Picturesque Dunedin: or Dunedin and its neighbourhood in 1890
Geology of Dunedin
Geology of Dunedin.
E.G.S. (Loud.); Mem. of Council, G.S.A.; Ord. Mem. North of England Inst, of Mining and Mechanical Engineers; Government Inspector of Mines, and Lecturer on General Geology at the Otago University.
In writing the following paper, the author has taken full advantage of the various publications dealing with the subject, and desires to mention the progress reports of the New Zealand Geological Survey (Sir Jas. Hector, K.C.M.G., E.R.S., Director); the "Geology of Otago," by Profs. Hutton and Ulrich, and several papers in the Transactions of the New Zealand Institute.
Eor convenience of reference, the subject is divided as follows:—
|I.||Introduction and sketch of geological structure.|
I.—Introduction and Sketch of Geological Structure.
It is possible that while to the casual observer the geology of the neighbourhood of Dunedin might not prove so interesting as that of many other portions of the colony, still, for the general student, there are many instances of geological phenomena which would rivet the attention, and furnish' ample food for reflection. page 77At the same time, there are various points capable of affording matter for investigation and discussion.
It would, perhaps, be best in giving a general sketch of the structural features of a locality to ascend in a balloon, and while suspended in mid-air, to take a bird's-eye view of the surface of the country, noting the configuration and character of the land, and observing how each formation has left its impress on the present features of the soil. An opportunity of this nature seldom, however, offers itself, and failing such easy means of ascent, the next best thing is perhaps to climb, by a very easy path, it is true, to the summit of Flagstaff—a basaltic hill 2192 feet in height, and situated to the north-east of the city. To the admirers of scenery alone, their reward is immediate and ample Fain would the writer linger a moment in description of the grandeur and scope of the view, but regretfully he is compelled, for fear of trespassing, to leave any description of it to his colleagues in the congenial task of describing the surroundings of the capital of Otago.
Looking away to the west we see the peaks and pinnacles of the interior of the Province, part of the great mica-schist country through which the Central Railway is now slowly fighting its way to the inland plain of the Maniototo, the Ida Yalley, and the fertile banks and river flats of the Taieri, the Clutha, and the Manuherikia. Turning round more to the south, the dark and rounded form of Maungatua forms a landmark, from which the eye follows the horizon down to Quoin Point and beyond to a distant glimpse of the Nuggets. Between Maungatua and Saddle Hill, which is another basaltic peak, come the fertile plains of the Taieri, laid out in true chess-board pattern, and marking an ancient lake, into which the river of that name flowed, no doubt as a muddy stream, and whence it pursued its limpid course towards the ocean, until in the fulness of time the lake became a swamp, and the swamp, under the fostering care and industry of man, grew into a fertile tract of land, dotted with homesteads and clustering clumps of trees. To the immediate east of Saddle Hill, we see the Kaikorai Eiver, which marks about the centre of the Green Island coal-field, the soft rocks composing which have been hollowed out and eroded until the sea flows at high tide far up the flat. Slightly to the east of the river mouth—which page 78is gradually filled up in fine weather with a sandy barrier, through which the water finds its way, and which is destroyed by the first flood—we again meet with the basalt, this time descending to sea-level, and forming what is known as the Green Island Bluff—a picturesque headland, with fine examples of columnar structure, and forming a protection to the soft sandstone cliffs further on, of which, however, we can, from our present position, see nothing-Could we look over the top of the cliff, we should see very striking evidences of marine erosion, pinnacles and caves—one of the latter of considerable dimensions—with natural arches through which the green seas rush and swirl in their hungry efforts to devour the land. This elevated stretch of country is terminated at the Forbury Head by a sharp descent to sea-level, the base of the hill forming a charming site for the sheltered health-resort named St. Clair; and from this point the eye follows the ocean level along a line of sandhills to another basaltic promontory known as Lawyer's Head, which forms the commencement of the Otago Peninsula. It has been proposed, at this point, where the distance between the open sea and the waters of the harbour is but small, to cut a channel through the isthmus, and form a shorter and better entrance to the Dunedin wharves. At present the idea is not in great favour, but like other extensive public projects, it will no doubt return to popularity, and perhaps eventually be carried out. The Peninsula is a tract of land about 15 miles in length, formed principally of basaltic and rachitic rocks, and indented with several deep and sheltered bays. As our line of sight is necessarily straight across the harbour, looking due east, the nearer elevation of Signal Hill intervenes between our stand-point and Harbour Cone; but it is only when the greater height of Mount Cargill shuts out the distance, that we lose sight of the Peninsula. Still turning round, we pass the isolated peak of Mihiwaka, and following the coast, the character of which is indicated by several basaltic hills, we come again to the mica-schist hills of the interior. Merely stopping a moment to say good-bye to the beautiful panorama around us, we must descend to the flat, and resume our observations in a more detailed manner.
