Art. XXXV.—The Geology of the Bluff, New Zealand.
[Read before the Otago Institute, 3rd October, 1911.]
1. Introduction and Description of the Topography of Area.
The generally even surface of the Southland Plain is broken on its coastal margin by a range of hills extending in a south-easterly direction from the mouth of the New River Estuary for a distance of seven miles, and terminating in Bluff Hill. The height varies much from point to point, but the outstanding feature is Bluff Hill, which attains an elevation of 860 ft.
Several geologists have visited the locality, or have examined specimens of rock from it, but the area has never been submitted to accurate and systematic geological examination.
Hutton,* in 1872, referred to the Bluff Hill in describing the geological structure of the Southland District. He also described the relative positions of some of the rocks found there, and such of their characters as can be detected in the field. In his “Geology of Otago,” published in Dunedin in 1875, he repeated the conclusions he had come to.
In 1888 the general structure and physiographical nature of the district was described at some length by Park,† who also went into the evidence as to the age of the rocks; but the writer offered neither chemical nor microscopical descriptions of the various rock types.
At a later date Hutton‡ named and described sections of specimens of rock from Bluff Hill, but subsequently, with more material at hand, published additional notes,§ in which he expressed a change of opinion with regard to the nomenclature of some of them. It is rather unfortunate that the localities from which these specimens were obtained have not been recorded more definitely.
Hamilton∥ has also contributed to the literature on the subject, and the locality is also mentioned several times in. “The Geology of Otago,” by Hutton and Ulrich. The references in the latter publication will be discussed below.
Thomson¶ has recently published notes on some rocks which are “the result of a few hours' collection along the shore south and west from Bluff Harbour,” and “from a small headland about half a mile round the coast to the south-west just beyond the mouth of the harbour.” A glance at the map will show that “west” must be a misprint for “east.”
[Footnote] * Hutton, “Report on the Geology of Southland,” Rep. N.Z. Geol. Surv., 1871–72, p. 89.
[Footnote] † Park, “On the Geology of Bluff Peninsula,” Rep. N.Z. Geol. Surv., 1887–88, p. 72.
[Footnote] ‡ Hutton, “Notes on the Eruptive Rocks of Bluff Peninsula,” Trans. N.Z. Inst., vol. 23 (1891), p. 353.
[Footnote] § Hutton, “Corrections of the Names of some New Zealand Rocks,” Trans. N.Z. Inst., vol. 31 (1899), p. 484.
[Footnote] ∥ Hamilton, “Notes on the Geology of the Bluff District,” Trans. N.Z. Inst., vol. 19 (1887), p. 452.
[Footnote] ¶ Thomson, J. A., “Notes on some Rocks from Parapara, Bluff Hill, and Waikawa,” Trans. N.Z. Inst., vol. 42 (1910), p. 33.
The present paper will aim principally at an accurate description of the rocks that outcrop on the portion of the foreshore of Bluff Harbour lying between the wharves and Starling Point, together with the related rocks of Tewaewae Point, since this area exhibits in most striking manner the different rock types and their mutual arrangement.
The more detailed investigations herein described point to conclusions somewhat opposed to previously accepted ideas of the geology of the district.
The Bluff Range forms the backbone of a prominent peninsula of the south coast of the South Island of New Zealand, in latitude 46° 32′ S. and longitude 168° 23′ E. It extends for seven miles from north-west to south-east. It is united to the mainland by a narrow strip which projects to the east from the flank of the range at its north-western end, and separates the waters of Bluff Harbour on the south from the Mokomoko Inlet on the north. At its north-western extremity the range terminates in somewhat abrupt cliffs impinged on by the New River, the mouth of which has been driven east by the sands of the Riverton Beach, which are constantly travelling in this direction under the influence of the seas and currents caused by the prevailing westerly winds.
Origin of the Land-forms.
The range consists of a mass of igneous rock which was originally a deep-seated intrusion. Subsequent denudation acting more readily on the intruded than on the intrusive rocks has exposed the intrusive mass as a range of hills. The area of contact, which is fully described in the following pages, is thus a metamorphic aureole. The resulting metamorphic rocks outcrop in places as the base of the range on the north-east side, the most extensive outcrop being on the foreshore of the harbour from Henderson Street for a distance of 37 chains towards Starling
Point. A study of the rocks in this locality, together with those of Tewaewae Point, reveals the nature of the metamorphism, though the gap in the series occupied by the mouth of the harbour cannot be bridged in a manner absolutely satisfactory.
The origin of Bluff Harbour and Awarua Bay, as well as Waituna and other lagoons along the south coast between Bluff Hill and Fortrose, requires some explanation.
The sand and shingle driven by the prevailing current through Foveaux Strait came to rest at the lee side of Bluff Hill. Thence, after the manner commonly described, a sandbank extended outwards in a direction slightly north of east. At length it reached the headland of Waipapa and Slope Points, which attains a height of 800 ft., or slightly more, some twenty miles to the east. Thus a considerable area of water was cut off and enclosed by the sandspit. As the sandbank received further additions it increased in height and mass most rapidly at its eastern end, where its onward progress was stopped by the headland already named. Finally, at its lowest end—that is, immediately adjacent to Bluff Hill—the waters broke over the barricade, and restored communication with the ocean. The channel was made across the intruded rocks, possibly along a groove commenced by the ancient rivers, and long since filled in with sand and alluvium. This channel, once begun, was rapidly deepened by the inrush and outflow of the tide, which in the middle of the channel travels at the present time at the rate of eight knots an hour, such is the size of the basin to be filled.
Subsequently this large lagoon was subdivided by lateral sandspits, and Waituna Lagoon was separated from Awarua Bay, and a separate outlet was formed.
II.Description of the Rock Types and their Geological Occurrence.
For the purpose of description the rocks of the Bluff district are conveniently divided into two main divisions, distinguished here as A and B. These divisions are—A, the igneous rocks; B, the metamorphic rocks.
Each of these divisions may be further divided into classes, the rocks of each class in the division being entirely distinct. The igneous rocks, Division A, consist of—I, igneous rocks of plutonic origin; II, igneous rocks of hypabyssal origin. The term “hypabyssal” is here used in the same sense as Harker * uses it, hypabyssal rocks corresponding in a general way, though not precisely, with the group “gangesteine” or “dyke-rocks” of Rosenbusch.
The metamorphic rocks, Division B, contain two classes, as follows: I, acid metamorphic rocks; II, basic metamorphic rocks.
An attempt will be made below to show that there is an intimate connection between the acid metamorphic rocks (Division B, Class I) and the igneous rocks of hypabyssal origin, the metamorphic rocks being derived from the hypabyssal ones. This is not the view taken by previous investigations—e.g., Hutton and Park—but the present theory is the outcome of the examination of material that had not previously been brought under inspection. The point will be fully discussed in the course of this paper.
