482 Transactions. — Geology. 



I examined the Orakei tuff to see whether it would throw 

 any light on the question. It is chiefly composed of fragments 

 of basalt (Mr. Park mentions trachyte ; I could only find hard 

 black scoriaceous lava, with olivine), but here and there are 

 fragments of grey sandstones and shales, and occasionally of 

 the Wairau tuffs. There are no fragments, as far as I could 

 see, of the Parnell grit. If this thick bed had been below the 

 Orakei greensand, fragments would in all probability have been 

 thrown out. A beautiful illustration of such an event oceurs 

 in the case of a similar outburst at Tamaki Gulf, where a 

 modern volcano has broken through the Parnell grit and con- 

 tains large fragments of it scattered everywhere through the 

 tuff. The Parnell grit is an easy rock to recognise, and as it 

 is hard it would not be blown to dust if the Wairau tuffs 

 escaped. But fragments of these tuffs are included. Now, 

 these are below the grit, so that the grit was, no doubt, all 

 denuded away before the outburst — at least, that portion of it 

 above 5. In that case it must have been at a good eleva- 

 tion, and therefore probably above the Orakei greensand. It 

 was at C that Hochstetter collected from the greensand. 



The outcrop of the Parnell grit, forming a reef in the 

 harbour near the Bean Eock Lighthouse, throws no light on 

 the stratigraphy. 



The next outcrop is at the west head of the Tamaki. 

 Before referring to this it will be advisable to trace the Orakei 

 greensand in the same direction. At the bridge which crosses 

 the outlet of the sunken Orakei crater to the sea the rocks 

 exposed in the low cliffs are the Orakei tuff beds. These are 

 seen a little further on to be quite unconformable to the 

 Waitemata sandstones. Interbedded with these is the Orakei 

 greensand. Here it is a greenish sandy bed which thins out 

 completely in both directions — a lenticular mass ; but it 

 appears again at a little distance on both sides. This patch 

 is the most fossihferous outcrop, yielding more than forty 

 species of fossils in a few yards, though the bed is only a 

 couple of feet thick. Proceeding round the cliffs towards 

 St. Helier's Bay the bed is next seen, beyond a fault, as a 

 reef separated from the shore by 30 yards of deep mud 

 and covered at hightide — here, again, richly fossihferous. 

 A little beyond the west head of Okahu Bay the 

 strata have a westerly dip, which brings up the green- 

 sand in a gentle slope across the face of the cliff. Pecten 

 zittelli, Pecten fischeri, Gastropods, and Bryozoa are the com- 

 monest forms contained in it. A fault exists in the middle 

 of Okahu Bay, or the dip changes, for on the east side of 

 the bay the bed is seen dipping easterly, and is again seen 

 at the Bastion, where it has an easterly dip and passes down 

 below the water. From this point to Tamaki West Head I 



