Art. LIV.—On the Geology of the Country between Oamaru and Moëraki.
[Read before the Philosophical Institute of Canterbury, 16th July, 1886.]
The Hon. W. Mantell was the first geological observer in this district. In 1850 he described the Ototara limestone of Oamaru, the Onekakara clay of Hampden, and the volcanic ash of Kakanui. The fossils collected by him were examined by Dr. Mantell, Professor Morris, and Professor Rupert Jones. The Ototara limestone was referred, with doubt, to either the cretaceous or the eocene period; while the Onekakara clay was considered to be either pleistocene or newer tertiary; but, at this time, it must be remembered, the recent fauna of the New Zealand coasts was very imperfectly known. Mr. Mantell remarks that he had no opportunity of ascertaining the relative positions of the Ototara limestone and the volcanic ash of Kakanui.*
In 1865, Dr. Hector placed the Moëraki series (including the Onekakara clay) below the Oamaru series, and considered both to be miocene; the volcanic rocks he considered to be pliocene.†
In 1869, Mr. C. Traill, after examining the fossils from Hampden and Awamoa, came to the conclusion that both were probably miocene.‡
In 1870, Dr. Hector placed the Hampden and Awamoa beds in his Upper, or Struthiolaria series, and the Oamaru limestone (including the Hutchinson's Quarry beds) in his Older, or Ototara series.§
In my “Catalogue of the Tertiary Mollusca and Echinodermata of New Zealand” (1873), as also in my “Report on the Geology of Otago” (1875), I followed Dr. Hector, but called his upper and his older series the Pareora and Oamaru formations respectively. In this latter report I also pointed out that the volcanic rocks of Moëraki overlie the Onekakara clay, thus belonging to a later period of volcanic activity than those supposed to be associated with the Hutchinson's Quarry beds at Oamaru.
In December, 1876, and January, 1877, Mr. A. McKay examined the district, and made several important alterations.∥
[Footnote] * “Quar. Jour. Geol. Soc. of London,” vol. vi., p. 324.
[Footnote] † “Quar. Jour. Geol. Soc. of London,” vol. xxi., p. 128, and section.
[Footnote] ‡ “Trans. N.Z. Inst.,” vol. ii., p. 167.
[Footnote] § “Cat. Col. Mus.,” 1870, pp. 178, 179, and 189.
[Footnote] ∥ “Rep. of Geol. Expl.,” 1876–77, p. 41, etc.
He ascertained that volcanic rocks underlie the Ototara limestone; while others, he thought, were associated with the Hutchinson's Quarry beds. He maintained that the Hutchinson's Quarry beds are unconformable to the Ototara limestone, and stated that the fossils prove them to be members of the same formation as the Awamoa beds (l.c., p. 58). Dr. Hector, also, in his Progress Report for the same year, says: “These higher [Hutchinson's Quarry] beds it has been impossible to separate, either stratigraphically or otherwise, from the Awamoa series which overlies them” (l.c., p. ix.). Nevertheless they are always separated in all the classifications of the Geological Survey, the latest of which will be found in the “Reports of Geological Explorations for 1883–4,” p. xiii.
In 1881, Dr. Hector says that the Ototara limestone is separated from the Hutchinson's Quarry beds by a series of volcanic rocks which belong to the upper part of the cretaceotertiary (= Waipara) period.*
In 1882, Mr. A. McKay again visited the district, and extended his observations as far south as Moëraki† In his report, the blue clay and dark-green sandstones of Hampden and Otepopo (Moëraki series) are stated to underlie the Ototara limestone: thus returning to the first arrangement of Dr. Hector.
Last November I re-examined the district, and arrived at the following results:—(1.) Mr. McKay is right in saying that volcanic rocks underlie the Ototara limestone. (2.) He is probably right in his conclusion that an unconformity exists between the Hutchinson's Quarry beds and the Ototara limestone, although wrong in the reasons he adduces for it. (3.) He is wrong in his opinion that the rocks of Hampden and Otepopo are older than the Ototara limestone; and (4.), we are probably all wrong in supposing that any volcanic eruptions took place between the deposition of the Ototara limestone and the Hutchinson's Quarry beds, or during the deposition of the latter.
