Art. VIII—The Younger Limestones of New Zealand.
[Read before the Otago Institute, 7th December, 1915.]
In previous papers attention has been drawn to the great differences of opinion which have been expressed and published in regard to the age of the younger rocks of New Zealand. It has also been stated that it is very generally the fact that in all those districts where the younger rock-series is well developed there is a conspicuous bed of limestone forming one of its members. In various parts of the country this limestone has been placed in different chronological divisions by geologists. In some localities it has been classed as Cretaceous, in others as Oligocene or Miocene. Identical beds of limestone which have by some authors been classed as Cretaceous have been placed in the Oligocene or Miocene by others. The statements about the age of the limestones have frequently not been based upon the internal evidence found in the limestones themselves, but upon external evidence found in their stratigraphical relations, or in the palaeontological remains found in the beds above or below the limestones in various localities. In some instances both of these methods have failed, and reliance has then been placed upon the lithological resemblance of some outcrop of limestone to another, often situated in some distant part of the country. Up to the present time no attempt has been made to describe with any degree of accuracy the minute structure of these rocks, or to state even generally the nature of the organisms of which they are composed.
It is intended in this paper to make a general statement of this nature, though it is recognized that a full description of the rocks, with a specific identification of the organic remains which occur in them, would require the work of a specialist in each particular group of organisms. In the case of the Echmoderms, which are very abundantly represented, the remains are merely spines or isolated plates, which even in the hands of an expert would not admit of accurate identification. The Polyzoa also require very expert knowledge in order that they should be identified in chance sections. In the case of the Foraminifera generic classification alone is attempted. The Sponges and Radiolaria, which occur more rarely, are referred to in a general way only. Notes are also given on the classification of these limestones by various geologists who have examined them in the laboratory or in the field.
1. Whangarei – Kaipara – Bay of Islands Region of North Auckland.
Within this district limestones cover a large area of country, and two distinct types have been generally recognized— the so-called hydraulic limestone and a semi-crystalline type generally called the Whangarei limestone. Near Whangarei the latter occurs at Horahora, Kamo, and Waro. Near the Bay of Islands the same type is found at Waiomio, and in the Kaipara area at the so-called Gibraltar Rocks at the end of the Pahi Arm. The hydraulic limestone has a much wider occurrence. Typical localities from which specimens have been obtained and are here described are Mahurangi, Port Albert, Wellsford, Kaiwaka, and Limestone Island, in the Whangarei Harbour.
The earliest attempt to classify the limestones of this district was made by Cox.* He calls the hydraulic limestone of Limestone Island the “lower
Sketch-Map Of New Zealand, Showing The Localities Where The Limestone Rocks Described Here Were Obtained.
limestone,” and correlates it with the Lower Greensand formation of England. The other type—Whangarei limestone—is classed in the Cretaceo-tertiary, and is correlated with the Amuri limestone of North Canterbury.
A further description of the district was written by McKay.* He correlates the hydraulic limestone with the Amuri limestone in the Cretaceo-tertiary division, but he places the Whangarei limestone in a lower horizon both because of stratigraphical relations in the Whangarei Harbour and because of a clear sequence in a section at Waiomio, where he describes the superposition of the Whangarei as undounted. He classes all the limestones in the Cretaceo-tertiary division.
In the Kaipara district the first description again was written by Cox,† who this time classes the hydraulic limestone as Cretaceo-tertiary. The Whangarei limestone of the Gibraltar Rocks is said to overlie the Cretaceo-tertiary unconformably, and it is therefore placed in the Upper Eocene.
Park‡ was the next geologist to describe this area. He makes the hydraulic limestone an equivalent of the Amuri limestone of Cretaceo-tertiary age, and the Whangarei limestone is placed below it conformably and interstratified with the greensands.
The next geologist to visit the district was McKay,§ and in his report he entirely agrees with Park, and also gives a diagram showing the inferior position of the Whangarei limestone to the hydraulic limestone at Waiomio. This opinion has been strengthened by J. A. Bartrum, Lecturer on Geology at the Auckland University College, who assures me in correspondence that the semi-crystalline limestone (Whangarei limestone) lies interstratified in the greensands below the hydraulic limestone near Pahi. Hector and Hutton do not appear to have made any specific references to the stratification or age of the limestones of this North Auckland district.
