass of the time. To assign causes for such scarcity is rather 

 speculative. It is suggestive, however, to note that in three cases the red 

 argillites of Wellington Peninsula are accompanied by bands of diabase 

 tuffs which in the two cases examined appear to underlie the green argil- 

 lites; and similar association of tuffs and green argillites has been noted 

 elsewhere (14 ; 17, p. 2 ; 35 ; 36). The tuff-beds are proof of energetic 

 volcanic action on the land surface of that time, a condition which world be 

 inimical to plant-growth during and immediately succeeding the time the 

 tuff-beds were laid down. 



The presence of carbonaceous matter explains why the argillites, where 

 other conditions are favourable, are not more generally red-coloured. That 

 no oxidation of ferrous iron can take place in the presence of organic 

 matter has been pointed out by J. S. Newberry (32, pp. 7 and 8) and 

 H. Newton (33). These writers refer to oxidation of ferrous iron at the 

 time of deposition of the sediments ; the reasoning may equally be applied 

 to the case of later oxidation. 