Before proceeding to give the evidence on which these conclusions rest, I wish to remark that my mistake as to the true position of the volcanic rocks at Oamaru arose from supposing that the pieces of limestone found in these rocks were fragments of the Ototara limestone which had been altered by heat; a defective observation, which led me to assume that the limestones which rest on volcanic rocks at Kakanui and the south-west end of Cape Wanbrow must be younger than the Ototara limestone, and consequently must belong to the Hutchinson's Quarry beds. I now find that these pieces of limestone are parts of veins in the volcanic rocks which have been formed after consolidation of
[Footnote] * “Rep. Geol. Expl.,” 1881, p. xxvii.
[Footnote] † “Rep. Geol. Expl.,” 1883–84, p. 58, etc.
the rocks. If I had examined the Waireka Valley during my first visit to the district I should probably have found out my mistake. As the Ototara limestone is younger than the volcanic rocks, the inference naturally follows that the Hutchinson's Quarry beds, which also rest on volcanic rocks, may be unconformable to it; but the stratigraphical evidence is not conclusive, as the Ototara limestone may, perhaps, never have extended so far to the east. This question must be solved by palæontology, as I will presently point out. The specific gravities mentioned in the paper were all taken by Walker's Specific Gravity Balance, and by Jolly's Spiral Balance.
I noticed four principal centres of eruption, but no doubt there are others.
1. Oamaru Volcano.—In passing along the shore from the breakwater at Oamaru towards Cape Wanbrow, we first find rocks dipping 25° N. The upper beds (Pl. XXVI., Section I., a), under the Flagstaff, are basaltic agglomerate and ash, the former with bands and pieces of fine-grained limestone. It is this limestone that in 1874 I mistook for included fragments of Ototara stone, altered into a kind of lithographic limestone. By Mr. McKay they are shown as regular beds, interstratified with the agglomerate. A careful inspection, however, has convinced me that they are all veins running between blocks of lava in the agglomerate. They are segregation veins, formed from the calcareous cement in the agglomerate and ash beds, and are of later age than the main body of the rock. The volcanic rocks in contact with these veins are not in the least altered, and the veins are usually compact and solid throughout, often with a banded structure parallel to the margin. In one instance I noticed that there was a compact layer on each side, while the central portion, varying from 6 to 12 inches in thickness, was filled in with broken shells and corals; the two inner surfaces of the limestone were quite smooth, and the organic fragments appear to have been washed in from above. Associated with these beds are tachylyte breccias, consisting of angular fragments of glossy tachylyte, rarely exceeding an inch in thickness, cemented together by crystalline calcite. Round their margins the fragments are often altered into a rich yellow-brown palagonite. The basalt of the agglomerates is compact, bluish-black in colour, finely crystalline, and with olivine more or less abundant; S.G. = 2.80. Under the microscope it is seen to consist of a microcrystalline ground-mass of felspar laths, magnetite, and pinkish brown augite grains, containing here and there crystals of slightly dichroic olivine, much decomposed round the margins
into a dark-brown mineral. The tachylyte is formed of a pale smoky-brown glass, in which are numerous felspar laths. Here also the olivine has undergone much decomposition, a description of which I reserve for another occasion. Its specific gravity is 2.72.
Below the agglomerates comes a series of thin-bedded greenish-brown sandstones (b) containing fossils, and inter-stratified with ash beds. Below these is a coarse scoriaceous sandstone, which is underlain by grey current-bedded sandstones. Then, at the next point, comes a remarkable agglomerate (a1) formed of large basalt bombs, the interstices between which are filled up with compact fossiliferous limestone. These bombs vary from one to six, or more, feet in diameter, and some of them on the lower surface curve round those below, showing that they were soft when they fell into their places. Each bomb is encased by a coating of tachylyte about 1 inch thick, which is decomposed in places into reddish-yellow palagonite. The basalt of these bombs is rather coarser in texture than that of the agglomerate first mentioned, and I could detect no olivine with the naked eye; but under the microscope both the basalts and the tachylytes are much alike. Beyond the agglomerate, in descending order, comes (3) a series of thin-bedded sandstones and clays, dipping 20° N. Next below are (2) coarser scoriaceous sandstones, dipping 30° N., and then (1) agglomerate, (a2 in section), which gradually changes round to an easterly dip, so as to look nearly horizontal in the cliff. Then comes a fault with a hade to the north. On the south side of this fault the beds dip 25° S.E. At the top of the cliff are the thin-bedded sandstones and clays (3), underlain by the coarser scoriaceous sandstones (2), so that the downthrow of the fault is to the south, or, in other words, it is a reversed fault; the throw, however, is small. The sandstones and clays (3) extend to the next point, which is quite low; and in the following bay all the rocks are obscured by the silt deposit, which here comes down to the sea. The next point is Cape Wanbrow, formed of grey scoriaceous sandstones, dipping 10° S.E. I have given a somewhat detailed account of this section, for I am under the impression that it is at the fault that Mr. McKay supposes an unconformity to exist between his upper eocene and cretaceo-tertiary formations; but, if so, he is undoubtedly wrong. At the same time, I saw no other place where any break occurred at all.
