Oamaru Formation.
Ototara and Trelissick groups. (Hutton).

This formation is scattered about in patches in various parts of the Province, but principally near the coast.

Oamaru and Waitaki.—In this district the formation extends from the Kakanui River to the Waitaki Plains, and stretches inland up the valley of the Waitaki, as far as Kurow Creek. The lowest beds in the series are thin bedded dark sandstones, upon which lies the calca[gap — reason: illegible] free-stone so well known as the Oamaru limestone. It is an earthy limestone, containing 90 per cent. of carbonate of lime, and is followed by a hard crystalline shelly lime-page 47stone. These constitute the Ototara group. Upon them rest quite conformably, calcareous conglomerate, containing fragments of slate and volcanic rocks, and green and yellow sandy clays, sometimes interstratified with volcanic rocks, the whole being again covered with limestone. These represent the Trelissic group. These beds lie, on the whole, nearly horizontal; for the rocks up the Waitaki are but little, if at all, higher than those at Oamaru; but the strata have various local dips. At the Awamoa railway cutting, beds belonging to the Trelissic group, dip 15° N.E., while in the valley of the Waireka, near Cave Valley, beds belonging to the Ototara group dip 5° E by North. Up the Maruawhenua, the beds are nearly horizontal, the lowest one visible being grey sandy clay, followed by calcareous sandstone, which is overlaid by a brownish red sandstone. Higher up the Waitaki there is an outlier of tertiary green sandstone, which appears to be capped by calcareous rocks. These probably belong to the Oamaru formation. Below them good brown coal occurs.

Otepopo.—Further to the south soft green sandstones, dipping to the east, are seen in the Otepopo River. These probably belong to the Trelissic group, but no fossils have as yet been found in them. They are overlaid by the dolerites of Mount Charles.

Shaq Valley.—Grey, green, and yellow sandstone, associated with shelly beds, are found near Palmerston, and form the base of Puketapu, Mount Royal, Smyler’s Peak, &c., where they are horizontal. But these beds are not very extensively developed here, and the boundaries of the different tertiary formations, are not easily made out. These rocks are again seen at Deepdell Creek, sixteen miles up the Shag River, from whence they extend pretty continuously to Pigroot. Here they consist of green sands, covered by calcareous sandstone and limestone, the whole being often capped by basalt. (See Sec. VI.) Still higher up green sands with marine fossils, belonging probably to this formation, are found in the Kye-burn Creek at the coal mines, lying in the Maniototo plains, at the base of the Kakanui Monntains. Some of the green sandstone in Shag Valley, contains small specks of alluvial gold.

Dunedin.—Still going south along the coast, the next place that these greensands are met with is at Merton, between Waikouaiti and Blueskin, and from here they appear to run in a narrow band at the head of the Waitati Stream, and west of Swampy Hill, to Flagstaff Hill, where they are cut off by the basalts which run from Flagstaff Hill down to Silver Stream. At the head of the Waitati the rocks are grey calcareous sandstone and bituminous shale. In the Valley of the Kaikorai the Caversham sandstone again appears, and runs through Look-out-Point to the sea. It is overlaid by basalt, and below it is a thick series of greenish gray sandstone and clays with brown coal, forming the coal series of Green Island and Saddle Hill. (See Sec. IX). A narrow belt of page 48these rocks runs across the Dunedin Peninsula, north of Harbour Cove, between Boat Harbour and Macandrew Bay, and appears again on the north side of Dunedin Harbour. The sedimentary rocks here consist of sandstone, and yellow and brown limestone, and are overlaid by the trachytes and basalts of Harbour Cove. The dark brown limestone, which contains about 87 per cent of carbonate of lime, overlies the yellow limestone, but I was unable to ascertain the position of the sandstones which are found on the Dunedin Harbour side of the Peninsula.

Tokomairiro and Kaitangata.—Again going south an isolated patch of limestone, belonging to this formation, is found on both sides of the road at Waihola Gorge, and still further south rocks of the same age are extensively developed between the Tokomairiro River and the Clutha, forming the Tokomairiro and Clutha coal field. These rocks consist chiefly of conglomerates, sandstones, and shales; but at Tokomairiro a black bituminous limestone occurs, which is said to overlie the coal.^* During my visit to Tokomairiro I was not fortunate enough to find this limestone although I made many enquiries about it, so that probably it is of very small extent. The conglomerates over the coal at Wangaloa contain alluvial gold.