Park's latest statements in regard to these limestones show a considerable change of view ∥ The Whangarei limestone, including the stone of the Gibraltar Rocks, is said to rest conformably on the hydraulic limestone which is still correlated with the Amuri limestone, but both the hydraulic and the Whangarei limestones are now placed in the Cretaceous system.
2. Cabbage Bay.
A limestone similar to the Whangarei stone comes from Cabbage Bay, on the Coromandel Peninsula. McKay, in 1885 and 1897, correlated it with the Whangarei limestone of Cretaceo-tertiary age. Park, in 1897, placed it in the Lower Eocene. McLaren (Parliamentary paper, C.-9, “Geology of Coromandel Goldfield”), in 1900, classed it with Hector's Cretaceo-tertiary or Hutton's Oligocene. In Bull. N Z. Geol. Surv. No. 4, 1907, p. 56, Fraser and Adams placed it in the Lower Eocene, and described it as a hard compact limestone consisting largely of bryozoan corals, among which Foraminifera are sparsely scattered. Park, in 1910, placed this limestone as an equivalent of the Ototara stone (Oamaru) of Miocene age.
Another similar limestone comes from the Wiwiku Island, Kawhia Harbour. McKay, in 1883, describes it as a hard subcrystalline limestone of Cretaceo-tertiary age. He says that it lies below the ordinary limestone of this locality. At Morant Island, half a mile away, there is a greensand
which lies below a flaggy limestone that is apparently identical with that of Wiwiku Island. The greensand contains a variety of fossil Mollusca, and of these all that were collected belong to Miocene species. Morant Island is not mentioned by McKay. The ordinary limestone of this locality is well developed on both sides of the entrance of the Rakaunui River.
4. Tata Islands, Nelson.
The limestone found here is also a coarse-grained type, somewhat less crystalline and less compact than the Whangarei limestone. Hutton, in 1885, placed this stone in his Oligocene system. Park, in 1889, correlated it with the Ototara (Oamaru) stone of Cretaceo-tertiary age. This stone contains a number of fossil Mollusca which indicate a Miocene age.
5. Mount Somers.
From this South Canterbury locality a coarse limestone has been procured for use as a building-stone. It is still less crystalline and compact than the stone from the Tata Islands. Haast, in 1873, said that this limestone was the equivalent of the Weka Pass stone—the highest member of the Saurian formation (Cretaceous). Cox, in 1884, placed the limestone in the Eocene.
The limestone formation here has generally been called the Ototara stone by the Geological Survey. It has been largely used as a building-stone, with great success. It is not conspicuously crystalline, and is far from compact, but in practice it is found to possess satisfactory resisting-powers to weathering influences. The references to this stone are numerous. Hector and the officers of his Geological Survey consistently placed it in the Cretaceo-tertiary; Hutton always included it in his Oligocene system; and Park has of recent years always considered it of Miocene age. The last observer has described two beds of this stone separated by a bed of fossiliferous greensand.
7. Raglan, Mokau, Te Kuiti.
Limestone occurs in a thick stratum at all of these localities. It appears to have been correlated with the Ototara stone by all observers, and it has therefore been placed in the Cretaceo-tertiary by Hector, in the Oligocene by Hutton, and in the Miocene by Park.
8. Cobden Limestone Of Greymouth.
This, again, has on all hands been correlated with the Ototara stone.
9. Amuri Stone.
This is the limestone which occurs in beds of great thickness over a large part of Marlborough and North Canterbury. Very generally it has been correlated with the hydraulic limestone of the North of Auckland. This correlation appears to have been based on its fine - grained lithological nature, for, like the hydraulic limestone, it is practically destitute of molluscan fossils. Hector and the officers of his Geological Survey consistently classed it in the Cretaceo-tertiary, but Hutton always considered it a Cretaceous horizon. Park also has classed it in the Cretaceous system.
Descriptions Of Limestones From The Localities Named Above.
A. Polyzoal Limestones.
(a) Waiomio Limestone, near Kawakawa, Bay of Islands. Plate VIII, Fig. 1.
