Marshall. — The Hampden Beds and N.Z. Tertiary, Limestones. Ill 



AUT. XIX. — The Ha))ipden Beds and the New Zealand Tertiary 



Limestones. 



By P. Marshall, M.A., D.Sc, F.G.S., F.N.Z.lnst., Hector and 



Hutton Medallist. 



[Read before tJie Wanganui Philosophical Society, 3rd December, 1919 ; received by 

 Editor, 31st December, 1919 ; issued separately, 10th June, 1920.] 



The facts that have recently come to light in regard to the palaeontology 

 of the Hampden Keds may be of some use in determining the relative ages 

 of the Amuri and Oamaru limestones. The number of fossils that have 

 been found actually in the Amuri limestone up to the present time is small, 

 but such as have been recorded suggest a Tertiary rather than a Cretaceous 

 age. Within recent years, however, Thomson (1916, p. 51) and Speight 

 (1917, p. 344) have found a fauna in tufE-beds interstratified with the upper 

 portion of the Amuri limestone in the Trelissick Basin, in Canterbury. 



The moUusca of this tufi-bed, so far as they have been collected up to 

 the present time, number thirty-seven species, of which 19 per cent, are 

 Recent. This is clearly a much later fauna than that of Hampden, for 

 there the Recent species are no more than 10 per cent, of the total of eighty. 

 If attention is focused more on the nature of the fauna than on the per- 

 centage the same conclusion will be reached, for the genera Dicroloma, 

 Trigonia',' Gilhertia, and even Exilia, of the Hampden fauna, have no repre- 

 sentatives, or even counterpart, in the collections that have been made in 

 the tuff-bed of the Trelissick Basin, the horizon of which is 10 ft. below 

 the upper surface of the Amuri limestone as developed in that locality. 

 It follows, if the palaeontological evidence is to be relied on, that the Hamp- 

 den beds are considerably older than the upper portion of the Amuri lime- 

 stone. If Thomson's statement is correct, that the Amuri limestone is 

 Cretaceous at the base and Tertiary in its upper, portion (loc. cit., p. 51), 

 the Hampden beds must represent some horizon in the middle or upper part 

 of the Amuri limestone. Thomson's statement, however, is based rather on 

 surmise than on actual fact, for up to the present time no fauna has been 

 found in the deposits immediately at the base of the Amuri limestone, 

 though at Amuri Bluff itself it is true that only some 200 ft. of strata 

 separate the beds with Cretaceous saurian remains from the base of the 

 Amuri limestone. 



Irrespective altogether of the accuracy of Thomson's statement, there 

 is reason to believe that* the Hampden beds are equivalent to some horizon 

 of the Amuri limestone, or possibly to an horizon actually below the 

 Amuri limestone. At Hampden itself there is no limestone, for on the 

 fossil-bearing beds, which are mainly formed of greensand, there is a 

 great thickness of submarine tuff, scoria, and other volcanic matter. The 

 eruption of this apparently affected the sea-floor so much, and for such a 

 long time, that all deposition of limestone was prevented. As a matter 

 of fact, the first occurrence of limestone in this neighbourhood is at All 

 Day Bay, fifteen miles farther north ; and even there the limestone 

 stratum is thin, and rests directly on submarine volcanic scoria. Since 

 the regular succession of the Oamaru system in its upper members cannot 

 be found at Hampden, some other neighbouring locality must be found 

 where it is more complete, and where there are strata recognizable by 

 their fossil contents as being of somewhat similar age to those of Hampden. 