The relationship among some classes and the absence of relationship among others thus briefly mentioned makes it difficult to suggest any perfectly satisfactory scheme of classification.
Division A.—The Igneous Rocks.
Class I.—Igneous Rocks of Plutonic Origin.
As has been stated above, igneous rock of plutonic origin forms the backbone of Bluff Peninsula. The mass is believed to be essentially one throughout as regards chemical and mineralogical composition. This Park † definitely states to be the case.
The present paper will deal with the rock as it is typified in Bluff Hill. There are numerous outcrops at the surface, as well as a continuous outcrop at sea-level along the south-east end and the south-west side of the range, except in the rare intervals occupied by sandy beaches. From specimens collected from different parts of the mass some thirty sections have been prepared and examined.
Hand-specimen, (specific gravity, 2·68).—The rock varies somewhat in grain in different parts of the mass, though this variation is apparently not systematic. In some parts it tends towards a pegmatitic structure; sometimes a dense black rock of fine grain is found.
The typical rock is a rather coarse-grained type, speckled black and white. With a lens the black grains may be distinguished as pyroxene or hornblende, according to the characteristic cleavage. The white specks are feldspar.
[Footnote] * Harker, A., “Petrology for Students,” 4th ed. (1908), p. 108.
[Footnote] † Rep. N.Z. Geol. Surv., “The Geology of Bluff Peninsula” 1887, p. 89.
Under the Microscope.—Examination of thin sections shows the rock to be composed essentially of feldspar, augite, hypersthene, and hornblende, with magnetite as an accessory constituent. The feldspar, which on an average forms half the rock, occurs usually in plates, ranging in size up to 1·4 mm. long by 1 mm. broad. It also is found in irregular pieces enclosed ophitically by the ferro-magnesian minerals. It is a triclinic variety, showing both coarse and fine albite lamellation. The extinction-angle ranges up to 27°, this angle being the one recorded most frequently in sections as nearly perpendicular as possible to the albite lamellae parallel to the brachipinacoid.
According to the statistical method of Michel Levy for determining the feldspars, this angle denotes labradorite. In some cases, however, an angle of 16° in found on each side of the trace of the twinning-plane. In such a basic rock this figure indicates andesine, and Thomson thinks this is the prevailing species, though he affirms that “probably more than one variety of feldspar is present.” Undulose extinction and the bending of twin lamellae in a number of the crystals give evidence of considerable crushing.
Of the ferro-magnesian minerals hornblende is the most prominent. It frequently occurs as a fringe of varying breadth bordering the crystals and masses of pyroxene. In these cases it is a pale-green colour, and rather feebly pleochroic on the inner margin, but in the outer portion of the fringe it becomes more compact and denser in colour and pleochroism, changing from yellow-green to browny green.
Often, again, the hornblende occurs in masses apparently independent of the pyroxenes. Under these circumstances it is compact, brownish-green in colour, and strongly pleochroic. Thomson * mentions that “the cores of the hornblende crystals generally consist of a paler variety in optical continuity with the green exteriors, so that the former presence of pyroxene is suggested.” This point will be further discussed (pp. 331–2).
Both monoclinic and orthorhombic pyroxenes are to be seen. Augite occurs in rounded grains and in irregularly shaped crystals. It is colourless and non-pleochroic, and, where fresh, shows brilliant polarization colours of the second order. But often it is cloudy, and shows signs of decomposition, which, no doubt, ultimately gives rise to the particles of chlorite recorded by Hutton † and Thomson.*
Diallage also occurs, though somewhat sparingly. It encloses minute tabular scales of a reddish-brown colour, arranged parallel to the basal plane, giving it the schiller structure, which distinguishes it from augite.
The orthorhombic pyroxene is hypersthene. In some cases it exhibits schiller structure like the diallage, but it may be distinguished from the latter mineral by its pleochroism and by the fact that it extinguishes straight. These characters also distinguish it from augite. The pleochroism showed a or ă brownish red, b or b reddish yellow, c or c green, pale watery colours in each case. To determine definitely that the mineral was not enstatite, many sections were examined in convergent light to secure an optic axial interference figure, and thus find its optical character, but these attempts were unsuccessful. However, the pleo-
[Footnote] * Thomson, J. A., “Notes on some Rocks from Parapara, Bluff Hill, and Waikawa,” Trans. N.Z. Inst., vol. 42 (1910), p. 33.
[Footnote] † Hutton, F. W., “Notes on the Eruptive Rocks of Bluff Peninsula,” Trans. N.Z. Inst., vol. 23 (1891), p. 353.
chroism is usually accepted as sufficiently distinctive. After hornblende, hypersthene is the chief ferro-magnesian constituent in this rock. It occurs in rather elongated crystals, some of which show cross-fractures, and in smaller rounded grains.
Magnetite is fairly abundant, in irregular masses, moulded on the other minerals. Thomson * thinks the iron-ore is probably ilmenite. He gives no reason, however, for thinking it to be ilmenite rather than magnetite. Hamilton † noticed considerable disturbance of the magnetic needle while he was in this district, but, as ilmenite also affects the magnetic needle, Hamilton's observations do not point conclusively to magnetite, though they are certainly significant.
Order of Crystallization.—Observation of the form and arrangement of the minerals in this rock does not support the theory suggested by Rosenbusch as to the normal order of crystallization. According to his theory, magnetite should have been the first mineral to crystallize, and in sections there should be at least some well-shaped crystals idiomorphic towards the other constituents. Next in order the ferro-magnesian minerals should have separated out, forming crystals idiomorphic towards the feldspar, the last mineral to crystallize.
The actual sequence of events, however, seems to have been as follows: First a small amount of feldspar crystallized out, for crystals of this mineral are enclosed in both magnetite and hornblende; then the remaining feldspar and the ferro-magnesian minerals crystallized out, and sometimes the latter constituents show an approach to idiomorphism, while sometimes the opposite is the case;, finally magnetite separated from the magma, and formed interstitially.
In accordance with recent theories of the dependence of structure on eutectic relations,‡ the explanation would be as follows: Feldspar was originally in excess, and an amount crystallized out sufficient to reduce the ratio of feldspar to ferro-magnesian minerals to a eutectic proportion. These minerals then crystallized together until a eutectic proportion between them and the magnetite was arrived at, when all crystallized together, but since a large percentage of the rock was already in a crystalline state the magetite had little chance of becoming idiomorphic.