Past Cape Wanbrow the dip changes gradually to 15° S.S.E.; then to S., then to 15° S.S.W.; and ultimately to 35° S.W. The grey scoriaceous sandstones of Cape Wanbrow occupy most of this section (Pl. XXVI., Section II.), but are overlain by a bed of pale grey tuffaceous limestone, 6 or 7 feet in thickness, containing minute fragments of coral. This is followed by
sandstones with fossils, and then the Otatara limestone, 45 feet thick. The details of this part of the section I will defer until treating of the Hutchinson's Quarry beds.
It will be seen that the beds all round the east side of the Oamaru Peninsula form a single periclinal curve, as shown by me in 1875.* Mr. McKay's section† is very different, and I am at a loss to account for it, as he gives no details.
2. Deborah Volcano.—Between the Deborah railway-station and Totara are the relics of another volcano, which has been almost entirely destroyed by denudation. The rocks are basic, but I neglected to collect specimens. So far as I could see, they always underlie the Ototara limestone, which surrounds the volcano on all sides but the south-east. Mr. McKay, however, mentions a lava flow overlying the limestone somewhere in the neighbourhood.‡ He gives no precise locality, and I failed to find it; but as I arrived late in the day I could not make a sufficiently careful examination. In the Waireka Valley, opposite Deborah, a tachylyte tuff, probably erupted from this volcano, underlies the Ototara limestone, but I will give its position when describing the sedimentary rocks of the Ototara series. This tuff is compact, grey in colour, and with a lens shows minute black shining spots, and occasionally small pieces of vesicular tachylyte. It effervesces freely with acid, S.G.=2.47. Under the microscope it is seen to be made up of minute angular fragments of vesicular tachylyte in a calcareous cement. The tachylyte is of a pale yellow-brown colour, without any felspars, but contains a few scattered microliths. The vesicles are ovoid, not much elongated. It is much like a tachylyte tuff, presently to be described, from Lookout Bluff.
3. Enfield Volcano.—The railway at Enfield runs through an old volcano which extends as far as Elderslie (Section III.). It is formed principally by lava flows, which are compact and finely crystalline. Some are dark grey in colour, with small white pearly flecks, and cavities filled with limonite; these rocks weather reddish-grey. Others are darker, and without white flecks. S.G.= 2.64. I could see no olivine in any of them. Under the microscope these rocks are seen to have a microcrystalline ground-mass of felspar laths, brownish augite grains and ilmenite, more or less decomposed into leuxocene. There are no porphyritic crystals. In the absence of chemical analysis, I feel inclined to call these rocks augite andesites. At the road cutting close to the Waireka Presbyterian Church, there is a palagonite tuff composed of fragments of tachylyte and fragments of black magma-basalt with olivine. S.G.=2.35. The tachylyte is altered in places into a yellow-brown or brownish-green
palagonite. I could obtain no direct evidence of the age of these rocks, and it is quite possible that the andesitic lava flows may belong to a later period than the palagonite tuffs. This is a point that requires more investigation than the time at my disposal would allow.
4. Kakanui Volcano.—The Kakanui River runs into the sea between two low hills formed of scoriaceous sandstone overlain by the Ototara limestone, here generally more compact than usual. (Section IV.) The sandstones of the northern hill form a periclinal curve, which extends across the river so as to include the rocks seen in the river-bed between the bridge and the sea. The south head is a separate and smaller periclinal curve, showing two foci of eruption; but I did not ascertain which of the two is the younger. I saw no lava streams.
