Art. XI.—Some Alkaline and Nepheline Rocks from Westland.
[Read before the Otago Institute, 12th November, 1907.]
The rocks about to be described were obtained from the gravels of the watershed of the New River and its tributaries. They embrace a series of hypabyssal rocks ranging from acid granite porphyries to basic lamprophyres and gabbro diabases. Very few of the examples have been found in situ, but there is every reason to believe that the whole of the series were set free by erosion from the northern slopes of the Hohuna Range and from the adjoining Te Kinga Mountain. The humidity of the climate and a heavy rainfall has clothed the hillsides of Westland with a dense forest growth and a depth of humus matted with roots which effectually conceals the rock-surfaces. It is only above the bush-line, or upon the precipitous side of some deeply incised creek, or in some artificial cutting, that exposures of the underlying rocks occur. These limited exposures afford sufficient evidence to permit the rocks of the different formations to be classified and the areas of the formations to be defined. They, however, give few opportunities to examine or locate any dykes which may traverse these formations. It is,
therefore, only from the detrital rocks derived from the erosion of the now bush-covered mountains that a knowledge of many of the dyke rocks can be obtained.
So far as is known, the Arahura and Kanieri formations of the new geological survey are, with the exception of the Pounamu series, remarkably free from intrusions; but the Tuhua formation—an intrusive mass itself—is seamed in all directions with narrow dykes. Already in the Hokitika sheet of the new survey outcrops of the following dyke rocks have been located and the rocks described: Pyroxene and hornblende camptonites, pyroxene and hornblende porphyrites, diabases, and an augite diorite.
The Tuhua formation is a great granitic intrusion, with its major axis roughly parallel to the axis of the main range. As the flanks of the granite hills which expose this formation are in many places covered with detritus to a height of several hundred feet above sea-level, the outcrops are not continuous, but appear as huge bosses and isolated groups of hills. The Tuhua formation has been subjected to glaciation. The ice-sheet at the period of maximum extension flowed around and frequently over the summits of the granitic mountains. Enormous erosion resulted, and upon the retreat of the ice-sheet deep deposits of morainic matter covered the depression between the main alpine range and the granite range, and between the granite range and the ocean. The rivers emerging from the retreating ice-cap immediately began the reassortment of the glacial drift, and this work, with decreasing intensity, has continued on to the present time.
The rocks herein described were collected from the fluviatile gravels in the beds of the present streams and from the auriferous gravels deposited at higher levels by streams no longer existing. They were gathered from the beds of the New River and its tributaries on the right bank, and from the higher-level gravels between the New River and Arnold River basins, but not from the Arnold basin itself.
A rough estimate of the rock contents of the gravels was made in three or four localities; they contain about 80 per cent of greywackes, some 10 per cent. of slates, phyllites, argillites, quartzites, sandstones, and conglomerates from the Arahura and Kanieri series, and 10 per cent. of rocks from the Tuhua series. Roughly, perhaps about 1 per cent. of the Tuhua rocks may be classed as of hypabyssal origin, of which the larger proportion are much-decomposed feldspar-porphyrites, and the balance consist of the rocks herein described. Locally, these rocks are known to the miners as ironstones. They are recognised by their dark-green to black colour, and by their tendency
to weather in concentric layers. The shells of decomposed rock surrounding the boulder illustrated were more than 2 ft. in thickness. In some of the smaller boulders only a kernel of fresh rock remains (Plate XVII, fig. 1).
The basic and alkaline basic rocks collected have a wide range, and include an interesting series of tinguaites, tinguaiteporphyries, vogesites, camptonites, diabases, and rocks approaching monchiquites associated with theralites and gabbrodiabases. The latter may possibly be deep-seated representatives of the other rocks: but, until their plutonic or hypabyssal origin can be determined from outcrops in the field, they will be classed with the dyke rocks.
The numbers under which the rocks are described are the field numbers of the specimens as collected. Only those which represent the different types, or show transitional characters of an interesting nature, have been described.
The specimens collected numbered 131, from each of which one or more sections were prepared, and only in exceptional cases were any two rocks found to be exactly similar. They grade gradually from one type to another throughout the whole series. The suggested inference is that the whole series are genetically the product of one alkaline magma, which has undergone a gradual differentiation during the period in which the dykes were injected.
