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

The Gneiss Granite Formation

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.

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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