Foraminifera of relatively large size frequent: Truncatulina, Nodosaria, a thick-walled Carpenteria, Globigerina, and a moderate-sized nearly round form of Amphistegina. Spines and plates of echinoderms in a perfect state of preservation are very frequent. Polyzoa are abundant. There is much glauconite in separate rounded grains. Occasional small grains of fine sandstone. Mr. F. Chapman, of Melbourne, writes, “The Waiomio limestone is of the typical Miocene character”
(b) Horahora, Whangarei. Plate IX, Fig. 2.
Very similar to the rock from Waiomio, but it contains much less glauconite. Polyzoa are somewhat more abundant at the expense of the Foraminifera. In particular, Globigerina and Carpenteria are not in my sections, but Amphistegina is rather more abundant.
(c) Waro, near Whangarei.
Echinoderm fragments are very plentiful, and, with Polyzoa, constitute nearly the whole material of the rock. There are occasional Foraminifera, including Amphistegina and Rotalia. Some secondary calcite has crystallized round the echinoderm fragments. A little glauconite is present, but there are no grains of foreign rock matter. The subcrystalline nature often mentioned in this and similar limestones is entirely due to the frequent occurrence of plates and fragments of echinoderms.
(d) Gibraltar Rocks, Pahi Arm, Kaipara Harbour.
A coarse-grained type, mainly composed of Polyzoa. Echinoderm spines and plates are large and perfect, with a little secondary calcite in the interstices. Foraminifera are not numerous, but Carpenteria, Globigerina, and Rotalia occur, and the small round Amphistegina is fairly common. No Lithothamnium can be seen. A little glauconite, but no grains of foreign rock matter. Mr. Chapman says, “The Amphisteginae of this rock are of a varietal form denoting rather deep water.”
(e) Limestone Band in Greensand, Pahi, North Auckland.
Polyzoa and echinoderm fragments are here of less importance, and Foraminifera and Lithothamnium are in considerable quantity. Of the Foraminifera, Spiroplecta, Truncatulina, and Globigerina are the most common; but there are many individuals of a smaller type of Amphistegina than in the preceding rocks. Very little quartz, but a considerable quantity of glauconite.
(f) Wiwiku Island, Kawhia Harbour.
In hand-specimens a coarse-grained type. In section remains of Polyzoa and echinoderms are seen to form the greater part of the material. Amphistegina, apparently the same species as at Waiomio and Horahora, is quite plentiful. Angular quartz grains are quite frequent. Several specimens of Rotalia calcar in the section. The ordinary limestone of the mainland varies considerably in different horizons. Amphistegina occurs in a band 100 ft above the base.
(g) Tata Island, Golden Bay, West Nelson.
Again Polyzoa and echinoderms make up the greater part of the rock. Foraminifera rather numerous, including Truncatulina, Rotalia, and a somewhat doubtful specimen of Amphistegina. A little Lithothamnium is present. There are a great many quartz grains.
(h) Mount Somers, South Canterbury.
This rock consists mainly of Polyzoa, though echinoderm remains are frequent. Foraminifera are less abundant, and only Globigerina, Operculina, and the small round species of Amphistegina could be distinguished. Grains of glauconite and occasional grains of quartz are to be seen.
(i) Castle Hill, Trelissick Basin, Canterbury.
Polyzoa are again the most frequent organisms in this rock, though echinoderm plates are common. Foraminifera are less common, and only Cristellaria could be distinguished. There is some Lithothamnium.
(j) Pahau River, Culverden, North Canterbury.
In some specimens shell fragments are very abundant, but in others the material is mainly polyzoal. The echinoderm plates are plentiful, and generally in a good state of preservation. Foraminifera are generally rather few in number, but a Truncatulina and a small round Amphistegina, apparently the same species as in the northern limestones, are quite distinct. There is some Lithothamnium. A little glauconite. Much secondary calcite and a single quartz grain.
(k) Oamaru, North Otago. Plate VIII, Fig. 2.
This is the Ototara stone of the Geological Survey, which has been largely used as a building-stone throughout New Zealand. The stone is less compact than the majority of the limestones, and during the preparation of the sections the organisms become separated to some extent. Polyzoa, echinoderm plates, and Foraminifera are present in about equal proportions. Rotalia is the most common of the Foraminifera Truncatulina is not infrequent, and there is an occasional Amphistegina. No Lithothamnium, glauconite, or quartz are to be seen in the half-dozen rock-slices that were prepared.