The Origin of the Hornblende.— The Rev. H. Baron in conversation with Captain Hutton § long ago expressed the opinion that all the horn-blende in this rock is of secondary origin. This opinion receives support in the fact that very many of the pyroxene crystals are bordered by a rim of hornblende, which first appears as a narrow fringe, pale green in colour, and rather faintly pleochroic. This fringe increases in width at the expense of the pyroxene, and as it widens it increases in depth of colour and in intensity of pleochroism. Finally the pyroxene is entirely replaced by amphibole. The examination of a very few sections shows this change in all its stages. These observations show definitely that at least a large amount of the hornblende is secondary.
[Footnote] *Thomson, J. A., “Notes on some Rocks from Parapara, Bluff Hill, and Waikawa,” Trans. N.Z. Inst., vol. 42 (1910),p. 33.
[Footnote] † Hamilton, “Notes on the Geology of the Bluff District,” Trans. N.Z. Inst., vol. 19 (1886), p. 452.
[Footnote] ‡ Vogt, J. H. L., “Physikalische-chemische Gesetze der Kristallisation folge in Eruptivgesteine,” Isch. min. u. petr. Mitt. 24, p. 437, 1905.
[Footnote] § Hutton, F. W., “Corrections of the Names of some New Zealand Rocks,” Trans. N.Z. Inst., vol. 31 (1899), p. 484.
The inference that all the hornblende in this rock is secondary is supported by descriptions of similar areas in various parts of the world. Such areas have been described by Irving * and by Williams † in America, by Phillips ‡ in Cornwall, by Reusch § in Norway, by Lehmann ∥ in Saxony, by Becke ¶ in Lower Austria, by Wadsworth ** and by Hawes †† in America, and more recently by Harker ‡‡ in the west of Scotland.
As regards the causes that produced the alteration of the pyroxene little is yet known. The experiments of Mitscherlich and Berthier (1824), Gustav Rose (1831), and Professors Fouge and Michel Levy, of Paris, and the recent researches of Vogt, Joly, Cusack, Doelter, Brun, Day, Allen, and others have shown that augite appears to be the stable form at high temperatures and hornblende at low temperatures. From this it may be assumed that any condition tending to facilitate molecular readjustment at ordinary temperatures must necessarily tend to facilitate the change from augite to hornblende.
These considerations inclined Williams §§ to ascribe the uralitization of some rocks to the action of great pressure, such as might be exerted by the upheaval of mountains, and Lehmann ∥ and Hatch ¶¶ reached similar conclusions. Subsequently, however, Williams † decided that, though pressure may, and doubtless does in many instances, assist in the para-morphism of pyroxene in rocks, it cannot in all cases be regarded as even a necessary adjunct.
In the case of the plutonic mass of rock forming Bluff Hill the following points are put forward merely as suggestions.
The magma was intruded at sufficient depth to allow of the formation of a holocrystalline mass by slow cooling. At the temperature of the mass augite was formed. When ordinary temperature was reached the augite would tend to change to hornblende if conditions should change so as to induce unstable equilibrium in the crystals so far as the molecular forces were concerned. Such a change of conditions would possibly be brought about by either or all of the following:—
- (1.) Diminution of pressure by denudation of the overlying rocks. This undoubtedly took place, but whether it would tend to induce molecular readjustment is a matter for speculation.
- (2.) Movements of depression and elevation described above.
- (3.) Lateral pressure due to the folding to which the whole country was submitted in late Palaeozoic or early Mesozoic times.
[Footnote] * Irving, R. O., “Origin of the Hornblende of the Crystalline Rocks of the North-western States,” Am. Journ. Sci., vol. 26 (1883), p. 32.
[Footnote] † Williams, G. H., “The Gabbros and Associated Hornblende Rocks occurring in the Neighbourhood of Baltimore, Md.,” U.S. Geol. Surv., Bull. No. 28, 1886.
[Footnote] ‡ Phillips, Quart. Journ. Geol. Soc., vol. 32-(1876), p. 155, and vol. 34 (1878), p. 471.
[Footnote] § Reusch, “Die fossilienfuhrenden krystallinen Schiefer von Bergen in Norwegen,” German translation by R, Baldauf, 1883, p 35.
[Footnote] ∥ Lehmann, “Untersuchungen uber die Entstehung der altkrystallinischen Schiefer-gesteine,” p. 190; Bonn, 1884.
[Footnote] ¶ Becke, “Mineralogische und petrographische Mittheilungen,” vol. 4, p. 357, 1882.
[Footnote] ** Wadsworth, “Bulletin Museum Comparative Zoology of Harvard College, Cambridge,” vol. 7, p. 46.
[Footnote] †† Hawes, G. W., Am. Journ. Sci. (3), vol. 12, p. 136.
[Footnote] ‡‡ Harker, A., Mem. Geol. Surv., Tert. Ign. Rocks Skye (1904), p. 319.
[Footnote] §§ Williams, G. H., Am. Journ. Sci. (3), 28, p. 266 (1884).
[Footnote] ¶¶ “Mineralogische und petrographische Mittheilungen,” vol. 7, p. 83 (1885). -11
In this paper the name “norite” has been adopted in accordance with the views of Harker and Rosenbusch.
As has been shown above, the Bluff rock consists essentially of a soda-lime feldspar, a monoclinic pyroxene, and an orthorhombic pyroxene. Both kinds of pyroxene are changing to hornblende, and in the case of crystals, where the change is completed, it is impossible to say whether the hornblende is derived from orthorhombic or from monoclinic pyroxene. However, so far as may be judged from what remains, the orthorhombic variety is dominant, and the rock is therefore a norite rather than a gabbro.
Chemical Composition.—If the chemical composition be appealed to, as some authors demand, it also will be found to support the classification here suggested.
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|H2O and loss on ignition||1.00|
|Specific gravity, 2.68|
At Starling Point the norite encloses a mass of a dark-coloured rather fine-grained rock. Specific gravity = 3·035.
The size of the mass cannot be made out definitely, as there is a coating of soil at this locality. The outcrop is small, being exposed in a cutting about half a chain in length and 6 ft. or 8 ft. deep. But, judging by the position of the outcrops of norite around it, the surface extent of the mass cannot be more than a chain in diameter. As has already been stated, the norite varies much in texture from point to point, and in some cases approaches to a material similar to that now under description. This led Hutton to describe several varieties of rock from the district.
Under the microscope the rock presents a similar assemblage of minerals to the norite, but there is a very noteworthy increase in the proportion of ferro-magnesian minerals. Hornblende forms more than half the rock, magnetite is abundant, there is a little pyroxene, and a basic plagioclase, sometimes containing needles of apatite, forms the rest. There is an approach to a rough gneissic structure.
Structure and Order of Crystallization.—The rock has a coarsely schistose or gneissic structure, and this tends to obscure the order of crystallization, so that it cannot be made out with any degree of accuracy. Hornblende seems to dominate, but the edges of its crystals are extremely ragged. In fact, no mineral can be said to be idiomorphic, and the
manner in which the crystals of the principal three constituents are intergrown suggests simultaneous crystallization. The fact that there is no sign of graphic structure, however, and the tendency towards gneissic structure that is observable suggest recrystallization.
