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Volume 19, 1886
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Art. LII.On the Geology of the Trelissick or Broken River Basin,
Selwyn County.

[Read before the Philosophical Institute of Canterbury, 3rd June, 1886.]

Plates XXIV. and XXV.

Introduction.

The Trelissick Basin lies among the mountains which separate the River Rakaia from the Waimakariri, and it drains into the latter. The West Coast Road from Christchurch to Hokitika, on leaving the Canterbury Plains, does not follow up the valley of the Waimakariri, but ascends to Porter's Pass (3,097 feet), between the Thirteen-mile Bush Range and Mount Torlesse; then descending, and passing through the Trelissick Basin, it reaches the Waimakariri at an elevation of 1,808 feet above the sea. The road then ascends once more to Arthur's Pass (3,013 feet), which lies on the watershed between the east and west coasts. The ascent to Porter's Pass is rendered necessary by the deep, narrow, and almost impassable gorge, six miles in length, by which the Waimakariri reaches the plains (PL. XXV., fig. 1). In this respect the Waimakariri differs from the Rakaia and Rangitata, further to the south, which enter the plains by broad shingle valleys. In the sequel, I will offer an explanation of this remarkable peculiarity.

The first notice that I can find of the geology of the district is in the “Catalogue of the Colonial Museum,” (Wellington, 1870), in which the fossils collected by J. D. Enys, Esq., are arranged in two groups—one in the middle tertiary or Cucullæa beds, the other in the lower tertiary or Ototara series. The fossils, however, had got rather mixed, and in 1872 Dr. Hector visited and mapped the district, dividing the rocks into three formations, which he called Lower Miocene, Upper Eocene, and Cretaceo-tertiary. The fossils in the Wellington Museum, coming almost entirely from the two upper of these formations,

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were rearranged by him, and in this new arrangement they were included by me in the “Catalogue of the Tertiary Mollusca and Echinodermata of New Zealand” (Wellington, 1873).

In 1879, Mr. A. McKay examined the district for the Geological Survey, and published, in 1881, a report which is illustrated by the map and sections made by Dr. Hector in 1872.* In this report, Mr. McKay retains the three formations originally established by Dr. Hector, but makes two important alterations—(1.) He places what was formerly considered as the base of the lower miocene into the upper eocene and (2) he places the fossiliferous tuffs and volcanic rocks of White-water and Coleridge Creeks into the cretaceo-tertiary instead of the upper eocene. Dr. Hector's map appears, also, to have been altered in conformity with this view, for it does not agree with Dr. Hector's statement that the volcanic outburst took place during the upper eocene and certainly in 1873 Dr. Hector did not consider the fossils from White-water Creek to belong to the cretaceo-tertiary.

Last January I spent ten days examining the district, the result being to confirm Dr. Hector's classification of the rocks made in 1872; the later alterations of the Geological Survey being mistakes, as I hope to show presently. But I differ from Dr. Hector in his correlation of the lower limestone with the Weka Pass stone, as well as in several details of structural geology.

In the present paper I have been greatly helped by Mr. J. D. Enys, F.G.S., who showed me the localities for fossils and for eruptive rocks, and went over the fossils with me and explained my difficulties. The lists of fossils will therefore, I hope, be found tolerably accurate. They are compiled from the collections I have myself examined in the Wellington and Christchurch Museums, and in Mr. Enys' private collection. I have also availed myself as much as possible, of the list given by Mr. McKay in his report already alluded to; but in this I have had to use great caution, as it contains many errors.§ The table of distribution in many cases does not agree with the fossil locality-numbers; and these locality-numbers often do not refer to the Trelissick Basin at all. Also, some fossils appear to have got into the list by mistake. This, I think, must be the case with Fusus enysi, (McKay, MSS.), which is said to have been obtained in localities No. 231 and No. 235. The first of these localities is Ngaruroro River, Napier; the second is “Plant beds at the

[Footnote] * “Rep. Geo. Expl.,” 1879–80, p. 54.

[Footnote] † “Rep. Geo. Expl.,” 1881, p. 123; loc. No. 237 and No. 238.

[Footnote] ‡ “Rep. Geo. Expl.,” 1879–80; prog. rep., p. xxi.

[Footnote] § I was much surprised to find this, after having read the excellent lecture on accuracy that Mr. McKay gave us in his report on the Curiosity Shop beds.

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Road Cutting, Thomas River,” collected by Mr. McKay in 1874. Mr. Enys, however, assures me that he has never seen this species from here, but that he gave a specimen, which he had collected at Pareora, to Mr. McKay, distinctly telling him at the time where it came from; and it is possible that in this way the species may have got into the list.

The map accompanying this paper is reduced from the topographical survey made by Mr. Adams in 1882, which Mr. Enys supplied me with. It is, of course, more accurate than the one Dr. Hector had in 1872. The geology will, I trust, be found correct in the main; but that portion bounded by the Porter River, the West Coast Road, and the fault south of the Thomas River is purely hypothetical, the rocks here being covered by a thick deposit of gravel, which is not cut through by any stream. I was also called back to Christchurch suddenly, before I could examine the inliers of the upper limestone lying on the western edge of the basin between Thomas River and Coleridge Creek, and without having sufficiently examined the eastern slopes of Castle Hill and Flock Hill.

General Geological Structure.

The physical features of this basin have been sufficiently described by Mr. McKay. It is a rock-basin, hollowed out of a massif of sandstones, mudstones, and greywackes belonging to the Maitai System. The rocks filling the basin are divided into three distinct formations, as follows, each resting unconformably on the rocks below it:—

3. Pareora System (Lower Miocene of the survey).—A series of blue clays, shales, and sandstones, sometimes unconsolidated, with a total thickness of 600 or 700 feet.

2. Oamaru System (Upper Eocene of the survey).—Coralline limestone, underlain by volcanic grits and tuffs, passing in the south into thick scoria beds. Thickness of sedimentary rocks, 150 feet.

1. Waipara System (Cretaceo-tertiary of the survey).—Argillaceous limestone and calcareous sandstone underlain by marl, below which are green and other coloured sandstones. Maximum thickness about 1,200 or 1,300 feet.