On this occasion it is best to commence some distance from the town, so with the reader's permission, we will take him to page 79Waihola Railway Station, 26 miles away on the main South line where, on the "borders of the lake, and well exposed in the cutting, we find a deposit consisting of moderately coarse beds of sand and silt, containing large angular blocks, and dipping to the south-east at somewhat low angles up to 30 deg. Within the township these sands have been invaded by volcanic rocks, which appear as massive dykes from four to fifteen feet across, considerably altering and in durating the sands and clays, and within the upper township breaking through the lower beds of the coarser upper part of the formation. This deposit, which is supposed to be of glacial origin, extends from Waihola in a north-easterly direction to Brighton, a distance of about twelve miles, and is in its upper part, composed of exceedingly angular material, the blocks in places being from five to ten and even twelve feet in diameter, loosely compacted together, the fragments being often surrounded with fine material. Passing the Taieri River, which drains a considerable area of the northeastern portion of Otago, and at the mouth of which, as also further down the coast, there are valuable deposits of manganese; we skirt the Taieri Plain, leaving an outline of the Green Island coalfield on our right, where the basaltic peak of Saddle Hill caps the coal measures, and plunge into the bowels of the earth at the Chain Hill tunnel, which is excavated through a formation of fine-grained laminated mica-schists or phyllites. These-form a ridge of hills which run out to the sea-coast at Brighton, and soon after leaving the heavy cutting, we are on the borders of the coal measures, which lie unconformably on the schists, and deserve more than a passing notice. The rocks here consist of sandstones, shales and clays, with seams of coal; these dip in an easterly direction at one in ten under the Caversham sandstone, which is a tertiary marine formation, and is again overlaid by the volcanic rocks of the Dunedin basin. The coal-seams extend along the western boundary in a line running from the mouth of the Otakia Creek for about nine miles to the valley of the Water of Leith. Starting at Brighton, the southwestern limit of the field, we find, not far above sea level, a seam of coal 14 feet in thickness, dipping to the east at six deg., and worked on a small scale by an adit. The natural cover of the seam has been in places denuded, and a recent page 80river-gravel takes its place.
The grits, sands and clays forming the proper coal measures are here overlaid by limonitic sandstones and a gritty calcareous rock full of shell fragments that almost without exception belong to a species of oyster that cannot be distinguished from the black oyster of the Malvern Hills, Canterbury. In the same beds occur numerous fusi form bodies, respecting the zoological affinities of which there has long been a difference of opinion, viz., whether they should be referred to the belemnitidæ or simply considered cidaris spines changed to aragonite, and thus acquiring a fibrous, radiated structure. Fortunately, among a considerable series of these fossils collected by Mr McKay on this occasion, there prove to be at least two specimens showing distinct traces of an alveolar cavity (no phragmacone being present); and there is also another specimen preserving the upper portion of the guard sufficiently well to show that originally there was a ventral groove or fissure, the lower part of which can be traced in the shell structure. This would indicate that the fossil is a species of belemnitella, and not a true belemnite. (Geological Survey report 1886-87, p. xxix.) As the actual determination of this organism is obviously a matter of great importance in determining the age of the coals, it has been thought better to quote Sir James Hector's remarks in full.
Above the locality here referred to, to the south-west, the hill rises to a height of 1560 feet, and is capped by the dolerites and basalts of Saddle Hill. Round the edges of the schists, the coal-measures follow the former land area, to an out-crop at the Halfway Bush, but the coal there has the character of a bituminous shale, and contains a large proportion of ash. In the "Water of Leith, fragments of coal have been found, which indicate that the seams in that direction have been altered by the igneous rocks; one specimen, stated to have been found in the Botanic Garden Reserve, had the property of caking, which is quite exceptional', even among the altered brown' coals.