Ototara Series.—This series consists of the Ototara limestone, known as the Oamaru building-stone, together with all the conformably underlying rocks. The Ototara stone is a rather friable and very pure limestone, capable of absorbing one-third its bulk of water. It is made up of minutely comminuted Bryozoa and Hydrocorallinæ, with Foraminifera in the interstices. The underlying beds differ in different places. In Cave Valley the rocks immediately underlying the limestone are obscured; but the railway passes through a bed of pale-yellow, non-calcareous, diatomaceous ooze, which is cut by a dyke 20 feet thick and running E.N.E., with a dip to N.N.W. (Section V.). This dyke is a compact, very dark basalt, without olivine, but with aggregations of greenish-brown augite grains with felspar laths, giving it a semi-ophitic texture. S.G. = 2.80. The dyke does not penetrate the Ototara limestone; but this cannot be taken as positive proof that it is older than the limestone, for its upper termination is not seen. I did not observe the chalk marl with flints, mentioned by Mr. McKay.
Further down the valley, at the School, volcanic rocks underlie the ooze. These volcanic rocks are seen in many places in the Waireka Valley below the Ototara limestone, and I have never seen any above it. In 1874, I observed, in a valley a little south of Cave Valley, thin-bedded, hard, dark sandstones underlying the Ototara stone. I believe that these beds come in between the limestone and the diatomaceous ooze, but I could not find them again this time. About a mile and a half south of Cave Valley the following section may be seen:—
The lower part of the limestone contains small rounded frag-
ments of volcanic rocks. The palagonite tuff I have already described when mentioning the Deborah volcano.*
A little north of Totara, on the west of the railway, the limestone rests on tuffaceous clays which dip 10° S.W., but which, towards the south, flatten to 4° S.W. In the railway cutting at Teschemaker's the limestone, here horizontal, passes down into a coarse rubble of broken shells, coral, sand, etc., and is underlain by about 8 feet of alternating beds of marl and limestone, below which is a brown volcanic sandstone, 10 or more feet thick, which appears to be derived from the degradation of volcanic ash, and not a true ash itself. It therefore appears, so far as my observations go, that volcanic action took place before and during the deposition of the marls and clays underlying the limestone, but there are no volcanic ashes in the limestone itself. The volcanic action was chiefly submarine, but the water was shallow.
The geographical distribution of the Ototara limestone has been described by Mr. McKay. The dips I observed were as follows:—At Cave Valley, 5° E.N.E.; near Totara railway-station, on west side of the road, 5° N.W.; on the east side, between the road and the railway, 4° to 10° S.W.; at north side of Deborah, 5° N.N.E.; on south side of Oamaru Peninsula, 35° S.W. Now, Oamaru Peninsula, Deborah, and Enfield are old volcanoes, consequently the Ototara series dips away from the nearest volcanic centre. This shows, in my opinion, that the Ototara limestone is the remains of several old coral reefs built up round small volcanic islands near the coast, and that it usually retains its original plane of deposition. Limestones are known to be forming at the present day, at angles as great as 33° and 35°, on the coral-reefs of Florida and the Solomon Islands.
Hutchinson's Quarry Beds.—This quarry is situated in the town of Oamaru, on the east side of Oamaru Creek, close to the path leading to the reservoir. It is now abandoned, but was formerly used for lime for burning. The following is the section displayed:—
|8. Dark-green sandstone||6 fossils|
|7. Calcareous sandstone||8–10 fossils|
|6. Conglomerate of volcanic rocks and compact limestone||10|
|5. Volcanic clay||1 ½–2|
|4. Compact limestone||0 ¾|
|3. Volcanic clay||0 ¾|
|2. Rubbly limestone||3|
|1. Volcanic clay, with calcareous veins||13|
[Footnote] * To find this interesting outcrop, take the road from the Deborah railway-station to the quarry, and, leaving the quarry on the right, strike across the fields to the crest of the ridge. Then, looking down into the Waireka Valley, the section will be seen on the left hand.
Owing to a slip, the relation of Nos. 7 and 8 to the rest is not clear in the quarry, but higher up the creek No. 7 is seen resting on No. 6. All the fragments in the conglomerate are well rolled, and the volcanic clays appear to be detrital only. The beds are surrounded to the west and south by the volcanic rocks upon which they lie; to the east they are covered by silt; while to the north they extend for some distance along the east side of Oamaru Creek. I could find, however, in this part of the district, no junction with either the Ototara limestone or with the Awamoa series.
At Deborah, in a small railway-cutting, a little north of the station, the following rocks are seen, dipping 15–20° N.E.:—
Conglomerate of rolled volcanic rocks and limestone.
The junction between Nos. 1 and 2 appears to be unconformable, but the cutting is too small to feel confident on this point.
