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Report on the Geology & Gold Fields of Otago

Mattai Formation. — Kaihiku Series, (Hector?); Wairoa series, (Hector); Richmond sandstone, (Hochstetter); Otapiri series, (Hector); Shaw s Bay series, (Lauder Lindsay)

Mattai Formation.
Kaihiku Series, (Hector?); [unclear: Wairoa] series, (Hector); Richmond sandstone, (Hochstetter); Otapiri series, (Hector); Shaw s Bay series, (Lauder Lindsay).

Distribution.—This and the next formation are only found filling the southern synclinal already described.|| Commencing north of Burwood, near the heads of the Mararoa and Oreti Rivers, it runs in a south-easterly direction; forming Coal Hill, Centre Hill, Mount Hamilton, the Wairaki Downs, the Moonlight Ranges, and the northern portion of the Hokanui Hills. Here it disappears under the Putataka formation, but reappears again in the Kaihiku

|| Ante p. 23, Fig. 1, C.

page 38Hills, running down to the Nuggets on the sea coast, and spreading north beyond the road from Balclutha to Clinton. Twinlaw and Bald Hill, lying north of the Longwood Range, also appear to belong to this formation.

Rocks.—The rocks are principally shaly slates, argillites, and green sandstones, with occasional beds of conglomerate and green-stone tuff. These conglomerates consist entirely of subangular fragments of slate and sandstone, and I have never found a single pebble of any eruptive rock in them.

Position of Strata.—Along its north-west margin, this formation is very much disturbed, being thrown into several bold plications. At Mount Hamilton, the dip is 40° to 65° E.; at Centre Hill, 50° W. by S.; and at Coal Hill, (a south-westerly spur from the Eyre Mountains,) it is 75° E. by N. Along the southern part of the Wairaki Hills, near the Lead of the Morely Creek, the dip is 20° N.E.; and at the gorge, where the river enters the plain, 15° S.W. At Castle Rock Station, on the north side of the Moonlight Range, the dip is 20° S.W.; while further south, near Dipton, it is 30° S.S.W. On the eastern side of the Oreti River, above Benmore, these rocks dip 20° S.S.W.; and on the north-west side of the Hokanuis, in the bed of the Waimea Stream, 60° N.E. by E.; while at the head of the Otapiri, they are vertical; and strike south-east and north-west. At Popotunoa Gorge, the rocks dip S.W. at high angles, to vertical; but between Clinton and Balclutha, the dip seems to be generally northerly, and in the Waitepeka I found it to be 55° N. At the Nuggets, these rocks are vertical, and strike N.W. and S.E.

Relation to Under lying Formation.—The junction between this and the last formation is not very clear in the Province of Otago, and I have not been able to find a section showing it resting on the rocks of the Kaikoura formation. But that it does do so is evident from the general geological structure of the district, and from the much less amount of metamorphism that these rocks have undergone, than those of the Kaikoura Formation. In the Clutha district, there appears to be an uniformity between the two, but it is not well marked. In the Province of Nelson, however, complete unconformity exists, as I have elsewhere pointed out.*

Thickness.—I am unable to give any estimate of the thickness of this formation on account of the foldings along its northern border, which will require very careful surveying to unravel; but it is certainly more than 15,000 feet.

Fossils.—At Coal Hill, a southern spur of the Eyre Mountains, and at Centre Hill, near Mount Hamilton, fragments of the shell of a species of Perna or Inoceramus are found in considerable quantity, but I found no specimen sufficiently perfect to shew the shape

* Geological Reports, 1872-3, p. 34.