108. Tinguaite. Megascopically, a semi-translucent green rock, with vitreous fracture, resembling pitchstone. Microscopically, a network of ægerine crystals, with occasional phenocrysts of anorthoclase distributed in a groundmass of anorthoclase, cancrinite, and nepheline. The ægerine is brownishgreen in colour, and occurs in crystals of blade-like habit, sometimes frayed at the ends. It gives straight extinction and moderate pleochroism, dark-green for vibrations parallel to the longitudinal axis, and yellow-green perpendicular to it. It also occurs as needles and microlites, without any approach to orientation. It is idiomorphic to all other minerals, the terminal ends of the individuals sometimes penetrating the feldspar. The feldspar occurs in two generations, the earlier being idiomorphic. Rectangular phenocrysts are sparingly developed; crystals of long blade-like habit are frequent. Although these consolidated after the pyroxene, they are only occasionally penetrated by it, but appear to have pushed the ægerine aside, or to have developed alongside the already crystallized pyroxene. A few of the broader crystals show Carlsbad twinning. The groundmass of the rock is composed of cancrinite, nepheline, and anorthoclase in allotriomorphic relations; the small plates of anorthoclase with ragged outlines give undulatory extinction.
The cancrinite is recognised by its low refractive index and its interference colours of the first order. It is completely dissolved in 40 per cent. HCl, and gelatinises when heated. In some parts of the rock it shows tendencies to form plates of rectangular outline, but is generally shapeless. The nepheline follows the same habit, and is difficult to distinguish by optical means; but when the two minerals are dissolved out of a slice with dilute HCl, and only the pyroxene and feldspar left, the quantity of nepheline present can be estimated by comparison with an untreated slice. It is present in about equal quantities with the cancrinite. Iron-ores are absent. A minute quantity of isotropic matter is present, which may be analcite, but more probably nepheline cut parallel to the base. Omitting the rectangular anorthoclase phenocrysts, which occur sparingly, the proportions of the respective minerals are approximately: Pyroxene, 10 per cent.; feldspar, 60 per cent.; cancrinite, 15 per cent.; nepheline, 15 per cent. Owing to the elongated and partly panidiomorphic habits of the feldspars in the groundmass, the structure differs from those of described tinguaites. It approaches most nearly to the Norwegian tinguaites described by Brögger, but is coarser textured, both as regards the development of the feldspars and the ægerine. The latter frequently attains a length of 0.5 mm., and the feldspars 0.75 mm. The structure somewhat resembles that of a fine-textured holocrystalline dolerite, and this structure is maintained in the groundmass with few exceptions throughout the whole of this series of tinguaites, tinguaite-porphyries, and some of the vogesites.
72. A finer-textured tinguaite, without phenocrysts of anorthoclase; it also differs from the former in that it contains less cancrinite, but has, in addition to the pyroxene, some almost completely resorbed crystals of an amphibole, probably hornblende, the iron from the alteration of which is present as granular magnetite. This rock also shows a fluxional arrangement of the ægerine needles and feldspars, which are rudely orientated in the direction of movement: this is most noticeable around the skeletons of the amphiboles. (Plate XVIII, fig. 8.)
114. A coarse-textured tinguaite, containing numerous phenocrysts of anorthoclase, and more rarely of microcline with corroded boundaries. The ferro-magnesian contents are represented by serpentine after augite and small plates of hornblende. Some of the augite cores remain unaltered. The order of consolidation is reversed in this rock. The feldspar nepheline and cancrinite in the groundmass are allotriomorphic to each other, but idiomorphic towards the serpentine and hornblende, which they penetrate in crystals with well-defined boundaries. As
the minerals of the groundmass present the same relations to both the amphibole and the pyroxene, the amphibole may be a secondary mineral, and possibly the augite sometimes passed over to serpentine and sometimes to hornblende.
34. Has a tinguaitic groundmass typical of this series, consisting of nepheline, cancrinite, and feldspar—much of which is perthite—with ægerine microlites. With the exception of the ægerine, the minerals of the groundmass are allotriomorphic. The rock contains well-developed phenocrysts of hornblende, in long needles and as prisms with polygonal boundaries, and also augite in prisms. There are large phenocrysts of perthite with marginal corrosion and of anorthoclase undergoing alteration to muscovite. (Plate XVIII, fig. 7.)
