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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

Chapter IV

page 252

Chapter IV.

The Gneiss Granite Formation.

Proceeding now to give a short description o£ the several formations according to their age, the first claiming our attention is the crystalline metamorphic formation, which, from the lowest visible rock, I have named the " Gneiss-granite formation"—(No. 12 in the Geological Map). The greater portion of this formation is confined to "West-land, and has been traced by me as far south as the Arawata river,* without break, in a north-easterly direction to the eastern side of Lake Brunner, where it enters the Nelson province. It consists of gneiss-granite, always the lowest bed when visible, syenitic gneissgranite, laminated and protogene gneiss, mica schists in many varieties, the latter often inter-stratified with gneissic schists. The mica schist zone forms the central portion of the whole formation, where it often assumes the character of graphite and talcose schists. It is followed or overlaid by chlorite and hornblende schists, which may be said to form the third zone, but here also mica schists are often inter-bedded. The whole is capped by a quartzitic zone, divided into many beds by the occasional occurrence of metamorphic schists, but they are not so much altered, being merely micaceous, or chloritic schists. Of accessory minerals, the gneissic schists often contain cyanite or disthene; the mica schists, chiastolite, actinolite and garnets; and the chlorite schists, magnetic iron ore, often in large quantities. From the latter, the black iron sands, found all along the coast and from which so much gold has been extracted, are doubtless derived.

page 253

The whole series stands at a very high angle, and almost invariably dips to the east. The thickness of these beds is enormous; in some sections they are at least six miles broad. However—as for instance in the fine sections observed along the banks of the Haast river—the dip to the east is not constant, western dips also occurring, which might either be caused by faults or reduplication made by folding of the strata Another point to consider is, if the foliation of the beds coincides with the original stratification. Though at present we do not possess the necessary data to settle this question definitely, there is great probability that this is the case, as the dip of the strata is generally towards the east, and at the same angle as the overlying formation which has not undergone so much metamorphic action. The boundary between this zone and the next is only, as far as my observations go, a conventional one, the strike and dip of both being generally the same, the division having only been suggested by the lithological character of the next series, which is less metamorphic. However, I have no doubt that breaks and unconformities between them exist in many localities; these, however, in the short time at my disposal when crossing the Central Range, I was unable to trace.

* From information kindly furnished by Mr. G. Mueller, the Chief Surveyor of Westland, and received since the Geological Map was printed, it appears that this belt, or at least a zone of igneous rocks, continues still further south, having been traced by Mr. Macfarlane, he Government Agent of the special settlement of Jackson's Bay. It strikes in the direction of the McKerrow mountains

Igneous Eruptive Rocks.

In the lower portion of this zone, on the western declivities of the Central Range, we first find a series of granite veins and dykes of which the oldest were doubtless formed when the crystalline metamorphic formation was uplifted and brought into its present position, the granitic axis to the west being doubtless the main nucleus from which these veins were ejected. The granites are of a great diversity of texture and variety of composition, younger dykes consisting of granites of a different texture, sometimes passing through older ones both being in some instances crossed by still younger greenstone dykes. However, in the upper portion of this extensive formation, I have not met with any eruptive rocks, either on the western slopes of the Southern Alps, or in the district around Lake Wanaka, of which I shall speak next.

Crystalline Metamorphic Formation East of the Central Chain.

Although in this zone the gneiss-granites and true gneiss schists-the lowest rocks visible on the West Coast—do not occur, the whole of page 254the rock approaches not only in character that of the upper beds of that series, but it is also overlaid in the same manner by the next or Waihao formation both east of the Central Chain, and on the West Coast. The rocks only fill a small corner in this Province, at the head of Lake Wanaka—the River Wilkin forming the northern and western boundary—but they are extensively developed in the Province of Otago, where they form a large belt about fifty miles broad, stretching in a north-north-east direction across that province to the Otago Peninsula. In this zone, named by Captain Hutton "the Wanaka formation," the richest and most extensive gold fields in New Zealand are situated.*

The beds of which this formation is composed consist of gneissic, mica, and chlorite schists more or less crystalline. They generally possess less inclination to the horizon than the same beds on the opposite slopes of the Central Range, their dip on the eastern and northern sides of the River Wilkin being on an average about 55 degrees, ranging from south-east to north-east; and on the western side, about 45 degrees, ranging from south to south-west. They thus form an anticlinal, being surrounded on all three sides by the same series of beds, which I have named the Waihao formation.

Near Lake Wanaka and along the upper course of the River Wilkin, the rocks consist of mica and chlorite schists; more towards the centre of the anticlinal, of quartzose gneiss and of a very crystalline mica schist, full of laminae or lentil-shaped grains of quartz, often of considerable size, which cause the rock, when broken vertically to the foliation, to show a peculiar face. These laminæ of quartz are not uncommon throughout the beds, and always give to the-schists an uneven appearance. Some of these mica schists are often much contorted, so as to suggest that they were very much pressed together, when in a soft or pasty state, by the quartzose or gneissic beds of a harder nature, between which they are enclosed. Corrugations on a large scale are also observable, having all the appearance of ripple marks, but they might, as Captain Hutton suggests, be explained by considering them due to the effects of expansion by heat on soft rocks under great vertical pressure, the compression thus produced having been relieved by numerous small corrugations, instead of by fewer and larger contortions—(Hutton and Ulrich's page 255Geology of Otago, page 30). Captain Hutton states, in the same publication, that nowhere in Otago a junction can be found between this formation, which he names the Wanaka formation, and his Manipori formation, extending all along the west coast of Otago, and as far east as the Te Anau and Maniporo lakes, but I have no doubt that his Wanaka formation is simply the upper portion of his Manipori formation, and that both ought to be united.

My own researches have shown that similar rocks to those found in the Lake Wanaka zone occur abundantly in the upper portion of my gneiss granite formation in many localities along the western slopes of the Southern Alps. Moreover, the absence of eruptive rocks, of which I could not find any trace in the Wanaka district examined by me, and which, according to Captain Hutton, are also wanting throughout the whole zone in Otago, is a further argument in favour of my view, as such rocks are also wanting in the upper portion of the gneiss granite formation in Westland.

I have already alluded to the fact that the greatest quantity of gold, both at the West Coast and in Otago, is derived from this formation, and I have no doubt that in many localities large lodes containing gold and other precious metals will be found. Nephrite, the ponamu or greenstone of the Maoris, is also found in this formation on the West Coast in Greenstone Creek, the Arahura, and some other localities; however, I have never observed it in situ, and I am therefore unable to say with what portion of the rocks forming this formation it is associated. The Canterbury Museum possesses, however, a specimen of nephrite, to which a small portion of the bed-rock, chlorite schist, is still attached.

* See Geological Map of Otago, in Hutton and Ulrich's Geology of Otago—Dunedin: Mills, Dick and Co. 1875.