Beneath the main outcrop on the north side of the Kakanui Valley, near Clark's mills, there are several narrow bands of limestone interbedded in the soft marls. The lowest of these has a small quantity of Lithothamnium, but in other respects these bands are not to be distinguished from the ordinary limestones of the main outcrop.
(l) Limehills, Winton, Southland.
Composed mainly of Polyzoa, though Foraminifera are rather more frequent than usual. Shell fragments and plates of echinoderms are common. Among the Foraminifera there are Globigerina, Rotalia, Truncatulina, and a very small Amphistegina. Some small grains of quartz and a few round grains of glauconite.
(m) Cabbage Bay, Coromandel Peninsula.
A great preponderance of Polyzoa and relatively little echinoderm material. Foraminifera consist of Globigerina, Carpenteria, and Amphi-
stegina. A small quantity of glauconite and one or two grains of foreign rock. This rock closely resembles the limestones from Waiomio and Whangarei described above.
(n.) Mokau, Taranaki. Plate IX, Fig. 1.
The specimen that I have consists mainly of Amphistegina of relatively large size, 2–5 mm. in diameter. In many of the individuals the canal system is most distinctly seen, because all the minute canals are filled with glauconite, as well as some of the chambers. A few Operculina are also present, as well as an occasional Spiroplecta. Very few Polyzoa and echinoderm plates. Lithothamnium is quite abundant. A little quartz and some shell fragments.
B. Foraminiferal Limestones.
(a.) Raglan, West Auckland.
Globigerina abundant, but larger individuals than in the true oozes of New Zealand. One large Cristellaria. Rotalia and Truncatulina not frequent. No glauconite and no detrital grains. Much of the material of the rock is extremely fine-grained calcite that shows no distinct characters. Mr. Chapman says, “This is a deeper-water type than the Milburn rock, and was formed at a fair distance from the shore-line.”
(b.) Waitetuna, near Raglan.
Not greatly different from the above, but it contains Bolivina as well as Globigerina, a fragment of a shell, and a small polyzoan. A small piece of brown hydrous mica. Many slender rods, probably calcified sponge-spicules.
(c.) Cobden, Greymouth, Westland.
A relatively fine-grained type, which shows an approach to Globigerina ooze. Most of the tests of Globigerina are broken. Small Truncatulina and Rotalia. Some carbonaceous matter and brown hydrous mica. A few small quartz grains and some glauconite. Numerous slender rods that may be calcified sponge-spicules.
(d.) Te Kuiti, South Auckland.
Very similar to the Raglan types, but containing several small fragments of Polyzoa. Globigerina, Truncatulina, and Rotalia are abundant. The internal cavities of the tests are filled with calcite.
(e.) Sandymount, Dunedin.
Globigerina is the most abundant organism, and the cavities of the tests are filled with crystalline calcite. Truncatulina and Spiroplecta occur as well. A small amount of carbonaceous matter is present, and there are some quartz grains.
(f.) Milburn, South Otago. Plate X, Fig. 1.
Remains of Polyzoa few and inconspicuous. Echinoderm plates fairly frequent, but thin and small material. Foraminifera rather abundant, but small. Globigerina, Rotalia, and Spiroplecta are the most frequent. A few grains of quartz and some glauconite. Mr. Chapman says, “The Milburn limestone is of moderately deep-water origin, and resembles some of our Miocene foraminiferal marls.”
C. Globigerina Ooze.
The first six limestones described here are generally called hydraulic limestones. The others are generally classed as Amuri limestone.
(a.) Kaiwaka, Kaipara Harbour. Plate X, Fig 2
The only distinct organism is Globigerina, which is present in great abundance. The chambers are generally filled with secondary calcite, though here and there with glauconite. Several sponge-spicules of perfectly isotropic silica. A few radiolarians. Some brown hydrous mica and a very few extremely minute quartz grains. This limestone is associated with a stratum of diatomaceous and radiolarian ooze.
(b.) Port Albert, Kaipara Harbour.
Globigerina very abundant, and a small Dentalina? Chambers generally filled with calcite, but occasionally with pyrite. One radiolarian and many calcified sponge-spicules. A small amount of glauconite and brown mica.