Park,* in 1887, described this mass as an inclusion of the metamorphic rocks in the norite. But this explanation now seems improbable, as a glance at the following list of analyses will show. A and C are the results obtained above on an analysis of the hornblende schist and the norite respectively. They are repeated here for comparison with B, an analysis of a specimen of the mass under discussion.
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|Loss on ignition||1.20||1.00||1.00|
The analysis shows that the inclusion is more basic even than the norite, so that it cannot be considered merely as an included mass of hornblende schists. For the same reason it is not likely to be due to the complete absorption of a portion of the schist in the norite magma.
A more probable explanation of the occurrence of this mass depends on the theory of differentiation, to which much importance is attached by many modern geologists. Thus Harker † says, “We are left free to conjecture that the settling-down of crystals, which seems to be generally ineffective in a sill or laccolite, may give rise to very important differentiation in a large intercrustal magma-basin, cooling at an extremely slow rate. Various special features observable in igneous rocks are susceptible of interpretation on this hypothesis, and serve in a measure to support it. The dark basic secretions or ‘clots’ which occur sporadically in many granites and other rocks may be taken as an example. These consist in general of the same mineral as the normal rock, but are much enriched in the darker and denser minerals or in those of earlier crystallization. It seems reasonable to regard them as portions picked up from a lower stratum of the magma-reservoir, where crystals of these minerals accumulated by settling down in the magma.”
This theory certainly seems to explain the case in point, where we have an inclusion which, compared with the norite, shows a decrease of 3·7 percent. of silica, and a total increase of 6 per cent. in the oxides of the banes iron, calcium, and magnesium. Its specific gravity, also, is 3·035, compared with 2·68 in the case of the norite.
[Footnote] * Park, J., “Notes on the Geology of Bluff Peninsula,” Rep. N.Z. Geol. Surv. 1887–88, p. 72.
[Footnote] † Harker, A., “The Natural History of Igneous Rocks,” p. 322, 1909.
Class II.—Igneous Rocks of Hypabyssal Origin.
The typical rock is found across the channel of the harbour, opposite Starling Point, where it forms a fringe bordering the tongue of sand which bears the name of Tawaewae Point, and which is really the north head of the harbour.
The outcrop extends below low-water mark, but above that line its width is only 15 or 20 yards. The rock is traversed by joints which divide it into more or less oblong blocks of a variety of sizes. One set of these joints strikes approximately north-west to south-east; the other set crosses at right angles. The dip varies from 0° to 30° N.E. The total length of the outcrop is about 16 chains.
Hand-specimen (specific gravity = 2·5).—The rock is dense, and when freshly broken is of a light-grey colour. The weathered surface, however, is of a dirty brownish-yellow colour, and from it project numerous crystals of feldspar.
Under the Microscope.—Thin sections show phenocrysts of feldspar in a groundmass consisting of feldspar, quartz, hornblende, and mica. Magnetite also occurs, partly in masses of irregular size and shape, and partly in small crystals.
The phenocrysts of feldspar vary considerably in size, some going up to as much as 2·4 mm. by 1·2mm., but the average size is 0·9 mm. by 0·6mm. They are chiefly orthoclase, and show twinning after the Carlsbad law in nearly every case. Less common are phenocrysts of a plagioclase variety. These show the albite twinning very poorly developed, and I have no section in which an absolutely satisfactory identification may be made. The available evidence, however, points to albite.
None of the phenocrysts are entirely fresh, while many bear in a marked degree the signs of decomposition, and all stages between the two extremes are represented. The first stage is a cloudiness which spreads irregularly over the crystal, and associated with it is the deposition of a very fine dark-coloured opaque dust. Then appear minute pale colourless microlites, which as they increase in size assume a pale-green colour, and are distinguishable as hornblende. As the microlites increase in size and number, larger and more definitely shaped crystals of magnetite appear. The needles of hornblende grow at the expense of the feldspar, for they penetrate through and through the crystals of this mineral, and also appear in great number round the edges of crystals, where they finally arrange themselves in aggregates. As mineral change becomes more and more complete small grains of quartz and flakes of brown mica appear. Finally we see a cloudy space, recognizable by its size and shape as the ghost of a feldspar, containing needles of hornblende, grains of magnetite, and quartz and flakes of biotite.
The groundmass is partly crystalline and partly glassy. The crystalline portion consists of grains of feldspar and of quartz, crystals of hornblende, and small flakes of brown mica. The grains of feldspar are rather rounded in shape, much decomposed, and many show undulose extinction. The decomposition is associated with the deposition of the fine dust above mentioned and with the formation of hornblende. Quartz is in rounded grains, ranging up to 0·08 mm. in diameter.
Chemical Composition.—It was found impossible to obtain an analysis of a true porphyry similar to that of the Tewaewae Point rock Comparisons with rocks from American and European localities are given
below. B is a type mineralogically similar to the Bluff porphyry; C and D are analyses of typical porphyries, one from “Analyses of Rocks,” U.S. Geol. Surv., the other from Rosenbusch.
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|Loss on ignition||0.15||0.42||0.90||0.83|
|Specific gravity, 2.5.|
A.Porphyry, Tewaewae Point, Bluff.
B.Porphyry, Missouri: Composed principally of orthoclase and quartz, with some microline, plagioclase, and biotite, and minor accessory minerals. (“Analyses of Rocks,” U.S. Geol. Surv., F. W. Clarke, 1904.)
C.Porphyry, Mount Zion. Contains orthoclase, plagioclase, quartz, biotite, apatite, magnetite, and zircon. (Anal., L. G. Eakins.)
D.Alkaligranitporphyr mit Einsprengl. von Orthoklas und Oligoklas; grundmasse wesentlich Quartz und Anorthoklas. (Rosenbusch, H., “Elemente der Gesteinslehre,” 1901, p. 205.)
A study of these analyses shows that compared with typical porphyries the Bluff type is relatively rich in the oxides of the bases calcium, magnesium, and iron, and correspondingly poor in silica, while the proportion of alumina and alkalies is about normal. Further, it compares favourably with the analyses of the quartz-porphyrites except as regards the percentage of alumina. In other words, the rock is mineralogically a porphyry, but chemically it tends towards the porphyrites.
We conclude, therefore, that from the evidence of both chemical and mineralogical composition the rock is a porphyry which has been enriched with the bases calcium, magnesium, and iron.
Further considerations are necessary before the method of this enrichment can be studied. (See p. 334, “The Origin of the Hornblende Schists.”)
Division B.—the Metamorphic Hooks.