107. Is a similar rock to 114, but contains some remarkable examples of leucocrasia. Megascopically the appearance of the rock is peculiar, the tinguaitic portion being dark-grey to black in colour, whilst the leucocratic patches are almost white. In the field the white portions were mistaken for parts of the intruded rock attached to or included in the intrusive, but a section cut across what appeared to be the junction shows that such is not the case. The white rock consists of a hypidiomorphic even-textured aggregate of anorthoclase, the plates frequently measuring 2 mm. in diameter, and enclosing ophitically but sparingly ragged crystals of green common hornblende. The larger phenocrysts of feldspar in the tinguaitic portion are undergoing alteration to muscovite. (Plate XIX, fig. 11.)
32. Tinguaite porphyry. Though the previously described rocks are porphyritic by a recurrence of the feldspars in two generations, this is the first example in which the recurrence takes place with the essential mineral, the nepheline. Without the nepheline phenocrysts the rock is similar in all respects to No. 114, including the serpentinous mineral after augite. The porphyritic nephelines form good hexagonal tables, 1 mm. to 1 ½ mm. in diameter. (Plate XVIII, fig. 6.)
With the disappearance of the nepheline and cancrinite the rocks pass into the family of the lamprophyres. Some of these have been classified as vogesites and others as camptonites. It will be noticed that those in which the dominant feldspar is anorthoclase have been classed as vogesites, which may not be strictly correct.
49. Vogesite. A groundmass of shapeless elongated and ragged crystals of anorthoclase, enclosing microlites, needles, lath-and blade-like crystals and polygonal tables of augite and hornblende. Many of the colourless augites and pale-brown hornblendes are surrounded by a resorption border of ægerine
or ægerine-augite in fine grains. This rock also exhibits the phenomena of leucocrasia. The patches consist of a hypidiomorphic aggregate of anorthoclase and a deep-green hornblende. In the normal rock the crystals of hornblende occasionally attain a length of 2cm. The pleohroism—(a) very pale-brown, (b) brown, (c) dark-brown—shows the hornblende to be barkevikite; but in the abnormal rock represented by the leucocratic patches the pleochroism of the hornblende is—(a) very pale-yellow, (b) olive-green, (c) yellowish-green—and the mineral is probably common hornblende.
101. Vogesite. The groundmass of this rock is largely composed of an alkali feldspar, with panidiomorphic development, associated with a small quantity of a colourless mineral with a higher refractive index and low polarization tints. A chemical test for mellilite was made, but did not result in the precipitation of any gypsum needles. The mineral is therefore not identified. The phenocrysts are colourles to pale-pink augites and pale-brown hornblendes, with perfect outlines, and exhibiting a great variety of sectional shapes. Both minerals occur in two generations. The augite is often surrounded by a thin green resorption boundary of crypto-crystalline ægerine or ægerine-augite. Some of the larger hornblendes attain a length of 4mm., and the pleochroism shows them to be barkevikite. (Plate XIX, fig. 12.)
95. A rock differing from the last described, in that the feldspar is distinctly determinable as anorthoclase, and occurs in smaller proportions. The augite in the groundmass is in shapeless colourless grains. Some of it is serpentinised, and the porphyritic augites are also attacked. Crystals of partially serpentinised olivine with a peripheral border of magnetite are not infrequent. The hornblendes, although not so prominent as in the former rock, posses good crystallographic outlines. (Plate XIX, fig. 14.)
116. This is an intermediate rock between the vogesites and the camptonites. It resembled No. 95, except that serpentinisation is more advanced, and plagioclase is present, but not to the same extent as the anorthoclase.
123. Is also an intermediate rock, the alkali and lime-soda feldspars being about equal in quantity. In addition to the hornblendes—which here assume a more tabular habit, with a tendency to lose their idiomorphism—there occurs an occasional plate of deep-brown biotite and a few large phenocrysts of perthite. The perthite is surrounded by a zone of partially untwinned plagioclase, and, in addition to simple Carlsbad twinning, some phenocrysts are Baveno twins. Pseudomorphs of serpentinous material after are present,
and a little ægerine more or less intergrown with the hornblende. The lime-soda feldspar is labradorite. A micro-chemical and staining test for nepheline was made without any results.