(c.) Port Albert Wharf.
Much glauconite and a few minute grains of quartz. No Foraminifera except Globigerina can be seen. There is a great deal of finely granular calcite and some of the brown mica.
(d.) Wellsford, North Auckland
Slice much broken by small veins of crystalline calcite. Globigerina is the only organism, but the greater part of the rock is formed of finely granular calcite. There is a little glauconite and some of the brown mica.
(e.) Limestone Island, Whangarei Harbour
A true Globigerina ooze, with crystallized calcite occupying the interior of the chambers. Very few minute quartz grains, and some glauconite and some pyrite.
(f.) Mahurangi, North Auckland
Many small veins of secondary calcite. Globigerina the only recognizable organism. The larger chambers are crushed. A little glauconite, but no detrital grains.
A pure Globigerina ooze, with a very occasional grain of glauconite. Some sponge-spicules and several examples of a radiolarian. A little brown mica.
(h.) Otaio, Canterbury
A fine-grained type, with many minute quartz grains and a good deal of glauconite. Mainly Globigerina, but one specimen of Amphistegina.
(i.) Amuri, Bluff, Marlborough.
The organic remains are nearly entirely Globigerina, the chambers of which are separated. A few grains o glauconite and sponge-spicules. Radiolaria are not infrequent. There is a little of the brown mica.
(j.) Esk River, North Canterbury.
A very fine-grained type, consisting mainly of fine-grained calcite, which does not appear to show any trace of organic origin. One specimen of Rotalia. Many Globigerinae of small size.
(k.) Amuri Limestone, Weka Pass, North Canterbury.
The chambers of Globigerina, which are generally isolated, are fairly numerous. By far the greater part of the rock consists of very finely grained calcite. This appears to be the general structure of the Amuri limestone over the whole of North Canterbury and Marlborough, and even in the highly siliceous and flinty varieties of Ward and the Ure River no remains of siliceous organisms can be distinguished, probably because they have been dissolved, for their presence is clearly shown in small number in the limestones of Amuri Bluff and Kaikoura.
A type of this rock has been found near Oxford, and in this locality it is soft and chalky. Hutton stated that this was the remains of an old coral reef, a most unlikely origin in the light of these descriptions of New Zealand limestones, for in none of them has the remains of any coral been found.
At the Weka Pass a glauconitic quartzose limestone rests on the Amuri limestone. A number of rock-slices from the immediate vicinity of this junction have been examined. It is found that near the junction the Amuri limestone contains a considerable number of grains of quartz sand and some glauconite, as well as some brown mica. The presence of these minerals is associated with the appearance of different and larger species of Foraminifera, including Cristellaria and Rotalia. These characters emphasized to a greater extent are the features that distinguish the overlying Weka Pass stone from normal Amuri limestone. There are small isolated nodules of the Weka Pass stone embedded in the Amuri stone without a sharp boundary between them, In addition, there are also inclusions of the Amuri limestone embedded in the Weka Pass stone. The microscopic structure and relations of these limestones, therefore, serves to indicate that there is a strong resemblance between these stones near their contact, and that such differences as there are would be a natural result of the shallowing of the water and of an increase in the velocity of the ocean-currents. There is independent evidence of the shallowing of the water at this time in the nature of the deposits that everywhere rest on the limestone. From a geological standpoint it is a most remarkable suggestion that a pure Globigerina ooze should be overlaid uncomformably, though without change of strike or dip, by another relatively deep-water deposit containing the same microscopic minerals and organisms as the underlying rock, especially when the one rock lies directly on the other continuously over a wide area of country.
Opinions suggested by a Study of these Limestones.
The examination of the structure of these limestones and the recognition of the general nature of their component organisms would naturally be expected to throw some considerable light upon the conditions under which they were formed, and on their age.
So far as the conditions under which they were formed are concerned, we gain a decided amount of information. The limestones are, as a rule, remarkably free from contaminating sediment, and this fact alone indicates that they were formed at some distance from land, or that the land area was small and did not support large rivers that could supply any important
quantity of sediment. This idea is further supported by the actual position of some of the limestone outcrops, which may be far inland, and are even in some cases situated within the recesses of the mountain-ranges. The sediment that does occur in the limestones is either extremely fine quartz sand or small fragments of volcanic rock produced by volcanic eruptions of a submarine nature. There is thus strong evidence that at the time or times at which these limestones were formed the land was reduced to comparatively small dimensions.