These are found along the southern shore of the harbour. The outcrop is exposed between the levels of high and low water. Above highwater mark, as has been pointed out above, is a plain of marine denudation covered now by recent alluvium. On the other hand, the lowest ebb of the tide fails to disclose the limits of the outcrop. The main outcrop begins at Henderson Street, and strikes 15° S. of E. for a distance of 15 chains. The strike then varies to E.S.E., and continues so for other 22 chains, when a southerly bend of the coast cuts off the outcrop.
The rocks are traversed by numerous nearly vertical foliation-planes, which divide them up into layers of varying thickness. The joints have been mistaken for bedding-planes by previous investigators, who have recorded a dip varying from 84° to vertical.
There is another outcrop of the rocks further up the harbour, at Green Point, but here they are less metamorphic. They strike west-north-west to east-south-east.
The rocks are readily divided into two main kinds. One is a coarse dark-coloured rock consisting, as may be seen in the hand-specimen, almost entirely of hornblende. It forms the basic class of this paper. In the other rocks hornblende is also apparent, but no well-formed crystals may been seen in hand-specimens, as the rocks are much finer grained and more schistose in character. They form the acid class in this paper.
Class I.—Acid Metamorphic Rocks.
These rocks vary much macroscopically in the amount of hornblende and biotite, but under the microscope all prove to be varieties of hornblende scist.
1. Hornblende Schist.
Two complete series of sections were made from varieties of rock obtained by crossing the strike at right angles. Series A was obtained along the line marked AB on the map (fig. 2). Series B was obtained along the line CD.
Notes of the microscopical examination of series A are appended.
A 1. —Low-water mark, 157 ft. from high-water mark. Feldspar phenocrysts fairly abundant; cloudy and decomposed; some contain needles of hornblende. Hornblende in small crystals; especially numerous in the vicinity of feldspars; parallel arrangement. A little biotite. Grains of quartz, magnetite, and feldspar form groundmass.
A 2.—97 ft. Much the same as A 1.
A 3.—67 ft. Phenocrysts of feldspar, some showing Carlsbad twinning, decomposing as in A 1 and A 2. Increase of hornblende relative to feldspar compared with A 1 and A 2. A little epidote. Groundmass as before.
A 4.—37 ft. Hornblende still more prominent. Feldspar phenocrysts much smaller and more decomposed. Schistose structure marked. Magnetite abundant in groundmass.
A 5.—7 ft. Hornblende dominant; longitudinal axes of crystals parallel. No phenocrysts of feldspar. Magnetite abundant. A little epidote. Groundmass grains of feldspar and quartz, feldspar predominant.
A 6.—High-water mark. Rock chiefly hornblende. Schistose structure perfectly shown. One section showed remains of a feldspar phenocryst. Grains of feldspar, quartz, and magnetite, and other accessories between crystals of hornblende.
As will be seen from the map, the B series is actually a continuation of the A series. It is not necessary to describe the rocks in detail. They are perfectly schistose in structure. Half the rock is hornblende. There are no phenocrysts of feldspar, but the grains of quartz and feldspar in the groundmass are clear, as though due to recrystallization. Biotite is more abundant.
Summing up the results of the examination of this series of rocks, we find that—(1) the phenocrysts of feldspar are more and more broken
down the nearer they are to the plutonic mass; (2) the decomposition of the feldspar phenocrysts corresponds to an increase in the amount of hornblende in the rock.
These facts will be made use of when we discuss the origin of the hornblende schists (p. 334).
Chemical Composition.—A sample of specimen No. 4, series A, was submitted to chemical analysis, with the result given in the following table. This analysis probably represents the average composition of the schists, though microscopical examination of sections leads one to expect more acid results in the case of the outer members of the series, and more basic results in the case of those nearer the norite.
|Loss on ignition||1.20|
|Specific gravity, 2.56.|
Class II.—Basic Metamorphic Rocks.
Parallel with the series of hornblende schists just described is a band of coarse hornblende rock. The outcrop commences at a point 330 ft. from the shore-line measured along the line CD on the map, and extends below the level of low water a distance of 27 ft.
Hand-specimen (specific gravity = 2.94).—A coarse-grained black or dark-green rock. The weathered surface is rough on account of the exposure of large crystals of hornblende. A freshly broken specimen shows the bright cleavage surfaces of the hornblende. The rock appears to be almost wholly crystalline, there being but a small quantity of a dark-coloured matrix. There is no appearance of schistosity.
Under the Microscope.—The rock contains a very little feldspar in small grains in granular masses of dark hornblende. The rest is hornblende, a pale watery-green variety, feebly pleochroic, and fibrous in structure, all of which characters identify it as the form known as uralite. Where the fibres of uralite are packed together into large groups it is easy to recognize some of the edges of former crystals of pyroxene, but more commonly the fibres have broken away from the mass, inducing a schistose character.
That uralitization has taken place in the amphibolite can be proved in a most satisfactory manner. At Green Point several dykes of a diabase, a rock mineralogically and chemically similar to but less metamorphic than this amphibolite, are found, striking north-west to southeast. One of the dykes has suffered to a very considerable extent from the effects of weathering. In consequence of this, crystals are found to
project from the weathered surface, and can readily be removed from the soft matter that encloses them. These crystals exhibit to perfection the form of augite, but when sections of some of them were made they all proved to consist of a core of colourless augite surrounded by a margin of uralite.
These facts are of great significance. If the strike of these dykes be continued it is found that they may be expected to appear as outer members of the hornblende-schist series lower down the harbour. There is, therefore, no doubt that the amphibolite is a continuation of these dykes, but that, being in closer proximity to the norite, it has suffered metamorphism and uralitization to a greater degree.
No lengthy explanation of the name applied to this rock need be offered, as all authorities use the term for rocks “more or less markedly schistose in which hornblende is the dominant mineral.”
The chemical composition is given below:—
|Loss on ignition||1.00|
|Specific gravity, 2.94.|
The Contact of the Amphibolite and the Hornblende Schists.—At the line of junction these two rocks are more easily recognized in hand-specimens than they are under the microscope, for the causes that produced the metamorphism in both tended to bring about an exchange of material between the two. Thus the amphibolite is richer in feldspar where it is in contact with the schists, and the schists are relatively enriched with hornblende.
The Process of Uralitization.
So far as I can ascertain, no writer has yet put forward an exact definition of uralite, and there does not seem to be a consensus of opinion as to what varieties of secondary hornblende are covered by the term. For instance, Harker,* in discussing the decomposition of augite, says, “Another common alteration is the conversion to hornblende, which may be light green and fibrous (uralite) or deep brown and compact.” On the other hand, Williams † mentions the fact that the uralite fringing the pyroxenes “exhibits a marked tendency to become compact along its outer edge.” Again, Geikie ‡ terms uralitization “the conversion of pyroxene into compact or fibrous hornblende.”