Speaking roughly, the rocks may be said to dip everywhere towards the centre of the basin; but as the basin is much longer than broad, they form a syncline which runs from the upper part of Coleridge Creek in a N.N.E. direction, west of Castle Hill, to Parapet Rock (where the Pareora System stops)

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and Cragieburn Saddle. The north-west corner of the basin, however, is formed by another short syncline which runs nearly parallel to the first. As the West Coast Road enters the basin on its eastern side, it does not run along the syncline until after passing the Thomas River. I observed three faults in the district, but there may be others. The first crosses the Porter River just above the first limestone gorge, south of Prebble Hill,* and runs north-westerly to the north side of Castle Hill; its downthrow is to the north-east; this fault is clearly seen on the left bank of the Porter River (Pl. XXV., Section IV.). The second fault runs from Parapet Hill south-west, and crosses the Broken River at the small gorge under Sugarloaf Hill (Pl. XXV., Section II.) with a downthrow to the south-east. The third fault runs east and west along Waterfall Creek, which is the first affluent the Broken River receives from the west after entering the basin; its downthrow is to the north.

River gravels are widely spread over the basin. They form the summit of Long Spur, and are found on the ridge behind Castle Hill, at an elevation of nearly 3,000 feet above the sea. I have, however, omitted them in the sections, as I paid no particular attention to them. I saw no marks of glacier action; indeed such marks could not be expected to occur, for during the last great glacier epoch the Trelissick Basin must have been a snow-field.

The following altitudes may be found useful:—

West Coast Road Feet.
Lake Lyndon 2,743
Crossing at River Porter 2,266
Terrace N. of River Porter 2,481
Terrace S. of River Thomas 2,285
Crossing at River Thomas 2,178
Terrace at Hotel 2,371
Terrace S. of Broken River 2,390
Crossing at Broken River 2,094
Terrace N. of Broken River 2,350
Craigieburn Saddle 2,619
Lake Pearson 2,085
Hills West of the Road.
Castle Hill 3,023
Long Spur 2,747
Hog's Back 3,391
Hills East of the Road.
Prebble Hill 2,959
Gorge Hill 2,614
Flock Hill 3,269
Junction of Porter and Broken
River (estimated) 1,948
Castle Hill Station 2,520

Waipara System.

This system is largely developed on the north and east sides of the basin: a detached portion also occurs at the most southerly point, in Coleridge Creek. Its upper member is a white argillaceous limestone (Amuri limestone) generally with a platy structure, breaking up into irregular flakes, more or less parallel to the bedding. Below this comes sandstone or grit, underlain by a thick bed of marl; whilst the lower part of the system consists of grey or green sandstones, very variable in

[Footnote] * This is the same as Ram Hill in Mr. McKay's Report. The name of “Ram Hill” is not known to Mr. Enys.

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colour, and sometimes with brown coal or lignite. Its greatest elevation above the sea is at Hog's Back (3,300 feet), and Flock Hill (3,200 feet); at Prebble Hill its highest point is under 3,000 feet.

Beginning at the southern part of the basin, in Coleridge Creek, we find a small exposure of green sandstone, followed by grey marl, the thickness of which I could not estimate, but it must be more than 50 feet. These are succeeded by 200 or 300 feet of limestone, there being no appearance of the intermediate sandstone or grit. The dip is to N.E., at an angle of 50° in the lower part of the marl, gradually flattening to 28° in the limestone. Descending the creek, we lose the Waipara rocks for some distance, and then once more come across the marls underlain by green sandstone on the eastern side of the basin. The limestone is absent here, and the marls and sandstones are not well developed and rather obscure.

White-water Creek exhibits the following section (Pl. XXV., Section III.):—

9.

Limestone. A few feet only, on the left bank of the creek.

8.

Green calcareous sandstone, with fossils. 40 feet (?).

7.

Pale grey or white marl; perhaps 300 or 400 feet thick.

6.

Grey shale. 150 feet.

5.

Dark greensands.

4.

Dark soft sandstone with plant remains and efflorescences of sulphur. 40 or 50 feet thick.

3.

Impure lignite. 3 feet.

2.

Carbonaceous shales and sandstone.

1.

Grey sandstone.

The dip of beds Nos. 5 to 9 is N.W., flattening from 55° in No. 5 to 40° in the upper beds. The beds Nos. 1 to 4 dip to the N. at angles from 70° to 55°, and there may be an unconformity above them. From this point northward the upper part of the Waipara System is covered by the gravel terrace along which the West Coast Road runs, and it does not reappear until the first limestone gorge of the Porter River, near Prebble Hill, is reached. The green sandstones, however, which underlie the marl, form the banks of the Porter between Table Hill and Prebble Hill, the river running more or less on the strike; just above the first gorge these sandstones, dipping 18° N.W., end abruptly in a fault, which has a downthrow to the north (Pl. XXV., Section IV.). This appears to be a reversed fault, the older beds overlying the younger ones, but, as a gully obscures the exact line of junction, I do not feel confident that appearances can be trusted.

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The following section of the upper beds of the Waipara System is exposed in this gorge:—

6. Limestone 30 feet
5. Marl 10 "
4. Limestone 50 "
3. Calcareous grit 4 "
2. Green sandstone 40 "
1. Marl 150 "

No. 3 contains rolled fragments of volcanic rocks, and in the lower part of the marl (No. 1) there are several layers of calcareous concretions. On the south side of Prebble Hill the limestone is divided into two parts, the lower of which is composed of comminuted fragments of Bryozoa, Hydrocorallinæ, etc., forming what is called a coralline limestone, thus differing altogether from its normal character, and resembling the upper limestone, presently to be described. The dip of the upper beds just above the gorge is 40° N., increasing at the gorge to 56° N.

