472 Transactions. — Geology. 



Lastly, such a source as I suppose explains anomalies of 

 distribution which I shall now describe. Mr. Park wrote 

 that the ash-bed had a linear extension from Parnell to 

 Whangaparaoa Peninsula, thirty miles from Auckland, and 

 I shall assume that the volcanic breccias of that peninsula 

 are really extensions of the Cheltenham bed. In that case 

 the coarsest outcrop of the breccia is at the White Bluff, 

 but at Wairau the breccia is coarser than at Cheltenham, 

 which lies midway between White Bluff and Wairau. Again, 

 it is coarser at Deep Creek than at Wairau or Okura, yet 

 Deep Creek lies midway between Wairau and Okura. Or 

 let us consider the variations in thickness. At White Bluff 

 it is 30 ft., at Cheltenham 25 ft., at Wairau 30 ft., at Deep 

 Creek 40 ft., at Okura 4 ft., at Whangaparaoa 20 ft., each 

 of these places being farther north than the preceding one. 



It seems to me that these wide variations in both thick- 

 ness and coarseness must be due to a variety of sources, in a 

 chain of vents. Until we have far fuller fossil evidence than we 

 at present possess it seems impossible, or at least inadvisable, 

 to attempt to separate these beds, which we may judge from 

 their coarseness to have ail come from the Waitakerei vents, 

 the fossils in the beds, from White Bluff to Onehunga, con- 

 firming this. To attempt the separation it would be neces- 

 sary to know the position of the vents, of which we are quite 

 ignorant. In the meanwhile we may group the beds together 

 as the " Cheltenham breccia," regarding this really as a num- 

 ber of breccias formed at much the same time under much 

 the same conditions, but derived each from its own vent or 

 group of vents ; for we know that the Waitakerei Eange ex- 

 tends northward at least as far as Whangaparaoa, and that 

 there are other andesitic volcanoes beyond that which may 

 be of the same age. If we accept this origin for the breccia 

 our difficulties vanish. The volcanoes of the Waitakerei 

 chain were not equally active, equally powerful, or exactly 

 synchronous. Some outcrops of the resulting breccias would 

 be thick and coarse, others thinner and finer, while here and 

 there breccias of different origin would be mingled together. 

 Nor would the vents be quite in a line ; some might be to 

 the east of it, some to the west. Some would throw out 

 mainly scoria ; others, with greater explosive energy, would 

 eject large blocks. The thickness would probably vary more 

 than the coarseness, since no outcrops are far from the main 

 line, and this we find is actually the case. I believe the 

 Okura breccia may pretty safely be separated from the Chel- 

 tenham, but beyond that it is impossible to go; and perhaps 

 it is better, on the whole, to use the term " Cheltenham 

 breccia " for the present in the wide sense I have given 

 to it. 