28, 51, 76. These are also fine-textured intermediate rocks, with very subordinate feldspar, and ultra-basic in composition. Microsocopically, they bear a strong resemblance to Brogger's farrisite, but a chemical test failed to discover the presence of mellilite. The feldspar is partly anorthoclase, and the balance a basic plagioclase with the extinction angle of bytownite. 28 contains pseudomorphs of serpentine after augite, with occasional cores of the original mineral remaining. The hornblende occurs in two generations, with sharply idiomorphic outlines. 51 has, in addition to a sprinkling of magnetite dust, some plates of pyrite, probably of secondary origin. In 76 the augites are fresh, but these are serpentinous pseudomorphs after olivine. Without the accessory feldspar the rocks would be monchiquites, which they most nearly resemble. (Plate XVIII, fig. 5.)
100. Camptonite. The hornblendes differ from those in the vogesites and some of the transitional rocks in that, although preserving the same sharpness of outline, they do not tend to assume the elongated and belemnitic forms. The augite is partly serpentinised, and is subordinate to the hornblende, which constitutes nearly one-half of the rock. Borders of ægerine surround some of the hornblende phenocrysts, and small independent crystals of ægerine are present. The feldspar is labradorite, with a maximum extinction-angle of 32o. (Plate XIX, fig. 13.)
40, 82, 84, 97. Diabase and olivine-diabase. In the group represented by these sections the hornblende has almost disappeared, and only occurs as small granules, whilst the crystallization of the feldspar has preceded that of the ferro-magnesian minerals in the groundmass. The feldspar labradorite is in columnar crystals, with an occasional rectangular plate, but without law of arrangement. The augite, partially serpentinised, is in small grains, without definite shape, and interstitial to the feldspar. In some of these rocks there is a recurrence of the augite, which appears as phenocrysts. Olivine is frequently present, partly serpentinised, but generally with good kernels of fresh mineral surrounded by serpentine, and this again by a thin margin of finely granular magnetite, which gives the outline of the original crystal, and shows that before alteration the olivine possessed good crystal forms. In some of the sections there is a little isotropic matter. These rocks are classified as diabase and olivine-diabase according to whether the magnesian mineral is absent or present; but some of them,
with the isotropic matter present, approach closely to the rocks named “teschenites” by Hohenegger.
57. A porphyritic diabase from the Te Kinga boss. Megascopically it appears to be a feldspar porphyry, but under the microscope the basic nature of the rock is apparent. The large feldspar phenocrysts are labradorite, with a maximum extinction-angle of 42°; they show incipient epidotization, and an alteration into muscovite. The groundmass consists of ragged crystals, needles, and microlites of a basic feldspar, with a tendency to crystallographic development, set in a micro-granular augite arranged interstitially to the feldspar. Iron-ore is plentifully distributed through the rock as dust, cubes, and longer rectangular sections; the two former are magnetite, but the longer sections may be ilmenite. Some of the augite is serpentinised.
91. A rock intermediate between the diabases and the basic lamprophyres. Here the feldspars have almost disappeared, but what there is still bear the same relations to the augite of the groundmass as in the diabases. Augite is the dominant mineral, and occurs in two generations, the intratelluric as plates idiomorphic towards the augites of the groundmass, but not possessing good crystal faces, and in the groundmass as granules. Anothe: form, as diopside in phenocrysts; is present with good outlines. There is a little hornblende, which appears to be a secondary mineral, as is also a clear serpentinous aggregate occasionally seen in small plates. A little secondary quartz fills cavities in the rock. The accessory feldspar is labradorite.
