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Volume 52, 1920
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Art. XVIII.—The Tawhiti Series, East Cape District.

[Read before the Wanganui Philosophical Society, 3rd December, 1919; received by Editor, 31st December, 1919; issued separately, 10th June, 1920.]

The true geological position of the strata that are exposed on the coastline between Tokomaru Bay and the East Cape has never been definitely ascertained, for we have had to rely on rather incomplete statements of McKay. Fossils of a true Cretaceous nature were reported as occurring in some abundance near Awanui. I visited the district in January, 1919, in company with Mr. J. A. Bartrum, with the primary object of making collections of fossils from the localities that were mentioned by McKay, in the hope that study of them would be of assistance in unravelling some of the debated points in regard to the relationship of the Cretaceous rocks to those of Tertiary age in New Zealand.

We spent two days on the coast near Awanui, but failed to find any of the ammonites and other fossils that were mentioned by McKay.* The only fossil remains that we found were some fragments of Inoceramus near the wharf at Awanui and a number of small worm-tubes a little to the north of that place. Inoceramus was very abundant in the concretionary boulders on the north side of Tuparoa Bay, and there were also fossils in the marls on the south side of the bay. These, however, were badly preserved, and seemed to be of a distinctly Tertiary nature. The stratigraphy is extremely involved, and the strata have slipped so much, while the sections are so discontinuous, that in the limited time at our disposal we were quite unable to come to any detailed conclusions on the question of the relationship of the strata.

At Tawhiti Point, on the north side of Tokomaru Bay, fossils are quite numerous, and they are also abundant in the shell conglomerate which occurs on Tawhiti itself at an elevation of 500 ft. or more. From the former of these localities over a distance of about a mile along the coast from Kotunui Point a representative collection of fossils was made. McKay collected from these beds in 1872, and he made a further reference to them in 1886, when he classed them of Upper Miocene age.

Hutton, in his paper on the geology of New Zealand, places the Tawhiti beds in the Pareora system, which is regarded by him as the equivalent of the Miocene of Europe.

No list of fossils that were found in these rocks has yet been published. The rocks are described by McKay as soft brown sandstones. We found that in their unweathered state they are of a grey colour and fine-grained. They are formed partly of small grains of quartz, with a good deal of partly-weathered feldspar, and black grains which seem to be volcanic glass. It is almost certain that the sand is of volcanic origin. The strata are considerably inclined, and strike 41° and dip 37° north-west. The thickness of the strata of which Tawhiti is composed is very considerable. Tawhiti itself rises to a height of 1,670 ft., and if the strike and

[Footnote] * A. McKay, Rep. Geol. Explor. dur. 1873–74, 1877, p. 124; 1886–87, 1887, p. 214.

[Footnote] † A. McKay, Rep. Geol. Explor. dur. 1873–74, 1877, p. 147; 1886–87, 1887, p. 210.

[Footnote] ‡ F. W. Hutton, Quart. Journ. Geol. Soc., 1885, p. 209.

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dip remain the same throughout as at Kotunui Point there cannot be less than 3,000 ft. of rocks. Our collections were made from practically the lowest rocks that are exposed near Tawhiti. The rocks vary a good deal in coarseness and are concretionary in many of the strata, but the fossils did not appear to vary much, and we have made no attempt to distinguish between the faunas of any of the different strata. Actually the thickness of the strata from which we collected was quite small.

The following is a list of the fossils that were collected by Mr. Bartrum and myself. In identifying the species I have had the invaluable aid of Mr. R. Murdoch.

  • Anomia huttoni (Sut.)

  • Astraea heliotropium (Mart.)

  • *Atrina sp.

  • *Calliostoma sp.

  • Calyptraea sp.

  • *Conus sp.

  • *Corbula canaliculata (Hutt.)

  • Corbula macilenta (Hutt.)

  • *Cucullaea worthingtoni (Hutt.)

  • *Cylichnella enysi (Hutt.)

  • *Dentalium mantelli (Zittel)

  • Dentalium nanum (Hutt.)

  • *Dentalium solidum (Hutt.)

  • *Diplodonta ampla (Hutt.)

  • *Divaricella sp.

  • Dosinia greyi (Zittel)

  • Dosinia lambata (Gould)

  • *Drillia aff. novae-zelandiae (Reeve)

  • *Galeodea senex (Hutt.)

  • *Galeodea sulcata (Hutt.)

  • Glycymeris laticostata (Q. & Q.) (thick form)

  • *Macrocallista assimilis (Hutt.)

  • Macrocallista multistriata (Sow.)

  • Mactra ordinaria (Smith)

  • Mactra scalpellum (Reeve)

  • Malletia australis (Q. & G.)

  • Myodora subrostrata (E. A. Smith)

  • Natica australis (Hutt.)

  • *Natica callosa (Hutt.)

  • *Natica ovata (Hutt.)

  • Nucula nitidula (A. Ad.)

  • Nuculana fastidiosa (A. Ad.)

  • *Olivella neozelanica (Hutt.)

  • Ostrea angasi (Sow.)

  • Ostrea corrugata (Hutt.)

  • *Panope worthingtoni (Hutt.)

  • Pecten convexus (Q. & G.)

  • *Pecten sectus (Hutt.)

  • Pecten zelandiae (Gray)

  • Protocardia pulchella (Gray)

  • *Sinum carinatum (Hutt.)

  • *Siphonalia conoidea (Zittel)

  • *Siphonalia costata (Hutt.)

  • *Siphonalia excelsa (Sut.)

  • *Siphonalia nodosa actuicostata (Sut.)

  • *Struhiolaria cincta (Hutt.)

  • Tellina eugonia (Sut.)

  • Tellina liliana (Iredale)

  • Turritella symmetrica (Hutt.)

  • Venericardia corbis (Phil.)

  • Venericardia purpurata (Desh.)

  • *Voluta corrugata (Hutt.)

Extinct species are distinguished by an asterisk. Several of the species that are unnamed are certainly extinct, but are not in a sufficiently good condition of preservation to allow of a satisfactory description being written.

This list contains fifty-two species, of which twenty-six are extinct—a percentage of 50. If this percentage of extinct species of mollusca is taken as a criterion for the correlation of the strata, the Tawhiti series must be almost exactly midway between the Target Gully series, where there are 35 per cent. of Recent species, and the Waipipi series, in which the percentage is 63. This position of the strata is suggested also by the very nature of the fauna. On the one hand Olivella neozelanica still persists, as well as Diplodonta ampla; while on the other hand a large number of species of Miocene occurrence appear, such as Cucullaea worthingtoni, Siphonalia conoidea, S. costata, Galeodea senex, &c. It is noticeable that there is no species of Limopsis in this collection, and that this genus is absent from those that have been made on the coast-line near Wanganui.