page 39or exterior sculpture of the shell. On the eastern slope of the Moonlight Range, on the spur of a hill about four miles north of Dipton Station, beds composed almost entirely of Monotis salinaria var. Richmondiana Zitt. and Halobia lomelli Wissm. occur, and at this spot I also obtained a longitudinally ribbed Mytilus, and an imperfect cast of a species of Perna. I was also informed that the same fossiliferous rocks are found on the hills to the north-west of Castle Rock. On the opposite side of Oreti River, at a place called Cowan’s Wash, I obtained Monotis, Halobia, a large ribbed species of Pecten, and many plant remains; and from the railway cutting in the neighbourhood, Spirigera wreyi Suess, and two species of Spirifera have been forwarded to the Museum. One specimen of Spirifera shews well the spirally rolled lamella that supports the arms. At Morely Creek, on the south side of the Waikari Hills. I got Halobia, and casts of Isocardia, Trochus, &c., and Dr. Hector reports Inoceramus and Trigonia from the same locality. At Omaru Creek, between Catlin’s River and the Clutha, Professor Black obtained Monotis salinaria. Pecten, and others; and at Shaw’s Bay, or Roaring Bay, near the Nuggets, Spirifera, and other fossils not yet described were found by Dr. Lauder Lindsay. Dr. Hector also found Spirifera, &c., in the Upper Otapiri, and Mr. J. Buchanan collected Halobia, Spirifera, &c., from the gorge of the Kaihiku River. Two of these fossils are analagous to, or even specifically identical with those of the Alps of Salzbourg: while Spirigera wreyi is, according to Dr. Zittel, most nearly related to the devonian Spirigera undata Defr.
Age.—No formation in New Zealand has suffered more ups and downs in the geological scale than this, although as it contains fossils identical with species in Europe, and determined by a European palæontologist, it might have been thought that its true position could be ascertained with greater accuracy than any other of our formations. In 1864 it was referred by Professor von Hoch-stetter and Dr. Zittel to the triassic period. In 1866 Dr. Hector classed the rocks of the Nuggets, and Kaihiku Range as upper palæozoic* And he repeated this in 1870 in his catalogue of the Colonial Museum. But in his geological map of New Zealand, published about the same time, he colored them "cretaceo-tertiary." Dr. Haast has always considered rocks in Canterbury, which contain the same fossills as these, to be of lower carboniferous, or devonian age; and Dr. Hector in his last report of the New Zealand Institute (1874, p. 564), says that some of the rocks at the Nuggets belong to the carboniferous period, thus agreeing with Dr. Haast. On this point I may remark that Inoceramus or Perna occurs in the lowest beds of the series, and below Monotis and Halobia; while the two latter are found below the spirifera, beds, and at the Omaru Creek Monotis salinaria occurs in beds that certainly

* N. Z. Exhibition Jurors’ Reports and Awards, p. 265.

page 40do not overlie those exposed at the Nuggets, When to this we add that reptilian remains (Ichthyosaurus australis, Hector,) are found associated with the same species of Spirifera at Mr. Potts in Canterbury, we have little doubt but that this formation is not older than the trias. The question will I hope soon be settled, as Dr. Hector has taken a large collection of these fossils to England with him in order that they may be described there, meanwhile I agree with Hochstetter and Zittel in referring them to the triassic period.

Contemporaneous Eruptive Rocks.—In Preservation and Chalky Inlets a considerable area is composed of a coarse grained pink granite, which I consider to belong to this formation. This granite consists of a matrix af red orthoclase and white quartz with small quantities of black mica., and is clearly eruptive and younger than the slates and sandstones of the Kaikoura formation which it pierces, and pieces of which are often seen enveloped in the granite. Fig. 2 represents a junction of the granite with slate on the east side of Isthmus Sound in Preservation Inlet. The junction here is quite abrupt and jagged, and two angular fragments of slate are seen embedded in the granite. The slate near the junction has been considerably altered, and converted into a finely crystalline rock of a dark grey color. The minerals are separatedf and mica and quartz grains can be recognised with a lens, but microscopical investigation is necessary before the changse can be satisfactorily made out. This alteration, however, does not penetrate very far, and the great mass of the slates in the neighbourhood are quite unaltered. The felspar of the granite generally gets white as it approaches the slates, and for about an inch from the junction gets very fine grained, but the mica flakes increase in size. But in some cases the granite preserves its character close up to the slates. This proves clearly the eruptive nature of the granite, and that the eruption took place later than the Kaikoura formation. It also proves that the highly metamorphic character of the gneiss rocks of the Manipori formation is in no way owing to the granite out-burst, as the granite has failed to alter the slate rocks in its immediate vicinity.*

Fig. 2.—Junction of granite and slate, Isthmus Sound.—a. Granite; b. Slate (Kaikoura formation); c. Fragments of slate in granite.