Lake Wakatipu —At Fews, or Twelve-mile Creek, on the east side of Lake Wakatipu there is a small patch of tertiary rocks belonging to this formation. The lowest bed is blue shale covered by green sandstone, which is again followed by calcareous sandstone with bands of conglomerate, the whole being capped by a hard grey limestone, containing 90½ per cent. of carbonate of lime, and about 3 per cent. of carbonate of magnesia. These beds are about 600 feet thick, and dip 15° S.E, towards the Lake. (Fig. 5, B.) They have, however, been a good deal underm[gap — reason: illegible]ed, and large masses of the limestone have fallen down into the Lake, giving the western portions a very broken appearance I have also been informed that other outliers of this formation exist on the north side of Afton Burn, on the west side of the Lake, and up the Shotover in Stony Creek. Limestone boulders, with fossils, are also said to occur at, Alexandra, but the rock has not yet been found in situ, and I do not know whether they belong to this or the next formation.

Limestone at Lake Waktipu
Fig. 5.—Limestone at Lake Wakatipu: a, river alluvinm; á lake terrace; b, limestone, &c., (Oamaru formation); c, schist (Kakanui formation).

Blue Spur, Tuapeka.—In this locality a small patch of conglomerates are found occupying the top of the ridge between Monro’s and Gabriel’s Gully. These conglomerates are formed of pebbles of quartz, green quartzite, dark purple jasperoid slate, and fragments of schist, bound together by a blue clayey matrix. They occupy a cup-shaped depression in the schist rocks, and dip to the east. (Fig. 6.) The reasons that lead me to think that these conglomerates are of eocene age, and therefore better arranged under the Oamaru formation than any other, will be better understood after the geological history of the Province has been discussed, and they will therefore be found in Section VI. I would, however, remark that if my views are correct, these conglomerates do not strictly belong to the Oamaru formation, but to a slightly earlier period.

Fig. 6.—Blue Spur, Tuapeka.

Southland.—In the Southland district a patch of limestone belonging to this formation is found in the Waimea plains, between Longridge and the Mataura. This patch does not form hills and cannot be distinguished in outline from the flat-topped alluvial terraces of the plain. Another isolated patch forms a long, narrow strip of low hills, running in a N.N.W. direction from Forest Hill, near Winton, to Centre Bush Hill, on the Oreti River; and two other small outliers occur in the valleys of the Moonlight Ranges, one forming the picturesque Castle Rock, and the other a small hill north-east of Castle Rock.

Another and more extensive area is found on both sides of the Longwood Range, extending north as far as Sharp Ridge and the Wairaki Downs, along the south base of which it spreads from the Night Cap Hill to Taylor’s Creek. It also crosses the Waiau River and probably covers an extensive district as far north as the Monowai River, but this country is but little known at present. Higher up another patch is found at Freestone Hill, near where the Waiau runs out from Lake Manipori. The Haycock Hills, also, between Centre Hill and the Mararoa, belong to this formation, and other patches may exist round the base of the Takitimus; but I have not been able to determine accurately the boundaries of the different tertiary formations in this district. At Orepuke, at the southern base of the Longwood Range, another small patch is found occupying the valley of the Waimeamea.

The rocks belonging to the Oamaru formation in Southland are principally shelly limestone and calcareous sandstone; but the Haycocks consist of blue fissile marls with calcareous bands, and reddish yellow sandstone. Round the base of the Wairaki Downs page 50the rocks are sandstone, micaceous sandstone, and shales with coal; and at Orepuki they are also shales with coal.

As in the northern part of the Province, the formation here must, when viewed on a large scale, be considered as nearly horizontal, but rising slightly to the north. Local dips are, however, very various. For example, at Castle Rock it is 10° N.N.W; at Lime Hill, near Winton, 30° S.E.; at Forest Hill, and in the Waimea Plains, the beds are nearly horizontal, but inclining slightly to the south-west: while on the northern base of Centre Hill the dip is 10° to 40° N.W.

Preservation Inlet—On the south-west coast of the Province, rocks, probably belonging to this formation, extend in a narrow belt from Green Islets, past Windsor Point to Preservation Inlet, between Puysegur Point and Otago’s Retreat. They also compose the greater part of Coal Island and Chalky Island, and are also found on the promontory between Preservation and Chalky Inlets. The rocks here are entirely sandstones and shales, with a few seams of coal, except at Chalky Island, where the upper beds are grits and and chalk-marl.

I could not observe the lay of the beds between Green Islets and Windsor Point, but at the latter place they dip 40° W., and going to the west a synclinal curve is seen opposite Marshall Rocks. (See Sec. X.) Further to the west an anticlinal curve occurs in the neighbourhood of Puysegur Point. At Otago’s Retreat the beds dip 25° West, and this direction of dip is continued without much variation through Coal Island and Chalky Island.

As no fossils have as yet been found in these rocks, it is doubtful whether they should be referred to the Oamaru or the Waipara formation, but I place them here for the present principally on account of the large quantity of resin found in the coal, which is characteristic of our tertiary coals.