[Footnote] * Harker, A., “Petrology for Students,” 4th ed. (1908), p. 79.
[Footnote] † Harker, A., “Petrology for Students,” 4th ed. (1908), p. 79.
[Footnote] ‡ Geikie, “Text-book of Geology,” vol. 2, 4th ed. (1903), p. 790.
Nevertheless, since perhaps the majority of authorities give prominence in their definition of this mineral to a fibrous or acicular structure, this distinction will be observed in the present paper, and the definition adopted here is as follows: Uralite, pale-green slightly pleochroic fibrous variety of hornblende, derived from pyroxene. The hornblende of the norite, therefore, though of secondary origin, is not in this paper referred to as uralite, for it is a compact variety, rather dark in colour, and strongly pleochroic.
Uralite was first described by Gustav Rose from a green porphyritic rock at Mostovaya, near Ekaterinburg, and at Kaminskaya, near Miask, in the Ural Mountains. It has since been observed from many localities. The microscopical study of rocks has shown the process of “uralitization” to be very common, and some authors regard many hornblendic rocks and schists to represent altered pyroxene rocks on a large scale.
The crystals obtained from the dyke at Green Point afford abundant and excellent material for the investigation of the changes that take place when augite is converted into uralite. These crystals are usually short and stout, and show an equal development of the unit prisms (110), the orthopinacoids (100), and the clinopinacoids (010), while the usual terminal faces, the plus pyramids (111), are also perfectly formed. Twinned forms, with the orthopinacoid (100) as twinning and composition plane, are also quite common.
The results obtained from an examination of sections of some of these crystals cut in various directions will now be given.
The Core of Augite: The internal core of augite is colourless, except where recrystallization has commenced.
Cleavage: The usual cleavage-lines are not very distinct in sections in the zone of the prisms, though they are seen well enough in cross-sections. What is very distinct in sections parallel to the ortho- and clino-pinacoids is a series of parallel lines which intersect the cleavage-lines at angles approximating 70° in sections parallel to the clinopinacoid, and at right angles in sections parallel to the orthopinacoid. These lines thus represent a series of parting-planes parallel to the base (001), a not uncommon feature in augite.
Refractive Index: A rough surface in polarized light indicates the usual high value.
Pleochroism: Not noticeable.
Interference colours: Bright tints of second order.
Extinction: In sections || a (100) = 37°.
b (010) = 0°.
Alteration - products within the Mineral. —These are feldspar and hornblende in about equal amount and a little olívine. The decomposition begins at points on the cleavage-lines and proceeds most rapidly in the direction of them. The hornblende is dark green and strongly pleochroic. It extinguishes when the cleavage-lines of the augite are parallel to the vibration-directions of the nicols. The feldspar extinguishes at small angles. Olivine occurs in small grains; it is very rare. There is no trace of calcite, epidote, or chlorite, minerals that are commonly reported as associated with such changes as are here described. The fact that hornblende and feldspar are always associated as decomposition-products in the interior of the crystals suggests that the material derived from the decomposing augite is divided between
them, the calcium and magnesium going towards the formation of the one, the alkalies and alumina to the other mineral.
The internal decomposition of the augite seems in no way associated with the formation of uralite on its margins. The border of uralite is quite distinct, and shows no gradation towards the products of decomposition in the interior of the crystals.
The Fringe of Uralite.—Dana says in his “System of Mineralogy,” “The crystals, when distinct, retain the form of the original mineral, but have the cleavage of amphibole. The change usually commences on the surface, transforming the outer layer into an aggregation of slender amphibole prisms, parallel in position to each other and to the parent pyroxene. When the change is complete the entire crystal is made up of a bundle of amphibole needles or fibres.”
Present Observations.—The fringe of uralite varies in width with the size of the crystal, indicating that the amount of change varies as the surface exposed. Usually, however, it is noticed that the change has taken place more rapidly in the direction of the vertical axis than in the other directions, for the fringe bordering the terminations of the crystals is wider than that bordering prismatic faces.
Statements about the parallel arrangement of the prisms of uralite do not find support in an examination of the sections of the Green Point crystals, for the fibres are seen to be arranged in radiating groups which show no signs of systematic arrangement. Between crossed nicols some of these groups, or parts of the groups, are extinguished, while other groups are not. Each fibre extinguishes at an angle that varies from 15° to 18° to the direction of the longest axis, so that a dark wave traverses the group as the nicols are rotated.
In sections parallel to the clinopinacoid the groups commonly make an angle of 45° with the edge of the crystal, measured either in a + or - direction. In sections parallel to the macropinacoid and to the base these groups are commonly parallel, and the fibres show straight extinction. This seems to indicate that the fibres are arranged in fan-shaped aggregates parallel to the clinopinacoid, and making angles of about 45° with the macropinacoid.
Rosenbusch * states that the fibres are parallel, and that the vertical axis is the same in the parent mineral as in the new one. Also, that in the case of a twinned crystal the fibres of uralite stand in twinned position on opposite sides of the twinning-plane. With regard to this latter statement, an occurrence in one section shows quite a different state of affairs. The twinning-plane is distinct enough in the augite, but disappears completely on the verge of the uralite fringe.
Chemical Changes.—So far as present knowledge goes, the composition of uralite is believed to conform nearly to that of actinolite. The most prominent change in passing from the original pyroxene is that corresponding to the difference existing between the two species in general—that is, an increase in the amount of magnesia and a decrease in that of calcium. Analyses of the Bluff minerals are compared below with results given in Dana's “System of Mineralogy.” In the absence of other means of separating the materials the following process was
[Footnote] * Rosenbusch-Iddings, “Microscopical Physiography of the Rock-making Minerals,” 4th ed. (1900), p. 271.
resorted to: Having made a sufficient number of sections to ascertain the thickness of the covering of uralite, crystals were ground down on all faces to remove this portion. The remainder provided material for an analysis of the core of augite. From other crystals thin flakes were cleaved, and the analysis of these was taken as representing the composition of the uralite.
|Loss on ignition||0.36||2.50||1.44||2.40|
A.Augite: From dyke of amphibolite, Bluff.
B.Uralite. Forming exterior of A.
C.Central portion of pyroxene crystal, Templeton, Quebec. (Anal., Harrington, Geol. Canada, p. 21, 1879.)
D.Amphibole forming exterior of C. (Anal., Harrington, Geol. Canada. p. 21, 1879.)
These analyses emphasize the change in the relative amounts of magnesia and calcium. There is also in the case of the uralite a rise in the percentage of alumina and iron-oxides corresponding to a fall in the total percentage of magnesia and calcium. This is what we might expect in the case of a mineral derived from another mineral by hydro-chemical processes. The process of uralitization is commonly reported to be accompanied by the separation of calcite and by the formation of epidote.* In the case of the Green Point minerals the augite undoubtedly loses calcium, but neither calcite nor epidote are seen as decomposition-products.
