326 Transactions. 



examined the section. A similar sequence also occurs in the Broken River 

 between its junction with the Porter and the Broken River limestone 

 gorge, but the clarity of the section is somewhat obscured by slips from 

 the dominating limestone scarp above it : there is, however, no evidence of 

 unconformity. 



Hector and McKay have correlated the marl with the Amuri limestone, 

 whereas Hutton placed it on a lower horizon, since he correlated the lower 

 Trelissick limestone with the Amuri limestone. There is strong evidence, 

 which will be referred to later, that Hutton's contention is incorrect. The 

 stratigraphical position of the marl, the fact that in places it is lithologic- 

 ally indistinguishable from typical Amuri stone, and its close resemblance 

 in parts to a rock formed from Globigerina ooze, owing to the numerous 

 Foraminifera that it contains,* render it extremely probable that Hector and 

 McKay were right in placing it as they did. The characters of the deposit 

 show that it was laid down in fairly deep sea, but the interstratifi cation 

 with sands in its lower portions shows clearly that some land was adjacent. 

 It is the higher parts, however, which have a true deep-sea facies. The 

 palaeontological evidence furnished by other localities in the neighbourhood 

 indicates that the marl and the beds immediately associated with it are of 

 Lower Tertiary age, which is not inconsistent with its correlation with the 

 Amuri limestone.' 



The beds exposed near the junction of Broken River and the Porter are 

 continued round the east and south of Prebble Hill imder the tussock-clad 

 slopes, and there is no clear-cut section till the neighbourhood of the upper 

 limestone gorge of the Porter is reached. In swinging round the hill the 

 beds are bent up into a bowl-shaped form, which is tilted towards the west ; 

 on the edge of the southern rim a magnificent section is exposed. Near the 

 river the beds strike N. 80° E., with a northerly dip at an angle of 50°, the 

 river cutting directly across them. 



The section exposed at the river consists of the following, in descending 

 order : — 



Limestone. 



Volcanic tuff. 



Marl, whiter in the upper portion and greyer in the lower. 



White quartz sand, with two concretionary bands similar to those in 



the section lower down the river. 

 Greensands. 



These last apparently form the solid base of the Avestern terrace of the 

 Porte] 1 River for some distance up above the gorge, but a fairly heavy 

 covering of shingle renders a proper examination impossible. The beds 

 just above the gorge are apparently involved in a small fault with a down- 

 throw to the north ; but the section is somewhat obscure, and the appear- 

 ance may be really due to a surface slip, the greensands having moved 

 forward. If a fault really occurs, the throw is small. 



Owing to the close covering of well-grassed soil, and talus from the 

 overlying limestone, no good exposure of these beds can be obtained on 

 the east side of Castle Hill ; the lower bed of limestone, however, outcrops 

 in a small slip north-west of Mr. Milliken's house. 



Following the basin round, the next important section where the lower 

 members of the formation are exposed is in Whitewater Creek, a tributary 



* P. Marshall, The Younger Limestones of New Zealand, Trans. X.Z. Inst., 

 vol. 4vS, 1 !»l(i. pp. 94, 95. 