Relation to underlying formation.—No well defined junction between this and the Waipara formation is found in Otago, but at both places where the Waipara formation occurs the general geological structure of the country leaves no room for doubt as to the relations between the two. In the Shag Valley, the Horse Range, which is the leading physical feature of the district, is composed in large part of rocks belonging to the Waipara formation (Secs. VI. and VIII.), which must have been deposited before the Shag Valley was eroded out, as they form the northern water-shed of that valley. But rocks belonging to the Oamaru formation are found lying horizontally in the bottom of the Shag Valley (Sec. VI. and Fig. 8), which must, therefore, have been hollowed out previous to their deposition. A complete unconformity must, therefore, exist between the two formations here.

Again, at Mount Hamilton, the rocks belonging to the Waipara formation enter into the internal structure of that mountain, and page 51have evidently partaken in the latest disturbances on an extensive scalp, that caused the New Zealand Alps (see Fig. 4); while the rocks belonging to the Oamaru formation skirt round the bases of these mountains, and fill up the old vallies, indicating evidently a considerable unconformity between the two. If also the Waipara formation formed the base of the Oamaru formation, as supposed by Dr. Hector, we should certainly somewhere in the Province find the former underlying the latter. But such is not the case, for although the Oamaru formation is extensively developed, in Otago it invariably, with one possible exception at the Otepopo River, rests directly on triassic or palæozoic rocks.

Thickness.—It is difficult to estimate approximately the thickness of this formation; but I think that between Saddle Hill and Forbury it cannot be less than 2,000 feet.

Besides the foregoing leaves of dicotyledonous plants, and ferns are found in the sandstones up Linton Creek, and leaves are also found at Morley Creek, and Green Island.

Age.—In 1850, Dr. Mantell, Professor Morris, and Professor Rupert Jones, considered the Ototara limestone, from the few fossils sent to England by the Hon. W. Mantell, to be either eocene or upper cretaceous. In 1864, Dr. Hector referred it with doubt to the miocene period.^* In 1865, Dr. Zittel and Dr. Stache, after examining the fossils taken to Europe by Professor von Hochstetter, considered these rocks in the North Island as oligocene, or upper eocene.^† In 1866, Dr. Hector seems to have altered his views as he then classed the rocks of Oamaru, Waireka, and Caversham, as upper pliocene (?); those of the Waitaki and Lake Wakatipu, as older pliocene; and those of Green Island only as miocene.^‡ In 1870, however, in his catalogue of the Colonial Museum, he considered the Oamaru rocks to be older tertiary; and those of Waitaki, Caversham, Tokomairiro, and Wakatipu, as middle tertiary and in 1871, he placed the Oamaru beds as the upper member of his "cretaceo tertiary" formation.^§ In 1872, in my "Synopsis of the younger formations of New Zealand," I considered the "Waitaki and Wakatipu rocks to be lower oligocene, and those of Oamaru, Caversham, Winton, &c., to be upper eocene; and in my catalogue of the tertiary mollusca, in the Colonial Museum (1873), I abandoned the term oligocene, in deference to the wishes of Dr. Hector, and called them all upper eocene. Whether they ought to be called upper eocene or lower miocene is doubtful. I think the latter, because among the fossils are teeth of Carcharodon megalodon; a shark characteristic of the miocene rocks of Europe, and the species of shark are generally widely spread. Dentalium giganteum and Cucullæa alta ^|| are also found in rocks of supposed miocene age in Chile. On the other hand the species of Aturia^¶ closely resembles A. zic-zac from the eocene rocks of Europe, but A. australis occurs in beds at Schnapper Point, in Victoria, which Professor McCoy considers to be oligocene or lower miocene, and which are probably the equivalents of our Oamaru formation.

Contemporaneous Eruptive Rocks.—No eruptive rocks are found associated with the older or Ototara group of strata, although it is possible that the earlier eruptions in the Port Chalmers district may have been contemporaneous with it. But at Oamaru Heads we have clear evidence that during the deposition of the upper or Trelissic group of beds volcanic action was going on.

Oamaru.—We see here an anticlinal curve formed by basalts, and basaltic tuffs and breccias (fig, 7), interstratified with sand-

Fig. 7.—Oamaru Cape: a, Pleistocene silt; b, Pareora formation; c, limestone; d, volcanic rocks; e, soft sandstone (Oamaru formation).

stone (e.) and covered by limestone (c.), all belonging to the Oamaru formation. These volcanic ash beds often contain fragments of limestone, which have been generally altered by heat into a very fine grained lithographic limestone, but occasionally the fossils are still visible in them, shewing that they also belong to the Oamaru formation. These beds also often contain fragments of trachylite, or vitreous basalt, and they are cemented together by a calcareous cement, sometimes containing fossils, which shews that limestone was being formed in the sea during the time that submarine volcanic action was going on. These ash beds, therefore, must have been originally deposited in a horizontal position. At the southern end of the cape, beds belonging to the Pareora formatio (b.) overlie those belonging to the Oamaru formation quite conformably, and it is, therefore, evident that the folding of the rocks which caused the anticlinal curve at Oamaru Cape, took place long after volcanic energy had ceased to exist in the neighbourhood.