It is now necessary to return to the point where we left the schists, and resume our course over the coal-measures, noting how the working of the seams at Green Island has caused numerous fractures of the surface. Shortly before arriving at page 81Caversham station, we enter a tunnel which is 950 yards in length, and excavated entirely in the sandstone, here remarkably compact, containing only five or six cracks in its total length. It is a soft calcareous sandstone, generally "bluish-grey, or yellow, and unfortunately not well adapted for building purposes; had it not been for a protecting cover of basalt, no doubt erosion and weathering would have obviated any necessity for taking the railway below ground. Before arriving at Dunedin we skirt the hills on our left, leaving the South Dunedin flat on our right, bounded by the sandhills in the distance, and at the Kensington Crossing a fine exposure of spheroidally weathered basalt presents itself.
From the Dunedin station the railway follows generally the outline of the harbour, and as we pass, attention is naturally directed to the great excavations in the volcanic rocks, from which enormous quantities of road metal have been extracted. Further on volcanic tuff and rachitic rocks appear, until at Sawyer's Bay sandstone is again met with. This extends right across the harbour to Broad Bay, below Mr Larnach's mansion, which forms a landmark for many miles, and on that side of the harbour the formation yields an excellent building stone. At Port Chalmers there is a large quarry of trachyte breccia, of which the graving-dock at that town is built, as are also partially many of the large public buildings in Dunedin. Over the Mihiwaka tunnel lie volcanic tuff rocks, in which thin seams of coal have been found. In June 1886, while prospecting operations were being carried on, the writer had an opportunity of examining these deposits, and observed a two-foot seam of carbonaceous shale, with thin seams of jetty coal, dipping S. 15 deg. E. at 9 deg. It was stated that in a short distance the thickness increased to 4ft. 6ins. Further down the hill the seam dips at 35 deg. to the south-west, and is much faulted and broken; it was here only 2 or 3 inches thick. Shortly after leaving the last tunnel, we creep along the Purakanui cliffs, looking down from a giddy height upon the green waters of the Pacific Ocean, lazily lapping the base of the rocks below, or surging and boiling with the fury of the storm. Prom the other side of the elevated portion of the railway, Blue skin Bay is exposed o view, at high tide a lovely bay, encircled by bush-clad hills, and almost enclosed by a sand-page 82bank, which runs out from the northern side. At low tide, when the inlet is principally mud-flat, the prospect is much less alluring. At Waitati Station we are again at sea-level, and to our left, up the valley, calcareous sandstones are seen, while further up [the creek are oil shales and a seam of coal. From the native village of Puketeraki, we see the harbour of Waikouaiti, the northern headland of which is formed of the Ototara sandstone; this is sometimes pointed out as the natural harbour of eastern Otago, and had fate willed otherwise than it has, there is no doubt that the rolling downs behind the town would have made a charming site for a city. On our left is now the great mica-schist formation, with the coal-beds and greensands lying upon its flanks, and here and there a solitary basaltic peak, the remnant of a once continuous sheet, which, in Miocene or post-Miocene times, was poured over the whole district.
Just before entering the town of Palmerston, we pass through greensands, which are economically important, as they contain considerable quantities of valuable iron ore. A sample taken from the deep cutting opposite Mount Royal Station, though apparently consisting of little else than greensand, yielded, on analysis in the Colonial Laboratory, 37 per cent, of spathic iron ore. Large masses of this stone exist, and no doubt some day it will be utilised for the manufacture of iron.