At the south end of Oamaru Peninsula, we get the following section, all the beds dipping 35° S.W.:—
|12. Blue sandy clay with calcareous concretionary layers.|
|11. Green sandstone, with calcareous concretions near the top||25|
|10. Hard compact limestone||4–5|
|9. Limestone, with rolled volcanic fragments||12|
|8. Ototara limestone||33|
|7. Clay, with three bands of Bryozoon limestone||7|
|6. Ototara limestone||5|
|5. Volcanic conglomerate, with calcareous matrix||3|
|4. Blue ashy sandstones, with shells||150|
|3. Thin bedded sandstones||12|
|2. Grey tuffaceous limestone||6–7|
|1. Scoriaceous sandstones||200+|
No. 12 belongs to the Awamoa series; Nos. 9, 10, and 11 to the Hutchinson's Quarry beds; and all below to the Ototara series; but I could make out no unconformity between any of them. Here, as elsewhere, I came to the conclusion that volcanic action had ceased before the deposition of the Ototara limestone, and that it was not renewed during the deposition of the Hutchinson's Quarry beds.
At the south-west end of the southern hill, at Kakanui mouth, where the plains begin which stretch to the Otepopo River, the Ototara limestone, dipping 20° S.W., is overlain unconformably by dark-blue sandy clay, dipping very slightly
to S.W. at the point of junction, and then getting horizontal as it passes south along Allday Bay. In this clay I obtained Dentalium mantelli, Ostrea edulis, Waldheimia patagonica, and Cellepora nummularia; but many more species could be obtained with plenty of time. I believe this clay to be the equivalent of the Hutchinson's Quarry beds, but more palæntological evidence is required. The unconformity between it and the underlying limestone is plainly to be seen in the coast section, not only in the difference of dip, but also in the denuded surface of the limestone (Section IV.).
The Hon. W. Mantell makes the following remarks on this locality: “A mile south of Kakanui, strata of tertiary blue clays first appear; they contain numerous shells of species that in-habit the neighbouring sea, corals, a few traces of fishes, and small portions of wood. In some localities the clay is capped by a thin layer of sandstone.”* If the Hutchinson's Quarry beds are the same as this clay, then they must no doubt be placed, for reasons that will presently be given, in the Pareora System with the Awamoa series. I have already mentioned that Mr. McKay formerly held the opinion that the fossils were the same in both, although the Geological Survey has never grouped them together.
Pareora System.—I have already mentioned the Awamoa beds on the south side of Oamaru Peninsula, so well known from the collections made by Mr. C. Traill in 1868. The same beds occur on the eastern side of the hills north of Oamaru, as far as the Waitaki Valley. The only other place in the district where I saw rocks which I should refer to the Pareora System was in the Waireka Valley. Here, in going from Elderslie to Windsor, we see blue clay, which, further north, passes upwards into white quartz sands and gravels, covered from Corriedale to Ngapara by a hard conglomerate, formed by well-rounded white quartz pebbles in a ferruginous cement (Section III.). These form conspicuous cliffs, which cap the hills on both sides of the railway. I did not find any fossils in these beds, and cannot, therefore, pronounce positively as to their age, but it was from somewhere in this neighbourhood that Mr. C. Traill made a collection of Pareora fossils some years ago. I cannot, indeed, conceive these beds to be older than the Ototara limestone, as supposed by Mr. McKay; and in November, 1873, I found the quartz pebble beds resting on the limestone near Mr. R. Gillies' farm, in the Awamoko District. The lignite, which lies also above the limestone, is here generally covered by the ferruginous conglomerate. These beds appear to me to be like the Pareora gravels and conglomerates of Waihao and other places, and to occupy a valley of erosion in the Oamaru System.
[Footnote] * “Quar. Jour. Geol. Soc. of London,” vol. vi., p. 324.
In his earlier report, Mr. McKay says: “The outlier of these rocks [Pareora] between the Kakanui River and the Upper Waireka shows them to be quite unconformable to the Ototara limestone and the tufas and greensands underlying the limestone, as the limestone is absent and the conglomerates and blue clay seem there to lie on the tufas and basaltic rocks which are the northern continuation of the Mount Charles rocks at Otepopo. This conclusion is quite irresistible, if we consider the upper part of the valley of the Waireka as due to denudation, and not to a fault, which latter it could not well be.”* It will be seen that my observations confirm this (except the relation to the rocks of Mount Charles); but in his last report Mr. McKay abandons his former views without any remark except the statement that these beds are overlain by the Maerewhenua limestone,† for which he adduces no evidence, and gives no section nor list of fossils. Mr. McKay, however, collected fossils in the Upper Waireka Valley, as well as beyond the first tunnel on the Windsor-Livingstone railway, which will, I hope, settle the question when they have been accurately named.