Both, sets of analyses emphasize the fact that the change of augite to uralite is not strictly a case of paramorphism, though usually so designated.
The causes that led to the production of uralite are discussed later under the heading “The Origin of the Amphibolite.”
III. Relationship between the various Rock Types.
It has already been stated that there is a close relationship existing between the porphyry and the hornblende-schist series. The relationship between the amphibolite and the hornblende schists also requires explanation, and it must also be shown what part the intrusion of the norite
[Footnote] * See, e.g., “Microscopical Physiography of the Rock-making Minerals,” Rosenbusch-Iddings, 4th ed. (1900), p. 271.
has taken in producing or altering the various rock types. Perhaps the best way of opening up these questions will be to discuss independently the origin of the metamorphic rocks, and a subsequent paragraph will deal with the relative age of all the rock types.
An attempt will now be made to deal with these problems.
A. Origin of the Hornblende Schists.
To an observer traversing the schist area described above, the solution of the problem seems evident. The rocks are hornblende schists, apparently well bedded, and inclined at various high angles. Intruded into them is a mass of igneous rock. The suggestion at once occurs that the rocks are the result of the metamorphism of a series of sediments, produced by the igneous intrustion.
Hutton* first put forward this view in 1872, referring to the rocks as slates and sandstones, some argillaceous and some arenaceous.
Park † made similar statements in 1887, and added, “Tewaewae Point, on the mainland opposite to the Pilot-station, appears also to be formed of sedimentary rocks, but I had no opportunity of determining this.” This is the view at present held as to the origin of the schists.
A visit to Tewaewae Point, however, and an examination of the rocks that actually do occur there, at once raises grave doubts as to the correctness of this view. For there we find not a sandstone, but a typical porphyry. Microscopical examination shows evidence of strain in the undulose extinction of some of the feldspars, and metamorphism is indicated in other ways. The feldspar phenocrysts are cloudy, they have irregular outlines, and new minerals are closely associated with their decomposition. The chief of these are hornblende, mica, and iron-ores. The rocks are traversed by joints striking in the same direction as those found in the schists. The outcrop disappears below low-water mark, and presumably reappears on the other side of the harbour. At any rate, I have the assurance of the Engineer of the Bluff Harbour Board that rocks outcrop in situ right across the channel.
Microscopical examination of the members of the schist series shows that the outermost members contain comparatively large phenocrysts of feldspar. These are much broken down, and are quite surrounded by microlites and crystals of hornblende. Associated with this is the separation of iron-ores, especially magnetite. As the norite is approached these residual feldspars are found to decrease in size, until finally with the innermost series they disappear completely. This gradual disappearance of the feldspars is found to correspond to a gradual increase in the amount of ferro-magnesian minerals, especially hornblende and iron-ores.
The groundmass of the porphyry consists of feldspar and quartz, and small flakes of hornblende and brown mica. The groundmass of the schist is essentially similar. The feldspar grains have been considerably comminuted, while an opposite process has taken place in the case of the hornblende and mica. The schists also contain secondary minerals, such as apatite and epidote.
[Footnote] * Hutton, F. W., “Report on Geology of Southland,” Rep. N.Z. Geol. Surv., 1871–72, p. 89.
[Footnote] † Hutton, F. W., “Report on Geology of Southland,” Rep. N.Z. Geol. Surv., 1871–72, p. 89.
The analyses of the porphyry, the norite, and an intermediate member of the schist series are repeated here for comparison.
|Loss on ignition||0.15||1.20||1.00|
These considerations—namely, the field relations of the rocks and their mineralogical and chemical compositions—lead to the conclusion that the hornblende schists are derived from the porphyry by metamorphism induced by the intrusion of the norite. The porphyry has become sheared by enormous pressure, so that it has become foliated, and its original character is masked.
The thermal metamorphism of igneous rocks has received comparatively little attention, and geological literature available to me presents no comparisons with the area to which this paper refers, and gives no description of the chemical changes that take place in similar circumstances.
In the case under consideration the principal changes to be accounted for are the destruction of the phenocrysts of feldspar in the porphyry, the devitrification of the glassy portions of its groundmass, and the great increase in the amount of the ferro-magnesian constituents.
The first two points can be explained by the ordinary processes of hydro-, thermo-, and dynamo-metamorphism, all of which would be active at the time of the intrusion of the norite. The water would be partly magmatic and partly meteoric.
The third point, however, involves the supply of large quantities of calcium, magnesium, and iron for the formation of the ferro-magnesian minerals, for the supply in the original porphyry was by no means sufficient, and, in any case, the chemical analyses show that a large quantity has been introduced.
The norite we may at once presume was the store from which the supply of these elements was derived, for the norite magma is very rich in them.
Transference of Material from the Norite to the Porphyry.—Percolating water is universally recognized as a most potent agent, especially at such high temperatures as would obtain in the case of a plutonic intrusion. The small percentage of water in schists—1 per cent. by weight or 2 per cent. by volume—is held to be sufficient to account for all the recrystallization that has taken place in rocks that are completely metamorphosed. The solubility of minerals increases greatly when they are in a state of strain. All these facts are well attested.
All the conditions for solution and recrystallization were abundantly present in the case under consideration. The intrusion of the norite produced a high state of strain in the porphyry. There is still evidence of this in the strain shadows observable in the feldspar phenocrysts at Tewaewae Point, and the strain must have been much greater in rocks nearer the intrusion. This is shown in the schists by the number of shearing-planes, often but a few inches apart.
The temperature of the intruded mass must have been very high, and the cooling must have been prolonged, for the norite is holocrystalline and of coarse grain.
Water would be present in sufficient amount, as percolated meteoric water in the porphyry perhaps, but more probably the supply would be the magmatic waters from the norite itself.
B. The Origin of the Amphibolite.
Of amphibolites Harker* says, “The name ‘amphibolite’ has often been applied to rocks, usually more or less markedly schistose, in which hornblende is the dominant mineral. Many of them are doubtless the results of dynamic action on diorites, and sometimes on dolerites and gabbros.”
Teall† describes the formation of a hornblende schist from a dolerite (or diabase) from two dykes which occur in the Archaean gneiss of the north-west of Scotland, near the Village of Scourie.
A comparison of his analyses with that of the Bluff amphibolite is instructive:—
A.Dolerite (diabase?), Scourie, north-west Scotland.
B.Hornblende schist derived from A.
C.Amphibolite, dyke, Bluff, derived from diabase.