In the lower part of Broken River the greensands exhibit their greatest development (Pl. XXV., Section I.). I estimate their thickness here at about 850 feet, the dip being tolerably uniformly 25° W. They are covered in the Porter River by about 200 feet of grey marl, upon which rests a stratum of brownish green sandstone 20 feet thick. Then comes 20 feet of arenaceous marl, and then the limestone, about 100 feet in thickness at the second gorge. All these beds dip 25° W., but south of the Porter River the direction of the dip rapidly changes, as the beds sweep round through a right angle to the first gorge, and form Prebble Hill. I did not measure these beds in the Broken River, neither did I examine them closely in their northerly extension, although from the top of Flock Hill I saw, in the valley to the east, what I took to be a good exposure of the marl. The dip of the limestone at Flock Hill is about 25° W. (Pl. XXV., Section II.).

At Parapet Rock, on the West Coast Road, the limestone is compact and flaky, grey in colour, but weathering first red and then white. In the bed of Murderer's Creek it is underlain by about 30 feet of laminated calcareous sandstone, containing a bed of shale about 1 foot in thickness. The marl is not seen here, for to the north the greensands have been faulted upward against it. The limestone at Parapet Rock, on the right bank of Murderer's Creek, dips 77° E.S.E., but on the left bank it dips 40° S., gradually turning round to the west towards Flock Hill. The next place where I examined these rocks was at the upper gorge of the Broken River, near Sugarloaf Hill, where the same fault that occurs at Parapet Rock crosses the river (Pl. XXV., Section II.). Here the rocks have been so much disturbed by the fault that I was not able to interpret them intelligently. At the

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gorge itself the limestone dips 25° S.S.E. Immediately to the north, on the right bank of the river, white sands with rounded calcareous concretions occur, dipping 60° N.W.; while a little further up the river the limestone, apparently horizontal, is underlain directly by green sandstone with ferruginous concretions. Probably the white sands have slipped from above the green sandstone, as they occur in that position more to the west; but the locality requires further examination. Further up the river the green sandstone with concretions dips 25° W.N.W.

In Waterfall Creek, which flows into Broken River from the west, there is a very fine section. A fault goes up the bed of this creek having a downthrow to the north, and in consequence the left bank is formed of grey marl, and the right bank of green sandstone with ferruginous concretions. The exposure of the green sandstone here is about 300 feet in thickness, and on it rests white sandstone with concretions, about 100 feet, followed by another 100 feet of marl. This is followed by the limestone, which is here not less than 300 feet in thickness. The stream runs through a narrow gorge in the limestone, forming two waterfalls, and the dip is 40° W.S.W. The limestone covers a considerable amount of surface between here and the Hog's Back, forming a syncline (Pl. XXV., Section II.), with the axis lying about N. by E. and S. by W. At the north end of the Hog's Back the dip is 55° E., but more to the south the dip gets greater, until in Hog's Back Creek, at the southern end of the hill, it is nearly vertical. In Trout Creek, a small stream lying a little north of Hog's Back Creek, the eastern arm of the syncline is also highly inclined, the dip being 80° W.N.W.; so that this syncline is narrowed and steep at its southern end, while it broadens and flattens to the north.

Craigieburn Outlier.—This patch of the lower beds of the Waipara System lies outside the Trelissick Basin, from which it is separated by a low ridge of palæozoic rocks called the Craigieburn Saddle. It belongs to the valley in which Lake Pearson lies, and is 300 or 400 feet below the Craigieburn Saddle. On the left bank of the stream two seams of good brown coal are exposed. The upper of these seams is 7 ½ feet or more in thickness; the lower shows 8 feet of coal, but the bottom has not been laid bare. These coal seams are separated by a bed of brown clay 5 feet thick. The coal seams are overlain by pale soft sandstone with streaks of coaly matter, and this by a ferruginous conglomerate containing rounded pebbles of palæozoic sandstones and quartz mixed with pebbles of liparite, like those of the southern side of the Malvern Hills and the Rakaia Gorge. The dip of the coal beds is 25° N.W., and the whole series is overlain unconformably by horizontal beds of silt, which were probably deposited during the last great

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glacier epoch in a lake formed by the Waimakariri glacier blocking up the valley. The occurrence of pebbles of liparite in the conglomerate is very interesting. Similar pebbles have been noticed by Dr. von Haast in the Big Ben Coalfield on the northern side of the Thirteen-mile Bush Range,* which is about half-way between the liparites at Malvern Hills and Craigieburn; and as no rocks of the same kind are known to the west or north of Craigieburn, it seems necessary to suppose that these pebbles of liparite were brought from the Malvern Hills by a river running to the north. Now the High Peak Range, in the Malvern Hills, attains an altitude of 3,000 feet; the conglomerates at Big Ben are 2,800 feet above the sea; Lake Lyndon is 2,743 feet; Craigieburn Saddle, 2,619; and Lake Pearson 2,085 feet. So that there is, even now, sufficient fall in this direction for a river; and I shall show in the sequel that this gradient was probably steeper in the Waipara period. We are, however, met with the difficulty that the Big Ben conglomerates are, according to Dr. von Haast, surrounded by hills 4,000 to 5,000 feet high, the drainage now being from there into the Kowhai. Probably this ancient river passed over the southern flank of the Thirteen-mile Bush Range; but we must wait for more information before a complete solution of the problem can be attempted.

Fossils.—In the beds above the coal at Cragieburn I found fragments of leaves of angiospermous dicotyledons, and ferns have also been obtained from here. Both ferns and dicotyledons have been found in Murderer's Creek, about a quarter of a mile above Parapet Rock, in connection with a thin seam of coal. The greensands at the lower part of the Broken River contain quantities of a large undescribed species of oyster, apparently identical with one found near the coal at Malvern Hills. Below these oyster-beds Mr. McKay collected, in 1877, Conchothyra parasitica, together with undescribed species of Perna and Corithium; and on the south side of Prebble Hill a Tellina. From this last locality, Mr. Enys has a tooth of Myliobatis, different from those found in the Pareora rocks. The marls contain Ostrea subdentata, Hutton, on the left bank of the Porter, near its junction with the Broken River; also just above the first limestone gorge of the Porter River. According to Mr. McKay this species was collected by Dr. Hector in the greensands, but Mr. Enys knows it only from the marl. The so-called “fucoid markings” are also common in the marl at the first limestone gorge, as well as scales of Teleost fish.