Silt Formation.—In my report on the Geology of Otago in 1875, I gave a section of the silt deposit, or loëss as it has been called, on the north side of Oamaru Peninsula, and stated that it rested upon gravels with marine shells. Quite lately Dr. Hector has called this in question. He says: “As far as I have observed, the presence of such shells under silt can always be accounted for by landslips of the slope deposit.”‡ I therefore paid particular attention to this point, and can state confidently that on the north side of Oamaru Peninsula gravels with marine shells undoubtedly underlie the silt conformably. The cliffs have here been cut back for some distance to form the railway to the port; all traces of raised beaches, if they formerly existed, have been removed, and a true section has been exposed; as is proved by the intercalations of gravel and silt. But more than this: at the place where the railway sidings commence at the port, the cutting has exposed a large cave in the volcanic rocks which has been filled up to the roof with silt. On the floor of this cave are the gravel beds with marine shells, and these are covered by well-stratified sandy beds, passing up gradually into the silt, which is continuous with that of the rest of the cliff, as is clearly seen in the cutting. In this case a landslip is impossible, for the beds are covered by the roof of the cave; and if the fossiliferous beds pass under the silt here they must also do so in other parts of the cutting. On the south side of the peninsula, slips have, no doubt, occurred in places; but even
here it is quite evident that the gravels have slipped with the silt, and that both retain their relative positions. North of Oamaru many road sections show the silt to be distinctly interstratified with gravel beds, but I observed no fossils in them.
Sedimentary Rocks.—Onekakara Bay lies between the Peninsula of Moëraki and Lookout Bluff, just south of the mouth of the Otepopo River. Hampden is a little to the south of the centre of the bay (Section VI.). The sedimentary rocks consist of blue clay (Onekakara clay), overlain by a soft dark volcanic sandstone, or rock-sand, which weathers greenish; the whole being covered by beds of gravel and silt. The sandstone is found chiefly north of Hampden, but it also occurs at Moëraki. It is in the blue clay, south of Hampden, that the Moëraki septaria are found.
The sandstone consists largely of well-rounded volcanic débris, and is black on first breaking, but soon turns greenish. A similar soft sandstone is largely developed in the banks of the Otepopo River, near the railway; it differs in being almost entirely a volcanic sand, and in weathering to a distinct green colour. The sandstone here is also underlain by blue sandy clay, with dark soft sandstone again below it. It is this latter sandstone which occurs at the Herbert tunnel. Small beds of lignite are associated with it, which were explored by Mr. Fenwick in 1875.
As I have already mentioned, the age of these beds is a matter of difference of opinion. In 1884, Mr. McKay divided them into three divisions, all of which he considered to belong to the cretaceo-tertiary or Waipara System. The stratigraphical evidence he produces in favour of this view is the mistaken idea that they are overlain by the tuffs below the Ototara limestone in the Waireka Valley; this relation depending entirely on the supposed equivalence of the volcanic rocks of Mount Charles and of Kakanui. Of palæontological evidence, Mr. McKay adduces none, for he gives no list of fossils; but to get rid of the evidence in favour of their miocene age, he makes two most extraordinary statements:—
(1.) Previous collectors have “imperfectly collected at points where slips have mixed them [fossils] with the recent shells of the coast-line.”
(2.) Previous palæontologists have examined a mixture of cretaceo-tertiary and recent shells, “hence possibly one reason why these beds have been by some previous observers referred to the miocene period.”*
[Footnote] * “Rep. Geol. Expl.,” 1883–84, p. 62.
With reference to the first statement, it is not easy to believe that either the Hon. W. Mantell or Mr. C. Traill mixed together fossil and recent shells. Mr. Traill's collection is still in existence, partly in the Wellington Museum and partly in that at Dunedin; and, as most of the fossils are still in their original matrix, it is easy to disprove Mr. McKay's statement in this case. Mr. Mantell's collection is not in the Colony, but his list does not contain any of the commoner shells found on the coast. South of Hampden there is a raised beach with recent shells (Section VI.,f), formed into a quartzose sandstone, which, at first sight, might be supposed to pass below the clay. The commonest fossils in it are Barnea similis, Mactra discors, Paphia spissa, Venus mesodesma, Venerupis reflexa, and Ostrea edulis; but as none of these genera, except the last, occur in Mr. Mantell's list, he could not have made any part of his collection here. The idea that a palæontologist, having before him a collection of cretaceous and recent shells, should, as it were, strike a mean and consider the whole to be miocene—although, of course, not a single characteristic miocene shell would be among them—needs no refutation.