By the gradual assumption of the hypidiomorphic structure these rocks pass into theralites and gabbro-diabases. From what is known of the Hohuna Range, a large granitic boss clothed with a dense mass of vegetation up to the winter snow-line, and with few exposed rock-surfaces below that altitude, it is doubtful whether the whole of the rocks herein described do not come to the surface in the hypabyssal form. Although many sections have been made of the granitic rocks from this and the Te Kinga boss by the author and others, so far no nepheline syenites with which tinguaites, camptonites, and allied rocks are usually associated have been found. It is possible the gabbro-diabases may form a marginal apophysis of the granite mass. As boulders the gabbro-diabases are no more common in the fluviatile gravels of the New River system than the vogesites and camponites, and do not occur so frequently as the tinguaites. In the beds of the mountain torrents draining the northern slopes of the Hohunas they appear to be more common, but there is no evidence that they outcrop at the surface in
large masses, and the inference is they occur as dykes. The fact that some of the gabbros are theralites, and carry nepheline, points to the possible association of the already described hypabyssal types of rock with a gabbro-magma rather than with a syenitic one.
In the Te Kinga railway quarry, near the Rotomanu Station, one of the very few excavations made in the Tuhua rocks, in a face some two chains in length, two decomposed dykes are exposed, in addition to a mass of soft basic rock which never came to the surface, the exposed upper portion of which is surrounded by a halo of contact metamorphism from 2ft. to 3ft. in thickness separating it from the overlying granites. Sections made from this intrusion are described under No.55, but the specimens collected were not fresh enough to enable a complete description to be written. In the “No.1 Bulletin” of the New Zealand Geological Survey, Dr. Bell says, “The top of the Hohuna Range and its southern slopes are seamed in places with narrow basic dykes.” The northern slopes above the bush-line have not been examined lithologically, but shepherds who have traversed this precipitous country say that black seams—probably basic dykes—are frequent in the granites. From the evidence in the Te Kinga quarry and on the Hohuna Range it would seem that in places erosion has removed so much of the granite that only a shell now covers a basic igneous rock.
The specification of the questionably plutonic representatives of the series is,—
119. Theralite. In the gabbro-diabases it is impossible to determine the presence of nepheline with certainty by optical means, and micro-chemical tests only revealed its presence in one rock. Half a gramme of the powdered rock was dissolved in dilute Hci, and the solution slowly evaporated in a beaker. Before dryness was reached water was added, and evaporation continued to remove excess of chlorine. When saturation was nearly reached, evaporation was finished on glass slides, with the result that quantities of crystals of sodium-chloride were deposited. Staining only revealed the presence of a mineral which, on the application of hydrochloric acid, formed gelatinous silica. The presence of nepheline thus determined places the rock in the family of the little-known theralites. It consists of large twinned crystals of schillerised ægerine, with good crystallographic outlines, a basic plagioclase, a brown hornblende, and a little nepheline in panidiomorphic relations, together with a small quantity of interstitial allotriomorphic augite converted into serpentine. A few cubes and grains of magnetite are present. The feldspar is labradorite, and appears both in columnar crystals and as rectangular plates with zonary banding.
Some of the hornblendes are ophitically enclosed in the large ægerines (Plate XIX, fig. 9).
3. Olivine gabbro-diabase. Large olivines and pink augites are relatively common; there is a quantity of biotite and brown hornblende in small narrow crystals. The feldspar-labradorite occurs in large plates and in columnar crystals. It is idiomorphic towards both the olivine and the augite, but the latter are allotriomorphic to one another. Some large columnar crystals of labradorite are ophitically enclosed in the olivine. Frequently where the feldspar penetrates an olivine one corner of the feldspar crystal is rounded. The iron-ore, which is plentiful and assumes crystallographic forms, is titaniferous. The rock is quite fresh.
121. Another olivine-augite-feldspar rock, with very subordinate hornblende. It has a typically diabasic groundmass of augite and labradorite, enclosing large grains of olivine without definite shape and phenocrysts of labradorite. One large feldspar crystal ½in. in length has a narrow margin of clear labradorite; but the interior of the crystal consists of augite and feldspar in about equal proportions, together with a little ilmenite. The augite assumes the disposition of a graphic intergrowth; but, as the whole of it does not extinguish simultaneously, it is evidently an inclusion in the crystal. Grains of ilmenite are plentifully distributed throughout the rock.
99. Is an augite-feldspar rock, with very subordinate olivine. The phenocrysts are augite in conspicuous plates, with a tendency to idiomorphism and crystals of labradorite-feldspar changing to epidote and calcite. The groundmass consists of feldspar and augite in diabasic relations. The olivine is serpentinised into a clear pale-green mineral, with hardly perceptible dichroism and isotropic. Titaniferous iron-ore is common both as grains and with rectangular outline, and pyrite occurs in large crystals associated with a pale-green serpentine, which is birefringent in polarised light, shows no definite structure, but is rather a confused aggregate of fibres.