[Footnote] * “Rep. Geo. Expl.,” 1871–72, p. 21.

[Footnote] † Potamogeton ovatus, figured in the “Catalogue of Geological Exhibits, Indian and Colonial Exhibition,” p. 61, probably came from here.

[Footnote] ‡ Perhaps the same as O. alabamamensis, Lea, from the Eocene of Alabama.

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Dr. Hector and Mr. McKay also mention Pecten zittelli (called P. pleuronectes) from the marl, but Mr. Enys has never seen this species in the Trelissick Basin, and it is not included in Mr. McKay's list of fossils at the end of his report.

In the sandy beds above the marl in White-water Creek, I noticed the cast of a Flabellum (?) and another of Cardita, (apparently C. patagonica). In his report Mr. McKay says that Pecten hutchinsoni comes from these beds at the first gorge of the Porter; but in the list of fossils at the end of the report it is said to have come from what he supposed to be the same beds in White-water Creek, but which I shall presently show belong to a higher horizon. I searched in vain for fossils in these beds at the first gorge of the Porter, and consequently I suppose that it is Mr. McKay's list and not his report that is correct. In the limestone at Coleridge Creek I noticed a Waldheimia (?), spines of an echinoderm, and a net-coral (Retepora).

Correlation of the Beds.—That these rocks belong to the Waipara System is admitted by all geologists, and the sequence is very like that at the Waipara.

Waipara, after Dr. von Haast.*
Feet.
9. Grey marl (Amuri limestone) 60–100
8. Sandy clays 60–100
7. Greensand 80–100
6. Blue marl 50–70
5. Calcareous greensand 80–100
4. Concretionary sandstone, with Plesiosaurus, etc. 200
3. Soft sandstone, with Ostrea and Conchothyra 70–150
2. } Sandstone and lignite, with
1. } leaves of Dicotyledons 30–60
Trelissick Basin.
Feet.
} Argillaceous limestone
(Amuri limestone) 100–300
Greensands 50
Grey marl 50–300
White sandstone 20–100
Green sandstone, with concretions and Myliobatis
Sandstones, with Ostrea and Conchothyra
Sandstone and lignite, with leaves of Dicotyledons } 300–850

In mineral characters the Amuri limestone at Waipara and Weka Pass closely resembles the limestone which forms the upper member of the system in Trelissick Basin, and I have elsewhere shown that in the Weka Pass District the Amuri limestone is the upper member of the Waipara System; so that, stratigraphically and lithologically, they appear to be the same. Both are equally destitute of fossils. The officers of the Geological Survey correlate this limestone with the Weka Pass stone, but I cannot see on what evidence they rely. Certainly it does not contain any of the fossils characteristic of the Weka Pass stone, which are similar to those of the Curiosity Shop beds,

[Footnote] * “Rep. Geol. Expl.,” 1870–71, p. 9.

[Footnote] † Quar. Jour. Geol. Soc. of London, vol. xli., p. 266.

[Footnote] ‡ Quar. Jour. Geol. Soc. of London, vol. xli., p.

Picture icon

Geological Map of the Trelissic Basin

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and these are represented in the Trelissick Basin by the upper limestone which forms Castle Hill.

The correlation of the Waipara System with any European equivalent at present presents considerable difficulties. The occurrence of Plesiosaurus above beds containing leaves of dicotyledonous angiosperms would seem to indicate an upper cretaceous age; but Myliobatis (which is here thought to be on the same horizon as Plesiosaurus) has never yet been found in the Northern Hemisphere in any mesozoic rock. I have often protested against the cretaceo-tertiary formation as defined by the Geological Survey; but this has been, not because I deny the possibility of the Waipara period extending into the tertiary era, but because I deny that the limestones, etc., of Weka Pass, Ototara, and other places belong to the Waipara System.

Oamaru System.

Sedimentary Rocks.—These rocks attain their greatest elevation at Flock Hill (3,269 feet). At Castle Hill they go to 3,023 feet, and at Prebble Hill to 2,959 feet.

In the upper part of Coleridge Creek, tuffs, covered by limestone, rest on the rocks of the Waipara System, and dip 55° N.N.W. To the west an apparently isolated portion of the limestone requires further examination, as it appears to rest on Pareora beds; but probably this is deceptive. To the eastward the beds curve round to the north, and the limestone rests on the palæozoic rocks; they then again cross the creek, dipping at a high angle to the west. This is a famous locality for fossils, the tuffs under the limestone containing numerous teeth of Lamna, Carcharodon, and Sparnodus. On the north side of the creek the limestone is absent, the Pareora rocks resting on the tuffs. In White-water Creek the limestone is about 40 feet thick, and dips 15° W.S.W. It is underlain by a bed of conglomerate, formed of rounded fragments of volcanic rocks and limestone in a calcareous cement, which is full of fossils; below it comes dark-green tufaceous sandstone. The limestone can be followed continuously from here northward to Castle Hill, where it is cut off by the fault already mentioned. Between White-water Creek and Castle Hill the dip is 8° W. to 12° W. At Castle Hill it is 25° W.N.W., and near the fault 32° N. The eastern slope of Castle Hill I did not examine sufficiently; possibly the Waipara System may form the lower part. The limestone at Castle Hill is not less than 100 feet thick.

The Oamaru System again appears on the north side of the first limestone gorge of the Porter River, dipping 33° N.W. The greensands are here about 40 feet thick, but the limestone is very poorly developed, having been largely denuded before the deposition of the Pareora System. At the junction of the Thomas River with the Porter the limestone is about 50 feet

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thick, and is underlain by 10 feet of volcanic grit, below which comes 40 feet of tufaceous greensands, much current-bedded, and known as the “Fan coral-beds,” from the occurrence of Flabellum laticostatum, Ten.-Woods. These rocks pass in an easterly direction up Prebble Hill, but they are not connected with those at the first gorge of the Porter. In Home Creek the limestone is 60 or 70 feet thick, and the tufaceous greensands 50 feet, with a bed of blue clay, 1 ½ feet thick, between them and the limestone. The dip in the Thomas River is 20° W. to 25° W., in the Home Creek about 20° W.S.W., and at the natural tunnel through which Murderer's Creek joins Broken River the dip is 10° N.W. In Broken River the tufaceous beds are thinner, but I did not examine them closely. At the natural tunnel they have passed into a calcareous tuff, which I did not recognize further north. An outlier of limestone occurs on Flock Hill, and two inliers on the west margin of the basin: one near the head of Moth Creek, the other a little north of the White-water Creek; but I did not examine them.