Fig. 2.—Junction of granite and slate, Isthmus Sound.—a. Granite; b. Slate (Kaikoura formation); c. Fragments of slate in granite.

I have already mentioned that in the conglomerates of the Maitai formation no pebbles of granite or other eruptive rock are to be found; but in the Putataka formation, next to be described, pebbles of this pink granite occur; consequently, the date of this granite must lie between the Kaikoura and Putataka formations. Further evidence of this is found in the numerous dykes of syenite,

* See ante, p. 28.

page 41diorite, &c., that penetrate the rocks of the Kaikoura formation, some of which pierce the lower beds of the Maitai formation, as I shall presently mention; but not a single dyke is as yet known in the Putataka formation. I have also elsewhere* shewn that volcanic action was going on during this period in the Nelson Province, and I think therefore that we need not hesitate to connect the granitic eruption of Preservation Inlet, with the injection of dykes of syenite and greenstone, and probably also with the volcanic action, in the surrounding district.

In Preservation Inlet the granite extends from Revolver Bay and Isthmus Sound to beyond Lady Bay, but occasional masses of slate are found amongst it. Great Island, in Chalky Inlet, is also entirely composed of it. How far it extends up Long Sound, Cunaris Sound, and Edwardson Sound I do not know, as I have not been up them; and I have filled in this boundary on my map from the remarks on the rocks in Dr. Hector’s narrative of his West Coast exploration. With the exception of a few veins, I saw no granite in any of the other Sounds that I visited, viz., Dusky Sound, Wet Jacket Cove, Breaksea Sound, Doubtful Sound, Bradshaw Sound, Thompson Sound, Bligh Sound, and Milford Sound; and I saw none on the west side of Te Anau Lake. At the Bluff Hill, a broad dyke of syenite, composed of white felspar and crystals of black hornblende, runs nearly parallel with the bedding of the slates, and the line of junction between the two rocks is very complicated, veins of syenite, isolated apparently from the main mass, appearing among the stratified rocks parallel with the bedding, and looking as if the syenite was here a product of metamorphism, and that the argillaceous rocks had been changed into syenite, while the more arenaceous ones had resisted the action; but these appearances are, I am satisfied, fallacious. This syenite sometimes passes into an almost pure hornblende rock; sometimes it is of a green color, caused by the dissemination of small particles of hornblende through the mass, but when exposed to the weather, these minute particles soon disappear from the surface, leaving the larger crystals studding the white felspar base with black spots. Several dykes of a similar rock are also seen at Port William in Stewart Island. Centre Island appears to be entirely composed of it, as also, Captain Fairchild informs me, is Ruapuke. Another dyke also occurs at the east end of Wakapatu Bay, at the base of the Longwood Range, and others probably in the Takitimus. On the east side of the Nuggets a dyke of grey porphyry, with white crystals of felspar, traverses the rocks of the Maitai formation at right angles to the bedding.

Nomenclature.—The fossils found in these rocks show that they are identical with the Maitai series of Professor Hochstetter. They may also be in part the Kaihiku series of Dr. Hector, as they

* Reports of Geol. Exploration, 1872-3, p. 34.

page 42form the Kaihiku Range. But this is uncertain, as Dr. Hector’s description of the rocks forming his Kaihiku series does not at all agree with those at present under consideration, and he places it below his Te Anau series, which certainly cannot be my Putataka formation. In view of this uncertainty it is, I think, better to retain Professor von Hochstetter’s name, which also has the priority.

In a former report on the geology of the north east district of the South Island, I separated the Monotis bearing beds from those with Spirifera and Inoceramus (?), under the name of the Wairoa formation; but a further examination of these rocks in Otago, where they are better displayed than in any other part of New Zealand, has convinced me that this distinction will not hold, and that all must be classed in one formation.