Prom the foregoing sketch it will be seen that in this portion of Otago, the following formations are present:—
|1.||Quaternary.—Recent Alluvium, æolian, and moa-beds.|
|2.||Cainozoic.—Caversham sandstone, volcanic series.|
I. Recent.—The recent deposits in the neighbourhood of Dunedin comprise the river-beds, harbour deposits, and the extensive. Æolian deposits on the coast. The last-mentioned are of considerable area and importance. They commence beyond Green Island, at Brighton, near the belt of schist, which here reaches to the sea, and forms the picturesque headland upon which the township is situated. Further to the east they increase in magnitude, and, as has been mentioned, the sand forms, at the mouth of the Kaikorai stream, a bar, which is swept away in times of floods, only to re-form as the volume of water becomes page 83gradually less. This forms a very good example of the opposing fluviatile and marine geological agencies. This point was evidently a favourite camping ground for the Maoris, for many good examples of their stone implements have been found in the locality. On approaching the Green Island Bluff, the sand has encroached very considerably on the land, and caused a good deal of damage; but from this point for about 3 miles we find no dunes, as the coast is formed of cliffs composed of the Caversham sandstone, which is easily worn away by the beating of the waves, assisted in stormy weather by an irresistible volley of basaltic boulders derived from the easily fractured columns at the Green Island Bluff. At St. Clair we again meet with sandy deposits, which rise into hillocks of moderate height, and are especially interesting because among them are found, in great profusion, beautiful examples of sand-worn stones having that peculiar triangular section which is so familiar and striking. About this point a short portion of the beach has been attempted to be reclaimed, and on it a marine esplanade has been laid out, but recently unusually severe storms have washed away the retaining wall, and greatly damaged the appearance of the locality. From the western extremity to Lawyer's Head, a distance of two miles, are wonderful natural facilities for the construction of one of the most magnificent promenades that could be desired; but the sandhills are at present by no means pleasant to walk upon, and the space between high and low water-marks is alone available.
Opposite the tram terminus, the isthmus connecting the Peninsula with the mainland, is only about three-quarters of a mile in width, and Maori tradition relates that in old days this was covered at high water, and the tides met. Now, however, the high-water mark on the seaward side is very much further back than even a few years ago, and several lagoons have been formed. For a long way round the Peninsula, after leaving the Tomahawk lagoon, high cliffs have prevented any Æolian formation, and it is not until near the Sandy mount district that deposits of this nature are again met with. The loose, drifting material is here carried up to an extraordinary height, and has given its name to the locality. In other places round the coast, large deposits of a similar nature are encountered, but only one of these, at the Maori Kaik, requires mention, and that on account page 84of the fact, that the rare phenomenon of sonorous or musical sand is stated to be occasionally heard.
The Moa deposits in the neighbourhood of Dunedin are not extensive; crop-stones are occasionally met with, and only recently a tolerably perfect skeleton of dinornis casuarinus was ploughed up at Green Island. Mr J. Buchanan, E.L.S., one of the early settlers, has informed the writer that very numerous bones existed on the surface of the ground on Maungatua, but that the first fires destroyed them.
II. Pleistocene, Newer Glacier Deposits.—Formerly it was believed that glacially striated stones, with boulder clay and grooved rocks, were to be observed in, or near Dunedin, but on closer examination it was found that the former were merely spheroidal masses of basalt, in which decomposition had revealed a previously hidden structure, resembling striation. The grooved rocks also were found to be caused by water running over calcareous sandstones, in which it had formed narrow gutters.
Older Pliocene (?)—The large glacier deposit described on p. 79, and extending from Brighton southward, is referred by Prof. Hutton to this period, but Sir Jas. Hector considers it to be either Miocene or Eocene, at any rate prior to the latest manifestation of volcanic agency within the Otago district.
III. Cretaceo-Tertiary.—This formation has been determined by Sir James Hector, on account of the impossibility of making a distinct division between the Tertiary and Secondary periods. It comprises a large series of strata, which are stratigraphically associated, and contain many fossils in common throughout; "while at the same time, though none are existing species, many" present a strong Tertiary facies from both the highest and lowest "parts of the formation, but even in the upper part, a few are "decidedly Secondary forms." (Hector "Guide to Geol. Exhibits, Ind. and Col. Exhibition," p. 55.) In the vicinity of Dunedin, the formation is commercially important, as in it are the coal deposits. As its area and character are described elsewhere, it need not be further referred to. In the appendix will be found some details of the more distant portions at Shag Point, and in the Olutha District.