Volcanic Rocks.—Scoria beds and tuffs are largely developed in Coleridge and White-water Creeks. In Coleridge Creek some of the scoria beds might almost be called agglomerates, and evidently we are here near the orifice of a volcano which was in eruption during the early part of the Oamaru period. Some of the tuffs are fine-grained, compact rocks of a blue-black colour, and when broken present a sparkling crystalline surface, so that they might, at first sight, be readily mistaken for lava streams. But they effervesce with acid, and when thin slices are examined with a microscope they are found to consist of fragments of a deep brown-yellow palagonite, held together by a crystalline calcareous cement; some of the larger fragments contain crystals of olivine, and occasionally there are separate olivine fragments. Other tuffs are finer in grain, and effervesce very slightly. The specific gravity of these tuffs varies from 2.10 to 2.70, according to the amount of calcite they contain. In Whitewater Creek, just above the Amuri limestone, there is a tachylyte lava stream. It is compact, black, dull, breaks up irregularly under the hammer, and has a bluish tinge on the surface of the joints. Its specific gravity is 2.20. Under the microscope, in very thin slices, it is seen to be a vesicular tachylyte of an olive-brown colour, studded with globulites arranged in groups, either as clouds or as blackish spots.

In Home Creek, resting upon the Amuri limestone, another similar palagonite tuff occurs, compact, and of a blackish-green colour. This bed is 8–10 feet thick; the lower part is granular, effervesces freely, and has a specific gravity of 2.10; the upper part is finer in grain and effervesces slightly, its specific gravity is 2.00. Under the microscope this tuff is seen to be composed of angular fragments of brownish-yellow, or yellowish-green,

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vesicular palagonite in a calcareous cement. Olivine is rare, but there are occasionally small angular grains of quartz mixed with the palagonite. In the finer upper portion the palagonite fragments are smaller and greener, and there is less calcite. All these rocks break up into irregular cuboidal masses when struck by the hammer.

Dykes.—Eight dykes are known in the basin, all but one being clustered round Prebble Hill. Beginning in the south, we find the first (A) on both sides of the Porter River, near Table Hill (palæozoic), running east and west; I could not ascertain its thickness. The second (B) is just south of the fault at the first limestone gorge; it is nearly vertical, and runs W.N.W. through the greensands of the Waipara System. Turning eastward, along the south side of Prebble Hill, the next dyke (C) forms the crest of a long spur which runs W.N.W. The fourth (D) is on the ridge forming the watershed of the Broken River; it also runs W.N.W. The fifth (E) crosses Broken River; it is nearly vertical, runs N.N.E., and is 15 feet thick. Going up the river, the sixth dyke (F) is on the north bank, and runs N.N.E. The seventh (G) is also on the north bank of the Broken River, but above its junction with the Porter; it is 12 feet thick and runs N.W. The eighth dyke (H) is on the north bank of the Porter, in the marl; a small fragment, 12 feet long by 8 broad, is all that is exposed: it runs N.W. None of these dykes can be traced higher than the greensands, except H, and this one does not penetrate to the top of the marl. They are all dark bluish-black in colour, and are all composed of a microcrystalline ground-mass of laths of plagioclase, rounded grains of pale-green augite and magnetite; but they can be divided into two groups. Dykes A, C, D, E, and H are basalt, with a specific gravity ranging between 2.82 and 2.95, the mean being 2.87. They vary from finely granular to crypto-crystalline. They all contain olivine, more or less abundantly, in rounded or broken crystals. This olivine is of two kinds: one is pale green, and shows brilliant colours with polarised light; the other is colourless, and when revolved between crossed nicols, either merely passes through grey into black, or else changes from pale bluish-green to pinkish purple. Dykes B, F, and G are augite andesite, with a specific gravity ranging between 2.59 and 2.70, the mean being 2.64, They contain no olivine, and have a finely granular texture. The position of these dykes, clustered round Prebble Hill and penetrating the green sandstones only, gives the impression, at first sight, that they may have been connected with a small volcano under Prebble Hill, and that they were formed before the marl and limestone were deposited. But there are no traces of contemporaneous volcanic action in the green sandstones, nor in the marl, while only one of the dykes has penetrated so far upward as the lower part of

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the marl. Between the marl and the limestone rolled volcanic pebbles are found, but these may have come from the same source as the liparite pebbles found at Craigieburn, which were certainly not erupted in the neighbourhood. Chemically these dykes appear to be closely allied to the tachylyte and palagonite tuffs of White-water and Home Creeks; so that we may, I think, conclude that they were erupted at the commencement of the Oamaru period, and that their failure to penetrate Prebble Hill was owing to the tough nature of the overlying rocks.*

