431 Transactions. — Geology. 



found nearer Onehunga, and the interbedded shales merely 

 lenticular masses between which the grit has been squeezed, 

 the thinning-out being obscured by the formation of the 

 ground and the vegetation covering it. In this case the con- 

 glomerate at the top would represent a later deposit. But 

 the presence of conglomerate on the outskirts of a con- 

 glomerate country is not surprising. 



From the similarity between the fragments included in the 

 grits and the composition of the grits themselves it is not un- 

 reasonable to conclude that they were ejected by vents which 

 had some connection with each other. Moreover, that there 

 were several vents there is not the slightest doubt. It is 

 quite inconceivable that fragments of rock such as those 

 found at Whangaparaoa, Takapuna, Cheltenham Beach, and 

 in the ash-beds along the northern shores of the Manukau 

 Harbour, could have been hurled for any great distance 

 through the air ; and the fact that they are not much water- 

 worn shows that they have not been transported far by the 

 agency of water. Hence we may conclude that there were 

 several centres of eruption and several showers of ashes, 

 some coarser than others, as in several places, notably at 

 Takapuna (see Section V., above), the coarser ash overlies the 

 fine. Moreover, the eruptions which caused the ash must 

 have occurred about the same time, and the showers them- 

 selves must have taken place at relatively short intervals. 



All traces of the localities of these old centres of activity 

 have long since disappeared ; but the evidence furnished by 

 the material composing the grit enables within certain limits 

 the loci of some of them to be established. One volcanic 

 centre undoubtedly existed at or near Whangaparaoa. Such 

 huge fragments as those found there must certainly have 

 been derived from a vent in the immediate vicinity of their 

 present position. Another centre evidently lay not far from 

 the Takapuna ash-beds, the fragments in the grits there being 

 too large to have been derived from Whangaparaoa, a distance 

 of about ten miles. Another vent probably had its site at or 

 near Cheltenham Beach. But in the case of the two latter it 

 is not possible to do more than conjecture. It may have 

 been that a single vent situated somewhere between them 

 furnished the .material for the deposits in both places, as well 

 as that for those on the southern shores of the harbour. That 

 it probably existed nearer to Takapuna and Cheltenham Beach 

 than to the southern side of the harbour is shown by the differ- 

 ence in texture of the material in these places, that found 

 in the beds of the former being much coarser than the other. 

 There is, however, nothing to indicate either the precise 

 locality or the number of these vents. It is quite conceivable 

 that much of the material was derived from vents in the 