Teall's conclusions are: “(1) That the hornblende schist of the Scourie dykes has been developed from a dolerite by causes operating after the consolidation of the dolerite, and that the metamorphosis has
[Footnote] * Harker, A., “Petrology for Students,” 4th ed. (1908), p. 326.
[Footnote] † Teall, J. J. H., “On the Metamorphosis of Dolerite into Hornblende Schist,” Quart. Journ. Geol. Soc., vol. 41 (1885), p. 142.
been accompanied by a molecular rearrangement of the augite and feldspar; and (2) that the molecular rearrangement has in certain cases taken place without the development of foliation.”
Other cases of the formation of hornblende schist from igneous rock have been described by Allport,* who, in his summary, expresses the opinion that “hornblende schists may be metamorphosed igneous rocks, some being derived from dolerites or gabbros, while others are very probably foliated diorites.”
These considerations, in conjunction with the chemical and mineralogical composition and the structure of the rock, suggest that our amphibolite is derived from the metamorphism of a basic igneous rock. Furthermore, at Green Point there actually does occur a basic igneous dyke rock consisting chiefly of augite in process of uralitization, and striking in a direction such as to indicate its identity with the amphibolite of the Lower Harbour series.
There yet remains to be shown the causes that produced the change to amphibolite.
Williams† points out that augite appears to be the stable form at high temperatures and hornblende at low temperatures. The change, therefore, must have been subsequent to the consolidation of the dykes, for at the time of intrusion the temperature would have been too high to admit of the formation of hornblende.
We have, therefore, to supply some conditions such as would facilitate molecular readjustment in the augite crystals after consolidation. Such conditions would certainly attend the intrusion of the norite. We need not assume, however, that the intrusion of the plutonic mass at once produced uralitization of the augite. On the contrary, the heat attending the intrusion may have been so high as to prohibit the formation of hornblende. The important point is that a state of strain was induced throughout the whole intruded mass. Evidence in support of this statement has been given above. This condition of strain would continue to exist after the consolidation of the norite, and when the temperature had again fallen to normal. Then would commence the process of uralitization, and it would be assisted by the percolating waters that aided in the metamorphism of the porphyry.
The conclusions here are, therefore, similar to those of Teall in the case of the Scourie dykes. The amphibolite is derived from a diabasic rock by metamorphism that most probably acted after the consolidation of the diabase, and was accompanied by a molecular readjustment of the augite.
C. Relative Age of the Rocks.
The porphyry and the rocks derived from it—that is, the hornblende schists—are the oldest rocks, for into them the other rocks have been intruded.
Of the intrusive rocks, we assume that the diabasic dykes are older than the norite, for the diabase is metamorphosed to an amphibolite, and the metamorphism is presumably connected with the intrusion of the norite.
[Footnote] * Allport, “On Metamorphic Rocks surrounding the Land's End Mass of Granite,” Quart. Journ. Geol. Soc., vol. 32 (1876), p. 407.
[Footnote] † Williams, G. H., Am. Journ. Sci., vol. 28 (1884), p. 259.
IV.Age of the Rocks.
A. Age of the Metamorphic Rocks.
Hector early classed the hornblende schists in his Te Anau series on account of their lithological resemblance to rocks of the typical area. He referred the Te Anau series to the Devonian period, because in Nelson Province the rocks were thought by him to underlie the Maitai slates, which were classed as of Carboniferous age.
Hutton,* in 1875, placed the rocks in his Kaikoura formation, corresponding to the Te Anau series of Hector. Of the age of the Kaikoura formation he says, “As it underlies quite unconformably the Maitai formation, which is of Lower Jurassic or Triassic age, we may consider it for the present as belonging to the Carboniferous period.”
In 1877 Hector† placed the Te Anau series in the Maitai system, to which he now ascribed a Triassic age. Subsequently, however, he gave up this correlation, and the Maitai system was referred back to the Carboniferous age.
In 1885 Hutton ‡ gave up his name of Kaikoura formation in favour of the nomenclature of Hector. At the same time he adopted the correlation of the Geological Survey for the Maitai system.
In 1877 Park § reported on the Bluff Peninsula at the instance of the Geological Survey Department. He says, “There is only one sedimentary formation represented in this area, and, although it contains no fossil remains, it is referred to the Te Anau series, to which the mineral character of its rocks have some resemblance.”
In his latest work Park ∥ refers to these schists as argillites. In one place (p. 42) he says they are “of the Wangapeka formation (Manapouri system, Silurian age)”; in another place (p. 46) they are “argillites that belong to Kakanuian or Middle series (Ordovician age) of the Manapouri system.”
Present Conclusions.—In the entire absence of palaeontological and stratigraphical evidence we have to rely solely on lithological evidence. Previous investigators have apparently failed to recognize the extremely metamorphic state of the schists, and have assigned to them a correlation that their original nature does not justify.
The dykes of diabase at Green Point (amphibolite in the schist series), however, are rocks similar to those of the Te Anau series—namely, greenstones, aphanite, breccias, or greenstone breccias in the Te Anau Wakatipu area, and diabase and diabase breccias in the Nelson District. In the absence of other evidence, therefore, we shall place the basic dykes in the Te Anau series of the Maitai system. The porphyry, therefore, and the hornblende schists will be somewhat older than the basic dykes, but there is at present no reason to remove them altogether from the same series.
B. Age of the Intrusive Rocks.
The evidence for the age of the plutonic rock is even more scanty. Park, in 1887, thought the mass was of late Carboniferous age, for he
[Footnote] * Hutton, F. W., “Geology of Otago “(1875), p. 36.
[Footnote] † Hector, Rep. N.Z. Geol. Surv., 1877.
[Footnote] ‡ Hutton, F. W., “Sketch of the Geology of New Zealand,” Quart. Journ. Geol Soc., vol. 41, p. 191 et seq. (1885).
[Footnote] § Park, J., “The Geology of Bluff Peninsula,” Rep. N.Z. Geol. Surv., 1887–88, p. 72.
[Footnote] ∥ Park, J., “Geology of New Zealand,” 1910.
mistook the rock for syenite, boulders of which were thought to be found in the Hokonui Hills, of Permian age. It is now known that there is no rock resembling the norite in the Hokonui conglomerates.
In his “Geology of New Zealand,” recently published, Park makes no definite statement of the age of the norite intrusion, beyond discussing it under his Manapouri system, which includes series of Cambrian, Ordovician, and Silurian age.
There is, in fact, no evidence that accurately fixes the age of this intrusion. We know definitely that it is younger than the intruded rocks—that is, late or post Carboniferous. Very probably the intrusion is connected with the widespread elevation that in Jurassic times enlarged New Zealand to continental dimensions. This movement resulted in rockfolding, and the main mountain-ranges were formed. The folding was associated with the intrusion of igneous rocks in various localities.