I have considered that the tufaceous beds in White-water Creek are of the same age as those at the Thomas River and Home Creek, where they join the Porter; but as Mr. McKay holds a different opinion, it is necessary to state the evidence more fully:—(1.) Stratigraphically, the positions of the two are identical. Mr. McKay, unfortunately, missed seeing the out-crop of the Amuri limestone on the left bank of White-water Creek, which is now quite plain, although it might have been covered up at the time of his visit. (2.) Lithologically, the palagonite tuffs are the same in both places, and are quite different to the beds between the marl and the Amuri limestone, at the first gorge of the Porter, with which Mr. McKay would compare them. (3.) Palæontologically, the fossils from the two localities are identical. If the reader will compare the list of fossils given by Mr. McKay from the tuffs at White-water Creek (locality No. 241) with those from the Fan coral-beds at Thomas River (localities Nos. 239 and 243), he will find that there are in the first list 23 named species, (the undetermined species not being available for comparison), all but three of which occur in the Fan coral-beds. And of these three, Triton minimus (= T. pseudospengleri, Tate) occurs elsewhere in New Zealand, only in the Pareora rocks; Calyptra maculata (= Trochita neozelanica, Lesson) is still living; and Pecten hectori (= P. yahlensis, Ten.-Woods) is a miocene shell of Victoria and South Australia. Consequently, none of these can indicate a greater age for the White-water Creek beds. Mr. McKay says: “The fossils collected from these beds at the first limestone gorge on the Porter River were too few to serve the purpose of this comparison; yet, as far as these may, they tend to show that those from White-water Creek belong to the lower tufas at present under consideration. The comparative list at the end of this report will show upon what grounds this opinion rests.” The locality

[Footnote] * Since the above was written, Mr. Enys has brought me a specimen from another dyke on the south-west side of Prebble Hill, between dykes c and d. I have called it dyke k. Its specific gravity is 2.81, and no doubt it is a basalt; but I have not made a microscopical examination. It runs nearly north and south, and may be a continuation of dyke e, which has, however, a specific gravity of 2.92.

[Footnote] † “Reps. Geo. Expl.,” 1879–80, p. 64.

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here mentioned is “No. 240, below Weka Pass stone, Porter River;”* but on looking over his list for the fossils collected here I was surprised to find that none are recorded. (4.) Also, if the large development of tuffs and scoria in Coleridge Creek belonged to the Waipara System, there would certainly be some indications of them in the upper part of the creek, between the lower limestone and the marl, which is not the case.

Fossils.—The following is a list of all the fossils I know from these beds:—

(1.) From the limestone—

Pecten hochstetteri, Zittel; and Waldheimia triangularis, Hutton. Both from the quarry at Castle Hill.

(2.) From the tuffs and greensands—

1.

Cylichna enysi, Hutton.

2.

Marginella dubia, Hutton.

3.

Marginella ventricosa, Hutton.

4.

Voluta elongata, Swainson.

5.

Voluta attenuata, Hutton.

6.

Mitra enysi, Hutton.

7.

Ancillaria hebera, Hutton.

8.

Triton pseudospengleri, Tate.

9.

Natica ovata, Hutton.

10.

Natica hamiltoni, Tate.

11.

Trochita neozelanica, Lesson.

12.

Crepidula striata, Hutton.

13.

Turritella ambulacrum, Sowb.

14.

Trochus nodosus, Hutton.

15.

Zizyphinus spectabilis (?), Adams.

16.

Cantharidus tenebrosus (?), Adams.

17.

Teredo heaphyi, Zittel.

18.

Panopœa orbita, Hutton.

19.

Panopœa worthingtoni, Hutton.

20.

Pholadomya neozelanica, Hutton.

21.

Paphia attenuata, Hutton.

22.

Cardium patulum, Hutton.

23.

Cardium serum., Hutton.

24.

Lucina dentata, Wood.

25.

Cardita patagonica, Sowb.

26.

Crassatella attenuata, Hutton.

27.

Arca decussata, Sowb.

28.

Limopsis aurita, Brocchi.

[Footnote] * “Rep. Geo. Expl.,” 1881, p. 123.

[Footnote] † Mr. Enys informs me that a very large shark's tooth—probably Carcharodon angustidens—has also been found here; but he has not been able to secure it.

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29.

Mytilus striatus, Hutton.

30.

Crenella elongata, Hutton.

31.

Lima jeffreysiana, Tate.

32.

Pecten hutchinsoni, Hutton.

33.

Pecten athleta, Zittel.

34.

Pecten yahlensis, Ten.-Woods.

35.

Pecten chathamensis, Hutton.

36.

Pecten polymorphoides, Zittel.

37.

Terebratula bulbosa, Tate.

38.

Waldheimia gravida, Suess.

39.

Waldheimia taylori, Etheridge.

40.

Waldheimia patagonica, Sowb.

41.

Waldheimia radiata, Hutton.*

42.

Terebratella sinuata, Hutton.

43.

Terebratella aldingœ, Tate.

44.

Terebratellina suessi, Hutton.

45.

Rhynchonella nigricans, Sowb.

46.

Rhynchonella squamosa, Hutton.

47.

Leiocidaris australiœ, Duncan.

48.

Echinus woodsii, Laube.

49.

Pericosmus compressus, McCoy.

50.

Brissus eximius, Zittel.

51.

Flabellum laticostatum, Ten.-Woods.

52.

Flabellum spenodeum, Ten.-Woods.

Of the 46 species of Mollusca here enumerated, ten have not been found elsewhere, nine have been found elsewhere only in the Pareora System, and six elsewhere only in the Oamaru System. But as the known Pareora species are more than two and a half times as numerous as the known Oamaru species, this leaves a balance in favour of the beds belonging to the Oamaru System. The four Echinoidea belong only to the Oamaru System. Flabellum laticostatum is not recorded from elsewhere, but F. sphenodeum occurs also at Mount Caverhill, in the Amuri District. Five or six species of the Mollusca are still living, that is about 10 per cent. I therefore agree with the Survey that these beds are the equivalents of the Curiosity Shop beds, which I have elsewhere shown to be the equivalents of the Weka Pass and Ototara limestones.

Relation to the Waipara System.—At the first limestone gorge of the Porter River, the Oamaru System is seen resting on the Waipara System quite unconformably, as has already been

[Footnote] * Waldheimia radiata, sp. nov. Shell broadly ovate, with a deep ventral ridge and dorsal furrow, but very irregular. Surface with strong longitudinal ribs—about 18 on the ventral valve, of which 4 or 5 are on the ridge—imbricated with coarse growth-lines. Beak prominent, acute, the foramen sub-triangular, the deltidial plates disunited. Length, 0.56; breadth, 0.5; thickness, 0.3 to 0.4 inch. A well-marked punctate shell.

[Footnote] † “Quar. Jour. Geol. Soc. of London,” vol. xli., p. 547.