It is very noticeable that in the microscopic examination of these limestones no remains of any kind of coral has been found. This is the more remarkable when it is stated that in the greensands which often occur above and below the limestones there are frequently a large number of species of Flabellum and related species of coral. It is thus evident that the statements which have frequently been made that the Tertiary limestones are wholly or in part the remains of former coral reefs are quite incorrect, so far at least as those localities are concerned from which the specimens described here were obtained. This is the more important when it is realized that the limestones from which these specimens have been described were collected in all parts of the country, and that they include the outcrops which have been referred to by other geologists as of coral origin.
The actual depth of water in which the deposit of these limestones took place is also indicated by the organic remains that occur in the rocks.
Those limestones that consist of Polyzoa and echinoid fragments and of the larger types of Foraminifera with occasional Lithothamnium were deposited in water of no great depth. The soundings round the New Zealand coast show the presence of so-called coral and coral sand at depths of 105–772 fathoms to the west of Cape Maria van Diemen, in places as much as 100 miles from the coast-line. Similar material occurs at a distance of twenty-five miles east of the Bay of Islands in water 305 and 325 fathoms deep. The only other locality in which there is a record of a similar bottom is near the Snares, in water 77–81 fathoms deep. It is probable that this material closely resembles the deposit that developed into the polyzoal type of limestones, such as Whangarei, Cabbage Bay, Wiwiku Island, Tata Islands, Mount Somers, Oamaru, and Winton. The echinoid remains are certainly more abundant in the rock than they are likely to be in the “coral sand,” but this may indicate warmer climatic conditions. Thus in the Funafuti report (Roy Soc Lon Rep of Coral Reef Com.), 1904, p 329, it is stated, “Detached spines of echinoids are in many cores so abundant as appreciably to contribute to the mass of the rock.” In those localities where “coral sand” has been found on the sea-floor in the New Zealand area the land is of small dimensions, there are no rivers near at hand to supply sediment, and the ocean-currents flow with greater swiftness than at other places on the coast-line. Here, then, we appear to have important evidence in favour of the idea that the land was of much smaller size at the time when this limestone material was deposited.
In those instances where the limestone contains much Lithothamnium, as at Mokau and the Kaipara, it is probable that the depth of water was much less, though it is noticeable that at Funafuti this alga grows at a depth of 200 fathoms. The glauconite, which occurs not uncommonly in the limestone, especially in the polyzoan types, also indicates clear oceanic water, but it also suggests a relatively steep coast-line and peculiar conditions of ocean-currents.
In those types of limestone such as occur at Milburn, Raglan, and Cobden, where the Polyzoa are much fewer and the Foraminifera more numerous and smaller, it is probable that the depth of water was much more considerable—perhaps some 500 fathoms. The fossil remains found in this type of limestone are relatively few, and they belong to genera which may occur at considerable depths. Pecten huttoni, Lima laevigata, Pericosmus compressus, and Magellania sp. are those that are most usually found. In these rocks, again, glauconite is usually found in small quantity.
The third type of limestone is represented by the hydraulic limestone of Auckland and the Amuri limestone of Canterbury. It is a pure Globigerina ooze, and may have been deposited in water of any depth between 600 and 2,500 fathoms, though it is not very frequent in water of a less depth than 1,000 fathoms. The soundings round the New Zealand coast are most numerous in the extreme north, and here it is found that such an ooze covers the sea-floor at a depth of 1,095 fathoms about twenty miles east of the North Cape; generally, however, it does not occur at distances less than 100 miles from the coast. Probably the hydraulic limestone, which contains a notable quantity of the most finely divided detrital material, is analogous to the “Globigerina w. cl.” of the charts—Globigerina ooze and white clay (?)—which has been found at depths of 600 fathoms and more near the northern part of New Zealand, and at 400 fathoms seventy miles to the east of the Bay of Islands.