To test the accuracy of these statements, I collected myself for an hour or two, in the blue clay north of Hampden, at the place marked “Fossils” in Section VI., with the following result:—
Trochus (?impression only).
Cucullœa, sp. (fragments).
Pecten hutchinsoni (right valve).
Pecten hochstetteri (? fragment).
Of these 16 species, the three in Roman are still living, and the six marked with an asterisk are characteristic Pareora (i.e. miocene) species. This is, I think, quite sufficient to show that Mr. McKay is in error, but I will give a list of all the fossils reported from this locality:—
Aturia ziczac, Sowb.
Fusus australis, Quoy. and Gaim.
Siphonalia nodosa, Martyn.
Siphonalia nodosa, var. conoidea, Hutton.
Cominella, sp. ind.
Nassa tatei, Tenison-Woods.
Ancillaria australis, Sowb.
Voluta pacifica, Solander.
Voluta corrugata, Hutton.
Pleurotoma fusiformis, Hutton.
Pleurotoma buchanani, Hutton.
Triton spengleri, Lamarck.
Natica neozelanica, Q. and G.
Natica suturalis, Hutton.
Cerithium cancellatum, Hutton.
Struthiolaria papulosa, Martyn.
Trochita neozelanica, Lesson.
Crepidula monoxyla, Lesson.
Turritella rosea, Q. and G.
Turritella tricincta, Hutton.
Turritella ambulacrum, Sowb.
Turritella ornata, Hutton.
Trochus(?), sp. ind.
Dentalium mantelli, Zittel.
Venus stutchburyi, Gray.
Cytherea multistriata, Sowb.
Trigonia pectinata(?), Lamarck.
Solenella funiculata, Hutton.
Pectunculus laticostatus, Q. and G.
Limopsis insolita, Sowb.
Cucullæa, sp. ind.
Mytilus magellanicus, Lamarck.
Pecten hochstetteri (?), Zittel.
Pecten hutchinsoni, Hutton.
Ostrea edulis, Linneus.
Entalophora zealandica, Mantell.
Notocyathus pedicellatus, Tenison-Woods.
Trochocyathus mantelli, M. Edw. and H.
Trochocyathus hexagonalis, Mantell.
Some of the recent shells of this list may have been wrongly named, but it must be noticed that of the 32 named species of Mollusca (including the variety), all but Struthiolaria papu-losa, Cerithium cancellatum and Trigonia pectinata have been found in Pareora rocks in other places in New Zealand; and, of the exceptions, the first two occur in the Wanganui System, while the third (doubtfully identified) has not been found in any other part of New Zealand. Of the 29 Pareora species, 14 are not known older than the Pareora, 5 are not known younger than the Pareora, 7 are found in the Pareora only, and 3 go through all our tertiary rocks. Aturia ziczac occurs in New Zealand at Waihao Forks, with numerous Pareora fossils. The genera Cominella and Ancillaria are not known from mesozoic rocks in any part of the world. Entalo-phora zealandica is found at Wanganui, and in miocene rocks in South Australia. Trochocyathus mantelli occurs in the Pareora beds at Mount Horrible, near Timaru.
The evidence is, I think, conclusive that the Onekakara clay belongs to the Pareora System. And this being so, it follows, almost certainly, that the clay overlying the limestone a mile south of Kakanui, which has been followed by Mr. Mantell through Allday Bay to the Otepopo River, also belongs to the Pareora System; and if it is the equivalent of the Hutchinson's Quarry beds, as I suppose, they too must be put into the Pareora. The only evidence wanting is the comparison of the fossils from Hutchinson's Quarry with those from the clay near Kakanui, and this I am not able to do as my lists are not sufficiently complete.