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pointed out by Mr. McKay. At White-water Creek the Amuri limestone had been denuded down to a few feet before the Oamaru System was deposited; and in the lower part of Coleridge Creek the Oamaru System rests on the marl, the Amuri limestone having been entirely removed, although it still retains a thickness of between 200 and 300 feet at each end of the basin. In the upper part of Coleridge Creek an unconformity can also be made out, the Waipara System striking N.W., and the Oamaru System, in contact with it, W.S.W. The unconformity is therefore well marked.

Pareora System.

This system rests on palæozoic rocks along the west margin of the basin, and extends eastward to the Oamaru System. More to the north it rests on the Waipara System. It attains its greatest elevation (3,390 feet) at the Hog's Back, while in the southern part of the basin it does not reach to 3,000 feet.

The following is the section, in descending order, seen in an affluent of White-water Creek from the north. (Pl. XXV., Section III.)

Feet.
7. Blue shales (plant beds) 150
6. Soft grey sandstone, current-bedded 200
5. Grey sandstone, or sand, with layers of broken shells, Struthiolaria spinosa, etc. 80
4. Sandy clay full of Lamellibranchs 2
3. Grey sandstone, with shells and concretionary layers 15
2. Sandy clay, full of Lamellibranchs 3
1. Grey calcareous sandstone, with shells 15

No. 1 rests upon the denuded surface of the limestone of the Oamaru System (Weka Pass stone), which dips 15° W.S.W., while the Pareora System dips 10° W.N.W. Further up the creek the dip of the Pareora System remains the same in direction, but increases to 25° in No. 7.

In Moth Creek the beds are obscure, but they consist of blue sandy clays with marine shells, probably representing No. 6 of the White-water Creek section. In the Thomas River, from the road crossing downwards, the following are seen:—

6.

Dark grey clay and shales, with plant remains.

5.

Pale grey-yellowish sands, and thin seams of shale, with plants.

4.

Grey sandy clays and shale.

3.

Lignite.

2.

Grey sandstone, full of Lamellibranchs.

1.

Grey sandstone, current-bedded—200 feet.

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The lower beds dip 23° N.W., but higher up the river they appear to be horizontal (Pl. XXV., Section I.), although much disturbed by slips. The plant beds on the right bank of the river, near the road, dip 65° N.N.W. This high dip is probably due to the fault which has thrown the beds down. On the right bank of the Broken River, where the road crosses, the following section is seen, the rocks dipping 12° S.W.:—

5.

Brown and blue shales.

4.

Grey sand.

3.

Lignite, 2 ½ feet.

2.

Clay, 3 or 4 feet.

1.

Sands, current-bedded.

The sandy beds with Struthiolaria spinosa, etc., were not seen by me in the Thomas River section, although they are well-developed in the Porter River between the two gorges; it is possible, therefore, that an unconformity may occur below the lignite. However, the lignite is found in the Porter River, between the two gorges, overlying the Struthiolaria beds, and I could see no evidence of unconformity; but the beds are disturbed and the sections obscure.

An outlier occurs on the Hog's Back, in the north-west corner of the basin, where the beds, resting unconformably on the Waipara System, dip 15° W.S.W. (Pl. XXV., fig. 2).

Relation to Oamaru System.—In White-water Creek I have already mentioned that the Pareora System is unconformable to the Oamaru System; on the north side of Coleridge Creek the Pareora rocks rest on the tufaceous beds of the Oamaru System the limestone having been entirely denuded away, and in the north part of the basin it rests on the Waipara System. In fact, the unconformity between the Pareora and Oamaru Systems is manifest, and admitted by all. Mr. McKay, however, takes the beds lying on the upper limestone, at the junction of the Thomas with the Porter, as the upper part of the Oamaru System, his reason being that boring molluscs have penetrated some of the shells after the matrix with which they are filled had consolidated, proving unconformity with the upper beds (i.e., p. 68). But this is not a good reason, as the same thing may be seen in many consolidated beaches at the present day; and as these particular rocks are very calcareous, they probably consolidated as fast as they were formed. The locality is very difficult, indeed dangerous, to get at, and the stratigraphical relations of the rocks cannot be easily examined; but the fossils (localities Nos. 237 and 238 of the Survey) are entirely Pareora, and I therefore include them in that system, as was done by Dr. Hector in 1872.

Fossils.—As the fossils have been collected from two different horizons, it is better to keep them distinct. The lower horizon

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includes 1 to 4 of the White-water Creek section, and 1 to 2 of the Thomas River section. The upper horizon includes the beds between these and the lignite or plant beds.

Lower Horizon.

1.

Cominella carinata, Hutton.

2.

Voluta Kirkii, Hutton.

3.

Triton pseudospengleri, Tate.

4.

Crepidula monoxyla, Lesson.

5.

Crepidula costata, Quoy and Gaim.

6.

Turritella gigantean, Hutton.

7.

Turritella rosea, Quoy and Gaim.

8.

Vermetus moniliferus, Hutton.

9.

Turbo superbus, Zittel.

10.

Dentalium giganteum, Sowb.

11.

Venus oblonga, Hanley.

12.

Venus yatei, Gray.

13.

Cytherea assimilis, Hutton.

14.

Dosinia magna, Hutton.

15.

Dosinia subrosea, Gray.

16.

Tapes curta, Huton.

17.

Cardium spatiosum, Hutton.

18.

Crassatella ampla, Zittel.

19.

Arca decussate, Sowb.

20.

Cucullaa ponderosa, Hutton.

21.

Cucullaa worthingtoni, Hutton.

22.

Cucullaa alta, Sowb.

23.

Pectunculus laticostatus, Quoy and Gaim.

24.

Pectunculus globosus, Hutton.

25.

Pectunculus cordatus, Hutton.

26.

Modiola australis, Gray.

27.

Lima crassa, Hutton.

28.

Hinnites trailli, Hutton.

29.

Rhynchonella nigricans, Sowb.

Upper Horizon.

1.

Stenorhynchus (?), caudal vertebra.

2.

Myliobatis, teeth.

3.