The occurrence of siliceous organisms in this limestone at Amuri Bluff, where the deposit is 630 ft. thick, and the highly siliceous nature of the limestone near Kaikoura, Ward, and the Clarence Valley, indicate that in these areas at least the water was of great depth—perhaps more than 2,500 fathoms—for the siliceous organic remains begin to displace the calcareous types. This is the same in the North of Auckland, where much of the hydraulic limestone is free from glauconitic matter, and it is often highly flinty or even siliceous throughout. At Kaiwaka this limestone is associated with radiolarian and diatomaceous ooze.
The nature of the organic remains in the limestones has not been studied with any exactness, though such as the results are they certainly throw some light on their age; but we still have to rely in the main on the nature of the organisms contained in the strata associated with the limestones. It is a matter of very general agreement that the limestones at Te Kuiti, Raglan, Wiwiku Island, Mokau, Tata Islands, Mount Somers, Oamaru, Milburn, and Winton are of Miocene age. In nearly all of these localities these limestones are associated with strata that contain a variety of Miocene fossils. These fossils are mainly molluscan, and they have been regarded as Miocene chiefly because of the high percentage of Recent species that are represented—20 per cent. or more.
The Whangarei limestone is not so easily dismissed. No molluscan fossils have yet been recorded in strata that are associated with it, and it becomes necessary to consider again the nature of the organisms that occur in it. The presence of Amphistegina is of great importance in this connection. This genus appears to be regarded as a typical Miocene genus. Thus Chapman says, “The inequilateral Amphistegina took the place of the equilateral Nummulites towards the close of the Oligocene, and was the predominant form in many foraminiferal deposits of Miocene age.”* The frequent occurrence of Amphistegina in the Whangarei limestone thus points decisively to a Miocene age for this rock, and this
[Footnote] * F. Chapman, Mem. Nat. Mus. Melbourne, No. 5, 1914, p. 23.
organism may be used to correlate all those limestones in which it occurs, for it appears to be the same species in all of them. The stones from the following localities contain this species: Wiwiku Island, Kawhia, Tata Islands, Culverden, Otaio, Mount Somers, Oamaru, and Winton.
It appears to be the general opinion of geologists that the Whangarei limestone is a lower horizon than the hydraulic limestone in the North of Auckland, or at least that the two limestones belong to the same series. If this is the case, the hydraulic limestone also cannot be older than the Miocene. The suggestion is in part supported by the occurrence of Cucullaea and Dentalium in this rock at Limestone Island, Whangarei Harbour, specimens of which were on view in the Auckland Exhibition of 1913. It is further supported by the occurrence of a varied Miocene molluscan fauna in the greensands below the hydraulic limestone at Pahi.*
The Amuri limestone still remains to be considered. It has already been stated that microscopic examination of the Amuri limestone at Weka Pass, and of the Weka Pass stone which rests on it, has shown that the limestones have an identical composition in kind both mineralogically and organically, and that the differences between these two strata, so far as specimens taken from the immediate neighbourhood of the junction are concerned, are due solely to the difference of proportions of the various constituents. Very few molluscan fossils have as yet been obtained from the Amuri stone. Pecten zitteli Hutton is practically the only species. In the Trelissick basin, however, Thomson and Speight have discovered a molluscan fauna in the beds beneath the Amuri limestone, and at the Weka Pass the Weka Pass stone, which rests on the Amuri limestone, also contains a Miocene fauna. Again, the Otaio limestone, which is always regarded as an outcrop of the Amuri limestone, and which is a very fine-grained type of rock with an abundance of Globigerina, also contains this Amphistegina, which is apparently the same species as in other parts of the country.
This Amuri limestone has always been correlated with the hydraulic limestone of North Auckland. It is quite reasonable to suppose that these two rocks are of the same age. Under the microscope the two cannot be distinguished, and their composition indicates that they were deposited on an oceanic floor in deep water. This implies great depression, which is likely to have taken place during the same earth-movements in localities that are relatively close to one another. If this correlation, which has been so very generally adopted, is correct, the Amuri limestone, like the Whangarei limestone and the hydraulic limestone, must be of approximately the same age as the Oamaru and other limestones that are so largely composed of remains of Foraminifera.