Volcanic Rocks.—The north side of Moëraki Peninsula is formed by dolerites (S.G. = 2.88), which are seen on the shore to overlie the Onekakara clay, here dipping 0° to 60° S.S.E. In one place I noticed, in 1873, that the blue clay had been altered by contact of a lava flow, and turned white for a distance of 2 to 4 feet. These volcanic rocks are therefore much younger than those described from the Oamaru District. Mount Charles, between Herbert and the Otepopo River, is also formed of dolerites, which appear to overlie the greensands and blue clay; at the same time the greensands are formed almost entirely of volcanic detritus derived from still older rocks. These dolerites closely resemble those from Moëraki, but are sometimes coarser in grain, and a less specific gravity (2.73), owing probably to their being more altered. They are compact, and dark greenish-grey in colour, or paler, owing to scattered greyish-white flecks which sometimes become very abundant. Under the microscope they are seen to be holocrystalline, without any older generation. The felspars are in lath-shaped crystals, usually polysynthetic. Sections, more or less parallel to the brachydiagonal, gave extinction angles up to 15° with the twinning plane; while long narrow sections, more or less at right angles to the brachypinacoid, gave extinction angles up to 45°. From this I judge the felspar to be labradorite. Augite of a pale olive-brown, sometimes with black margins, occurs in imperfect crystals; and in a slide from Mount Charles I found a well-defined crystal of rhombic pyroxene, giving straight extinctions. This pyroxene is slightly dichroic, the vibrations parallel to the macrodiagonal being pinkish-green, and those parallel to the brachydiagonal olive-green. This pyroxene is not striated, and, therefore, I suppose it to be enstatite. Ilmenite is abundant, generally in thin plates from Moëraki, but more irregular from Mount Charles; it is much altered into leucoxene, which makes the white flecks. No olivine was seen in any of these rocks.
Lookout Bluff (the “White Bluff” of Mr. Mantell's paper already referred to) is an old and much-denuded volcano, composed chiefly of agglomerate and ash beds. To the north, scoriaceous sandstones, dipping to W. at various angles up to
45°, reach the Otepopo River, and their relation to the blue clay in Allday Bay cannot be ascertained; but to the south a series of dark-green soft sandstones, weathering reddish-brown, dipping 25° W., and underlain by blue clay, are apparently interstratified with tachylyte tuffs. I could find no fossils in these rocks; and, unfortunately, they are separated from the Onekakara clay by a mass of silt and gravel, so that here also their relations are not quite certain, although I saw no reason to doubt their identity with the Onekakara clay. One tachylyte which I collected is a compact, dull, earthy, black rock, and looks like a dark clay; it breaks up irregularly into small fragments, generally with curved surfaces (S.G.= 2.14) It is minutely cracked in all directions, is not vesicular, and of a clear olive-brown, in places paler with globulites, sometimes scattered, sometimes collected into irregular but sharply defined patches, generally angular, but often in lines. Other parts are darker, with abundant globulites. There are no microliths. Another specimen was dark blue-black, with a crystalline texture, and rounded black globules among the crystals (S.G. = 2.38). Under the microscope this is seen to be a tuff, made up of fragments of a brownish-yellow vesicular tachylyte in a crystalline calcareous cement. The vesicles are elongated in the same direction, but there is no other fluxion structure. These tachylyte rocks resemble those from Waireka Valley and White-water Creek in the Trelissick Basin, all of which belong to the Oamaru System. Whether they are or are not of the same age must remain for the present an open question. Their peculiar structure is probably due to lava streams, which have run rapidly into water and have been shattered into minute fragments.
In a paper read before the Geological Society of London, in June, 1885,* I have taken it for granted that the Hutchinson's Quarry beds formed part of the upper eocene, or Oamaru System; and that the volcanic outbursts at Oamaru were contemporaneous with them. This was my former view, but I know now that I was wrong in one, and perhaps in both, of these points. The alterations, however, do not affect in any way the general drift of that paper, which is to show that the fossils of the upper part of the cretaceo-tertiary and of the upper eocene formations of the Geological Survey are identical, and that there is no stratigraphical break between them—i.e., between the Curiosity Shop beds and the Otakaika limestone as representing the upper eocene, and the Ototara and Maerewhenua limestones as representing the cretaceo-tertiary. My repudia-
[Footnote] * “Quar. Jour. Geol. Soc. of Lond.,” vol. xli, p. 547.
tion of Mr. McKay's arguments in favour of unconformity between the Hutchinson's Quarry beds and the Ototara limestone remains intact, although I now, for other reasons, think his conclusion probable. The only alteration necessary to make in my paper is to erase the words I have italicised in the following sentence:—” Cape Oamaru is formed by an old volcano, which has broken through the Ototara limestone, and was active when the marine beds of Hutchinson's Quarry were being deposited.” (l.c., p. 561.)