37. An olivine-augite-feldspar rock, contains the olivines in large shapeless grains, and has as phenocrysts large augits, perthites, and labradorites with idiomorphic outlines. The groundmass differs from any of the previously described gabbro-diabases in that it consists of a panidiomorphic aggregate of labradorite, partially serpentinised augite, hornblende, and ilmenite.
130. Another variety is interesting owing to the large development of the augite phenocrysts, some of which are more than ½in. in length. There are inclusions of brown hornblende in the augite crystals. The phenocrysts present good crystal
faces, and are set in a panidiomorphic groundmass of labradorite, serpentinised augite, hornblende, and iron-ore, much of which is titaniferous.
55. A coarse-grained basic rock from the Te Kinga quarry, where it occurs projecting some 15ft. up from the floor of the quarry into the solid granite, which covers it to a depth of 50ft. The exposed portion, some 12ft. high by 15ft. wide at the base, evidently increases in size in depth. The granites show no signs of having been displaced by the intrusion, but the intrusive is surrounded by an aureole of completely altered rock between 2ft. and 3ft. in thickness, the result of the metamorphism induced by the contact. So completely altered is this envelope that it is imposible to say whether it belongs to the intruded or the intrusive. Near the granite it consists of a granular aggregate of quartz and biotite, which becomes more quartzose close to the granite contact. Part of the mica is in long rectangular flakes, with longitudinal striation, which does not always extend from end to end of the crystal; the balance is in shapeless plates. Nearer the intrusive the rock consists of a colourless mica, with chlorite, some very subordinate biotite and quartz, and a little iron-ore, apparently magnetite. The micas and quartz are allotriomorphic; the chlorite appears in radiating sheaves, with a rude suggestion of flow structure, and the rock would certainly be classed as a schist if it occurred under any other conditions. The intrusive rock is soft and close to the contact-zone, partially decomposed, but much fresher samples can be secured from the centre of the mass. It consists almost exclusively of augite, hornblende, and calcite, with very subordinate olivine feldspar and iron-ores. Some of the calcite may represent feldspar, but much of it is of foreign origin, and occurs in cracks. The magnetite is present as dust, and as grains with crystal boundaries. The augite and hornblende have a strong tendency to crystallographic development, and are idiomorphic to any feldspar that occurs. What little feldspar there is is principally anorthoclase, and a few crystals of basic plagioclase, with high extinction-angles. Patches of serpentinous mineral are present, usually in contact with the calcite. It is birefringent, with lattice structure, but more probably resulted from the alteration of the olivine than hornblende.
In addition to the basic and basic-alkaline rocks already described, there is an acid-alkaline series, which possibly resulted from a variation of the acid granitic magma from which the granites of the Tuhua series were genetically derived. With our present knowledge of magmatic differentiation it is within the bounds of possibility, although hardly probable, that the whole of the described rocks, including the granites, are the
product of one differentiated magma, of which the granites represented the first phase. The differentiation would then appear to have been one of increasing alkalinity, followed by one of increasing basicity. The granitic magma was originally slightly alkaline, as microcline and microperthite are common accessory minerals throughout the intrusion, and become more important in the earlier variations represented by the aplites and the pegmatites. The next differentiation of which we have any evidence is that of a granite porphyry, a hypabyssal rock in which the alkalinity reached its highest point before it was masked by the increasing basicity, supposing the whole series to have the same genetic origin. Between the granite porphyry and any of the rocks already described there is a petrological gap both structural and mineralogical which may possibly be represented by the porphyries and porphyrites mentioned as occurring in the detrital rocks, but which are weathered too much for investigation. Chemically, the gap may not be so great as it appears. The author regrets that he had neither the time nor the means at his disposal to make a chemical analysis of the different rocks described, by which method alone could the extent of the apparent gap be determined. Whether further explorations in the field will throw more light on the subject remains to be seen. The granite porphyry is interesting in that it contains riebeckite, an amphibole the occurrence of which has never hitherto been recorded in New Zealand.
