Marshall. — Geology of the Central Kaipara. 433 



Art. XXXVI. —Geology of the Central Kaipara. 



By Professor P. Marshall, M.A., D.Sc, Professor of Geology. Otago 



University. 



[Read before the Olago Institute, 5th December, 1916 ; received by Editors, 30th December, 

 1916 ; issued separately, 30th November, 1917.] 



[Plates XXXII, XXXIII. 



The discussions that have so frequently taken place in regard to the age 

 of the lowest strata of the younger rock series of New Zealand have often 

 involved statements as to the stratigraphy of those outcrops that occur in 

 the north of the Auckland Province. The most recent of these statements 

 have been made by Park.* Morgan, f and Marshall. The first of these 

 geologists, in accordance with his later views in regard to the stratigraphy 

 of New Zealand, believes that there are Cretaceous and Tertiary formations. 

 The Cretaceous formation is supposed to terminate at the horizon of the 

 top of the so-called hydraulic limestone, the southern equivalent of which 

 is the Amuri limestone. Morgan appears to adopt a similar view, though 

 he does not make any precise statement. MarshallJ describes some points 

 in the palaeontology of the limestones of this district. He shows that the 

 lowest limestone (Whangarei type), generally admitted to be near the base 

 of the formation, contains a large amount of Amphistegina, and that its 

 characters in general are those of the so-called Miocene limestones of New 

 Zealand. On the other hand, the formation known as the hydraulic lime- 

 stone, a large part of which is not really calcareous, is shown to be a 

 Globigerina ooze when it has a calcareous nature. 



Since it appears that no collection of mollusca has been made in this 

 locality for nearly twenty-five years, it was considered advisable to visit 

 it and to study the stratigraphy so far as time would permit, and to collect 

 all the mollusca that could be found. The author has now visited the 

 locality on three occasions with those objects in view, and this paper 

 embodies the results of his work. 



The shore of the Kaipara Harbour between Port Albert and Matakohe 

 is the portion of the district on which the observations have been made, 

 for it is on this portion that earlier workers have found those sections on 

 which their conclusions have been based. The general physiography of 

 Kaipara Harbour is well known. The inlet penetrates deeply into the land 

 and ramifies far into the ranges' of hills along several drowned valleys. 

 Several of these drowned valleys have deep-water channels, and in places 

 the depth is as much as 20 fathoms. The arms of the harbour are generally 

 bordered by cliffs, which rise n places to 100 ft. ; but in many localities 

 there are extensive mangrove flats, ard the lines of cliff are much inter- 

 rupted by the full development of mature stream- valleys which are tribu- 

 tary to the drowned valleys now forming the main arms. The whole topo- 

 graphy is thus that of a maturely dissected lowland much depressed and 

 drowned but the depression took place at a time sufficiently remote to allow 

 of considerable cliff-erosion on the sides of the inlets that were formed by it. 



* J. Paek, Geol. Mag., dec. v, vol. 8, 1911, p. 546 ; also vol. 9, 1912, p. 493. 

 t P. G. Morgan. 10th Ann. Hep. X.Z. Geol. Surv., 1916, p. 11. 



% P. Marshall, The Younger Limestones of New Zealand, Trans. N.Z. Inst vol 48 

 1916, pp. 87-99 (see p. 91). 