Dunedin.—The neighbourhood of Dunedin, as far north as Blueskin Bay, is the most interesting volcanic district in Otago. The main mass of the hills between Port Chalmers and Blueskin is composed of trachyte tuff and breccia, intersected with dykes of basalt and phonolite. The peninsula on the eastern side of the harbour is very complicated in structure, basalts and trachytes being mixed in the most confusing manner; while apparently underlying both there is at Portobello an extensive development of a rather coarse textured propylite, composed of greyish green felspar with crystals of hornblende, and other allied rocks. The trachytes and basalts in the southern part of the peninsula overlie sandstones and limestones of tertiary age, as has been already mentioned. The relation of the propylites to the sedimentary rocks is not very clear, but I am disposed to think that they underlie them. This, however would not necessarily imply that the propylites are older than the sedimentary rocks, for the coarse crystalline texture of the former shews that they are true eruptive rocks. This, however, is a question that must be left for the present unsettled.

All round this centre of more or less trachytic rocks, from. Tairoa Head, Cape Saunders, Tomahawk Bay to Forbury, and from there, through Dunedin to Flagstaff and Swampy Hill, and on to Blueskin Bay and Purehurehu, runs a band of basalts. page 56And other outlying patches of basalt are found at Stoney Hill, Saddle Hill, East Taieri, and North. Taieri. The southern limits of the trachyte appears to be Anderson’s Bay on the east, and Pine Hill on the west, both of which are trachytic. Bell Hill in Dunedin is composed of phonolite, a rock intermediate between trachyte and basalt. Interstratified among the volcanic rocks are small seams of coal, but it is highly improbable that any of them can have sufficient extent or thickness to enable them to be profitably worked.

The general structure of this volcanic district has a considerable analogy to that of the Thames goldfields, but it is on a much smaller scale. The propylites of Portobello resemble in mineral character those of the Thames, but they are more coarsely crystalline than the larger portion of the rocks at the Thames. At the latter place also, the outskirting basalts, although not altogether absent, are much more feebly developed than in the Dunedin district, where the propylites take a quite subordinate place. These differences, in my opinion, show that the conditions have not been so favourable for the production of metalliferous veins in the Portobello district as at the Thames; still, the resemblance is sufficiently close to warrant the hope that auriferous veins may be found here also.

It is probable that these propylites underlie, at a considerable depth, the basalts of Dunedin, for fragments of a very similar rock are occasionally found embedded in the basaltic lava streams, and appear to have been brought up by them from below. The Dunedin basalts also contain several zeolitic minerals as well as aragonite and quartz.

North of Blueskin.—Basalts, probably belonging to this formation, are found at Waikouaiti, and capping several hills in the neighbourhood of Palmerston, such as Puketapu, Mount Royal, Janet Peak, Smylers Peak, &c. On the north side of the Horse Range, they occur at South Peak, lying on rocks belonging to the "Waipara formation; and Mount Charles, at Otepopo, is almost entirely composed of a coarse grained dolerite.

At the Maruawhenua there are two patches, and several more in the upper Shag Valley, where they rest either on rocks of the Oamaru formation (Fig. 8. g.) or on the schists.

On the eastern side of the Maniototo Plain they are extensively developed, from the Hound Burn to Taieri Lake, and also at Hamilton, and between the Sow Burn and Pig Burn; in all these cases lying apparently directly on the schists Lower down the Taieri they are found at the Mare Burn and Scrub Burn, near Hyde (see Sec. III.), also lying on schist.

South of Dunedin.—They are found at Otokaia, and on both, sides as well as at the head of Waihola Lake. A basalt dyke also crosses the northern base of Mount Misery (see Sec. VII). I know of no other basalts in the Province except at Mount page 57Pleasant, on the east of the Longwood Range, and another patch of dolerites and basalt at the north-east corner of the Longwoods. Basalt boulders, however, occur on the hills at Bendigo, near the junction of the Lindis with the Clutha; and basalt scoria has been picked up on the shores of Lake Te Anau, so that probably there are other localities where basalt rocks occur, "in situ" in the centre of the Province, which have not yet been found.

Nomenclature.—The name of Ototara limestone was applied by the Hon. W. Mantell in 1850 to the Oamaru building stone, and this name I adopted in 1872 for the whole formation, but in 1873 I divided the formation into two groups, naming them Ototara and Trelissic respectively, and calling the whole by the name of the Oamaru formation. It includes both the Aotea and Papakura series of my former papers on the geology of some parts of the North Island.