A description of one of the aplites and the porphyry is herewith appended.
115. Aplite. Megascopically, a fine-grained white rock resembling Carrara marble. Microscopically, that which for convenience of description may be called the groundmass exhibits granulitic structure, and consists essentially of roughly equidimensional grains of quartz, orthoclase, and microcline. In this mosaic, with a tendency to hypidiomorphic structure, occur somewhat larger grains of micrographic intergrowths of quartz and feldspar, microperthite, and microcline microperthite. A yet stronger development of microperthite is present in roughly rectangular plates without well-defined outlines, in which sometimes the mineral is twinned in accordance with the Baveno law. As accessory minerals, a brown biotite is sparingly distributed in small shapeless plates, and the iron-ores are represented by a little scattered magnetite dust and a few scales of hæmatite. (Plate XIX, fig.10.)
19. Riebeckite granite porphyry. A rock with a creamcoloured ground, with brown spots. A small lens reveals the spherulitic nature of the groundmass sufficiently to enable the rock to be classified as a granite porphyry in the field. Under
the microscope the groundmass is seen to consist of complete spherules and sectors of spherules, with the interstices between their peripheral boundaries filled in with micropegmatite and a quartz mosaic. Approximately the area covered by the spherulitic growths is about two-thirds that of the total groundmass. Some of the individual spherules attain a diameter of 3 mm. The peripheral boundaries of the best-developed spherules are not perfect circles. The final consolidation of the rock appears to have taken place before the development of the spherules was completed. In parts the spherules have impinged upon one another, and the boundaries of the two individuals are coterminous. Where this happened further development could not go on, and this seems also to have been the case where the space between the two individuals is filled with a fine granular mosaic of quartz. Here the growth of the spherule ceased when no more feldspar matter was available; but where the intervening space is filled with a micropegmatitic intergrowth the spherule has continued to advance, and at the final consolidation a portion of the micropegmatite arranged itself in delicate fern-like outgrowths around and continuous with the spherule. A radial growth corresponding to sectors of a spherule occur around the phenocrysts, and extend some distance into the groundmass. The spherules are composed of a pegmatitic intergrowth of quartz and orthoclase varying in texture from cryptopegmatitic* near the centre to micropegmatitic near the circumference. Arranged radially to the centre, and sometimes extending nearly the complete radius of the spherule, are needles, long blade-like crystals and peg-shaped inclusions of riebeckite. This mineral is present in larger crystals, with ragged outlines and no definite shape in the groundmass, between the spherules; and in many portions of the slices the larger crystals occur with their longer axis tangentially arranged around the outer edge of a spherule. Riebeckite also occurs as minute inclusions in the quartz and perthite phenocrysts. By transmitted light the colour of the mineral is indigo blue, and the pleochroism—(a) black, (b) dark-blue, (c) brownishyellow—appears to correspond most nearly with that of the riebeckite described by Le Verrier from Corsica; it also resembles that from Dongo Buro, described by Prior. The needlelike blades in the spherules attain a length of 1 mm. The phenocrysts are quartz and perthite. Some of the quartz crystals have sharp outlines and angles; in others the angles are rounded by corrosion. The quartz is relatively free from
[Footnote] * Perhaps this term is not strictly correct, as the structure can be resolved under a magnification of 400 diameters.