The presence of Amphistegina in so many of these limestones both north and south supports the correlation of the limestones from another and quite a different point of view. So far as our knowledge of the New Zealand Foraminifera goes at present, the genus Amphistegina does not occur in our waters. There is no mention of it in Hutton's “Index Faunae Novae Zelandiae,” nor in Haeusler's account of the Foraminifera in the Hauraki Gulf, nor in Chapman's description of the Foraminifera dredged in 110 fathoms west of the Great Barrier. The genus is, however, well represented at the present day in the warmer waters of the Pacific Ocean. Thus Brady† states, “In the living condition Amphistegina is distinctly a tropical genus, its home is among the shallow-water sands of warm seas.” He
[Footnote] * J. Park, N Z. Geol Surv, 1885, p. 168.
[Footnote] † “Foraminifera,” Chall Rep, vol. 9, p. 741.
further states that it commences at 30 fathoms and continues with some frequency to 300 or 400 fathoms. At Funafuti it occurs at all depths down to 200 fathoms.
The formation of these limestones must, therefore, have taken place during a period of warm climatic conditions. Such a condition of relatively warm climate must be regarded as exceptional in the New Zealand latitudes, and is not likely to have been repeated within any but relatively long-time periods. There is independent evidence that the Miocene period was one of relatively warm conditions in New Zealand, as in many other regions of temperate climate. Within New Zealand such evidence is to be found in the relatively large size of many species of Mollusca, of which mention has been made by several geologists. To this may be added the fact that such genera as Cypraea, Trigonia, and Chama then existed in New Zealand, and, with Trivia, occurred as far south as Oamaru.
That the younger limestones of New Zealand are all of the same age, and represent approximately the same horizon, has already been suggested by Marshall, Speight, and Cotton.* It was, however, stated in that paper that the probable age of the limestone was Oligocene, this opinion being based upon the classification of various groups of organisms that had been collected from the limestone and submitted to experts for description. The Miocene age, however, is supported not only by the nature of the molluscan fossils that have been found in the limestone horizon itself at Otiake, but in the molluscan fauna below the limestone at Pahi and in the Trelissick basin, and in a similar fauna in the beds resting on the limestone in many localities. The percentage of Recent species in all of these cases amounts to rather more than twenty, a fact that suggests that the age is really the Upper Miocene. As has been stated on many occasions, however, it is probable that too much importance has been attached to this percentage.
The very general occurrence of Amphistegina supports the idea that the limestones are of Miocene age. This genus, however, existed in the Oligocene, though it is extremely characteristic of the Miocene. Thus Chapman† says, “The inequilateral Amphistegina took the place of the equilateral Nummulites towards the end of the Oligocene, and was the predominant form in many foraminiferal deposits of Miocene age.”‡
I am greatly indebted to Mr. J. A. Bartrum, Lecturer on Geology at Auckland University College, and to Mr. G. S. Thomson, of the Whangarei High School, for many specimens of limestone from the North of Auckland; to Mr. R. Speight, Curator of the Canterbury Museum, for micro slices of many Canterbury limestones. I am particularly grateful to Mr. F. Chapman, of the National Museum, Melbourne, for the identification of the Foraminifera and for some comparative notes.
[Footnote] * Marshall, Speight, and Cotton, Trans. N.Z. Inst., vol. 43, 1910, p. 407.
[Footnote] † F. Chapman, Mem. Nat Mus. Melbourne, No. 5, 1914, p. 23.
[Footnote] ‡ Since the geological range of Amphistegina is a matter of prime importance in connection with this paper, I have made inquiries of Mr F. Chapman, of the National Museum, Melbourne, as to the occurrence of the genus. He has kindly sent me the following statement: “Amphistegina is (leaving out doubtful forms) Cainozoic and Recent. It occurs very sparingly in one or two localities in the Middle or Upper Eocene, and perhaps Oligocene, in Europe; but is increasingly abundant towards the Burdigalian (Middle Miocene); then moderately rare till the Recent formations, when it occurs in all tropical and subtropical seas. In India Amphistegina appears to be confined to the Miocene. In Australia it is fairly abundant in the Oligocene, excessively abundant in the Miocene, and thence to Recent more sparingly met with, when it is found living at or near the coasts in low latitudes. Whenever it is found abundantly in fossil deposits one may conclude we are dealing with Miocene strata.”