Purpura tetiliosa, Lamarck.

4.

Siphonalia mandarina, Duclos.

5.

Cominella carinata, Hutton.

6.

Cominella maculate, Martyn.

7.

Oliva neozelanica, Hutton.

8.

Ancillaria australis, Sowb.

9.

Voluta pacifica, Solander.

10.

Voluta gracilis, Swainson.

11.

Voluta kirkii, Hutton.

12.

Conus trailli, Hutton.

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13.

Pleurotoma sulcata, Hutton.

14.

Clathurella hamiltoni, Hutton.

15.

Natica darwinii, Hutton.

16.

Natica gibbosa, Hutton.

17.

Natica ovata, Hutton.

18.

Natica hamiltoni, Tate.

19.

Cerithium nodosum, Hutton.

20.

Struthiolaria spinosa, Hutton.

21.

Struthiolaria obesa, Hutton.

22.

Struthiolaria cingulata, Zittel.

23.

Trochita neozelanica, Lesson.

24.

Crepidula incurva, Zittel.

25.

Turritella tricincta, Hutton.

26.

Turbo superbus, Zittel.

27.

Cantharidus tenebrosus, Adams.

28.

Dentalium conicum, Hutton.

29.

Mactra discors, Gray.

30.

Cytherea enysii, Hutton.

31.

Cytherea assimilis, Hutton.

32.

Chamostrœa albida, Lamarck.

33.

Crassatella ampla, Zittel.

34.

Cardita patagonica, Sowb.

35.

Pectunculus laticostatus, Quoy and Gaim.

36.

Mytilus latus, Chemnitz.

37.

Perna, sp. ind.

38.

Anomia undata, Hutton.

In the plant-beds, above the lignite, casts of two small bivalves have been obtained. They have been referred doubtfully to Unio, but they are much smaller than any species known to me, and one of them appears to have been radiately ribbed; they have the shape of Callista.

Origin of the Trelissick Basin.

Mr. McKay appears to be of opinion that the form of this basin is due, in large part, to foldings of the rocks by compression, subsequent to the deposition of the Pareora System; and it is to these foldings that he would attribute the upheaval of the surrounding mountains.* This opinion is, perhaps, to some extent, due to the very exaggerated sections which accompany his report; but in reality there is no dip in either the Pareora or the Oamaru rocks which cannot be easily accounted for (1) by original deposition; or (2) as the effect of subsequent landslips or faults; or else (3) by being in the immediate neighbourhood of a volcano. The only localities where the dip is more than 30° are in Coleridge Creek, near the volcano; in the upper gorge of the Porter; and the plant beds at the Thomas where the road

[Footnote] * “Rep. Geol. Expl.,” 1879–80, p. 59.

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crosses, close to faults; or else in places where landslips have evidently taken place. There is no stratigraphical evidence of folding by lateral pressure of a general character, involving the palæozoic rocks; and if the tertiary rocks had been folded by compression they would have been to some extent altered by the heat and pressure, as are, the eocene and miocene rocks of the Swiss Alps and the Himalaya. Here, however, the tertiary rocks are quite like their equivalents on the plains and at Oamaru.

With the Waipara System some folding may have occurred, but I think the evidence is not much in favour of it. The steep dips at Parapet Rock and in the Broken River near Sugarloaf are no doubt due to the fault which crosses at both places. In the lower part of Whitewater Creek we find dips varying from 45° to 70°, but these may be owing to subsidence of the volcano which burst through them in the Oamaru period. At the Hog's Back true folding may have occurred; although even here the steep syncline at the south end may have been formed in connection with the fault; indeed, it looks much as if it had been squeezed together between two faults (Pl. XXV., fig. 2). But this movement, whatever may have been its cause, took place before the deposition of the Pareora System, which rests at a slight angle, upon the upturned edges of the Amuri limestone.

The valley in which the Trelissick Basin lies evidently owes its origin to a pre-cretaceous river, which ran in a northerly direction from Coleridge Creek to Craigieburn, and joined the Waimakariri. But the question arises: Was the present rockbasin, in which the Waipara and younger rocks lie, hollowed out by a glacier? Or is it due to unequal movements of lava? I was formerly of opinion that it had been hollowed out by a pre-tertiary glacier coming from the Waimakariri and emptying down the Acheron into the Rakaia; but I have now abandoned this idea, partly because of the great fall between Craigieburn Saddle and Lake Pearson, but chiefly on account of the discovery of pebbles of liparite at Craigieburn, which could hardly have been brought from the Malvern Hills if a lake had lain in the way. It now seems to me more probable that the northern part of the valley was elevated more than the southern part, during the elevation that followed the deposition of the Pareora marine strata; for such an unequal elevation would account for all three rock systems being now found at higher elevations in the northern than in the southern end of the basin, notwithstanding the northerly downthrow of two of the faults. This greater elevation of the northern or lower part of the valley would throw the drainage of the basin over the low eastern rim, and the present gorge of the Broken River would then be cut. This would have occurred during, or after, the last great glacier

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epoch. This same unequal elevation would also account for the narrow gorge, already mentioned, by which the Waimakariri enters the Canterbury Plains, and which, according to Dr. von Haast, has been entirely cut since the glacier epoch.* If this hypothesis be correct, it follows that the inland sea in which the Waipara, Oamaru, and Pareora rocks were deposited, must have entered the Trelissick Valley from the Waimakariri by Craigieburn; the Broken River gorge not having been cut until long afterwards; and as all these rock systems bear marks of an epoch of subaërial denudation following that of their deposition, it follows that the sea entered by this channel at three different times, each time followed by an epoch of upheaval.

[Addendum.]

Christchurch, 30th September, 1886.

Mr. J. D. Enys has informed me that, since my visit to Castle Hill Station, he has discovered a dyke nearly at the top of Gorge Hill—between Broken River and the Porter—which he believes to be a continuation of dyke D. This furnishes absolute proof that one of the dykes, at any rate, is younger than the Waipara System; and probably, therefore, all are younger.

F. W. H.

[Footnote] * “Geology of Canterbury and Westland,” p. 213.