inclusions; and fluid-pores, so typical of the quartz of granite porphyries, are absent. A few minute glass inclusions without bubbles and the riebeckite inclusions already mentioned are all that can be made out. The perthite phenocrysts show less signs of corrosion and a greater variety of shape. Some are rectangular to square plates; others of elongated blade-like habit, occasionally reaching 7 mm. in length by 0.4 mm. in breadth. Those with elongated habit have undulose extinction. In many of the sections of the mineral both the component feldspars extinguish simultaneously; in others there is a distinct angular interval between extinction; and there is no crystalline continuity between the feldspars of the phenocrysts and the sectors of spherules radiating from them. In some of the perthites the feldspars are orthoclase and anorthoclase; in others the nature of the triclinic feldspar has not been determined. In some of the slices there is an occasional crystal of sphene. Omitting the rare accessory sphene and the iron-ores which may possibly be of secondary origin, the sequence of crystallization in the consolidation of the rock is: (1) riebeckite; (2) quartz and perthite phenocrysts; (3) spherulitic growths; (4) the quartz mosaic and micrographic intergrowths representing the balance of the groundmass. It is just within the bounds of possibility that the riebeckite may occur in two generations, that in the interstitial groundmass being the prior, and that in the spherules a subsequent crystallization contemporary with the growth of the spherules; but the evidence is strongly in favour of the amphibole having conformed to the normal order of consolidation, and only occurring in one generation. The development of the spherules appears to have proceeded in the following manner: Starting from a centre, the nature of which cannot be distinguished, the formation and consolidation of the cryptopegmatite proceeded outwards, the process of crystallization pushing the larger and broader plates of riebeckite before it, but turning the needle-like blades—which it ultimately enclosed—in the direction in which they offered the least resistance to the process—that is, radially. Needles of riebeckite which escaped this first capture were seldom included in the outer zone of the spherule. Any mass too large to be moved by the energy of crystallization was included in the radiating body of the spherule, the growth of which proceeded on the further side of the inclusion as if the obstacle did not exist (see Plate XVIII, fig. 4, where a grain of quartz is included between the centre and the periphery of a spherule). After the process had continued outward beyond the length of the included needles of riebeckite, the micrographic growth of the spherule is com-
paratively free from inclusions, and without any traces of secondary iron-ore dust, so that resorption of the amphibole in this zone did not take place. The outer zone of the spherule consists of a delicate fern-like growth of micropegmatite. The change from crypto-to micro-pegmatitic structure is gradual from the centre of the spherule to the circumference. The branching of the outward creeping fern-like growths did not generally occur until two or three fifths of the radius of the spherule had been attained. Optically the spherules extinguish in sectors as the stage or the nicols are revolved; in some the sectors are narrow, in others considerable areas are extinguished at the same time. (Plate XVIII, fig. 4, and Plate XVII, figs. 2 and 3.)
The sections of the rocks described have been compared with sections of rocks from the most notable alkaline petrological provinces of the world—viz., the Arkanas rocks, described by Williams; the Christiania rocks, described in Brögger's famous memoir; and the rocks of the Serro do Tingua, in Brazil, described by Hussack. For the loan of slides of these collections, and for his ready assistance and advice, I am deeply indebted to Dr. P. Marshall, of the Otago University
Fig. 1. A gabbro-diabase boulder. The rounded water-worn appearance is due to spheroidal weathering. A part of the shell-like concentric weathered casing can be seen adhering to the stone beneath the right-hand end. Eight-mile Creek Diggings.
Fig. 2. (No. 19.) Grain of quartz enclosed in spherule; ragged crystals of riebeckite surrounding periphery of spherule.
Fig. 3. (No. 19.) Showing fern-like growths in outer part of spherules and sectors of spherules.
Fig. 4. (No. 19.) Riebeckite in granite-porphyry.
Fig. 5. (No. 28.) Lamprophyre approaching monchiquite.
Fig. 6. (NO. 32.) Tinguaite-porphyry, showing large crystal of nepheline.
Fig. 7. (No. 34.) Tinguaite, containing ægerine-hornblende-augite, nepheline, cancrinite, perthite, and anorthoclase.
Fig. 8. (No. 72.) Tinguaite, containing ægerine, nepheline, cancrinite, and anorthoclase, with some resorbed amphibole.
Fig. 9. (No. 119.) Theralite. Large twinned schillerized crystal of ægerine occupies the left-hand half of figure; the balance is labradorite, hornblende, nepheline and augite with iron-ores.
Fig. 10. (No. 115.) Microperthite in aplite.
Fig. 11. (No. 107.) Tinguaite, showing junction of normal rock with leucocratic patch.
Fig. 12. (No. 101.) Vogesite. The phenocrysts are hornblende and augite, the latter having a border of ægerine set in a groundmass of which the larger portion is an alkaline feldspar.
Fig. 13. (No. 100.) Camptonito. Augite and hornblende phenocrysts set in a ground of labradorite.
Fig. 14. (No. 95.) Vogesite. Crystails of augite and hornblende set in a groundmass of anorthoclase, with a little augite.
Note.—All the sections are to a magnification of 26 diameters.