Eruptive Rocks.—The eruptive rocks may justly claim a large share of attention in the consideration of the geological structure of the district we are discussing, as, but for their presence, the features of the country would have been entirely different. It is of the greatest importance, especially in a new country, to have materials for the construction of roads, and for this purpose, the compact basalts are eminently fitted. As settlement progresses, and the prosperity of the inhabitants increases, more substantial building materials are required than the timber which succeeded the wattle-and-dab order of architecture of the early settlers. Hardly any material can surpass as ornamental and durable building stones, the trachytes, breccias, and basalts of our immediate neighbourhood, which may be observed in tasteful combination with various sandstones in most of the public edifices of Dunedin. In the future, when massive stone piers and wharves are required to replace the present wooden structures, as our fine public buildings have replaced the timber and sheet-iron shanties of a few years ago; these natural advantages will enable them to be erected with a maximium of durability and a minimum of cost.
Geologically speaking, the volcanic rocks of the Dunedin basin are of great interest and importance. The main mass of hills between Blue skin and Port Chalmers is composed of trachyte tuff and breccia, intersected with basaltic dykes. The structure of the east end of the Peninsula is very complicated, 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, etc. Underlying the volcanic series in the locality of Broad Bay, are, as has been already mentioned, sandstones and limestones, belonging to the coal series. The relation of the page 86propylites to the sedimentary rocks is not very clear, but the former are supposed to underlie the latter, having probably the character of an intrusive volcanic outburst.
All around the centre of rachitic rocks, from Taiaroa Head, Cape Saunders, Tomahawk Bay, to Forbury Head, through Lookout Point above the Caversham tunnel to Flagstaff by Abbot's Hill, near Green Island, and past Swampy Hill to Blue skin Bay and Purehurihu, is an including belt Of basalts. In other places there are only outlying patches, as at Saddle Hill, Stoney Hill, East Taieri, and North Taieri. The sothern limit of the trachyte appears to be at Anderson's Bay on the east, and Pine Hill on the west. As has been stated, small coal seams are interstratified among the tufaceous volcanic rocks, but these have not, so far, been proved to be of a profitable character, nor is it likely that they will be. Attention has been drawn to the resemblance between this district and the Thames goldfields, where valuable mineral veins intersect the dioritic rocks, and the propylites of Portobello resemble those of the northern locality, but are much more crystalline, and at the Thames there is not such a strong development of the surrounding basalts. As we shall subsequently see, the parallel is not only in lithological and geological conditions, for at Harbour Cone, in the crystalline diorite rock, there has been found sufficient gold, if not to create a prosperous goldfield, at least to cause a hope that such a desirable consummation may some day be attained.
In places the basalts exhibit very beautiful columnar structure, notably at Forbury Head, Green Island Bluff, on Mts. Cargill and Mihiwaka, and on Stoney Hill near Brighton. In many instances incipient columnar structure is developed, and spheroidal concretions are very distinct in the rocks upon which the First Church is built. In some localities the bold cliffs formed by these hard rocks give rise to very fine scenery, the Forbury Head, upon which stands Mr. Cargill's mansion, and more notably on the Peninsula, a little to the eastward of Lawyer's Head, where there are vertical cliffs 800 feet in height.
At the Glen quarry and in the Woodhaugh Valley beautiful zeolitic minerals are found, with crystals of calcite and aragonite, and good specimens of hyalite may be obtained at the former of these localities, as also at the Forbury cliffs. In the Kaikorai page 87Valley is quartzite, no doubt altered from the Caversharn sandstones, and in many places halloysite and chloropal, with other decomposition products.
II. Economic Geology.
We cannot, in the immediate vicinity of Dunedin, boast of any great natural mineral riches; still the geological features of the district are not of an unfavourable character for the development of a city. There are, it is true, no deposits of true coal, but close to our doors is the excellent fuel from Green Island, known as brown coal, and existing in very large quantities; and on either side the Kaitangata and Shag Point coalfields (see appendix) are ready to supply us with pitch-coal of the highest quality. The coal at Green Island is hydrous brown coal, containing, as will be seen by the following analysis, about 16 per cent of water:—
(Mean of analyses of Green Island coal recently made in Colonial Laboratory for Railway Department.)
It occurs in a seam 19ft. thick, and has when first got a lustrous appearance and dark brown colour, but on account of the large proportion of water contained, dessicates and crumbles on exposure to the air. It burns freely, with a slightly unpleasant smell, and leaves a bulky incandescent ash; as a fuel for locomotion it does moderately well; and the slack and small coal are largely used for stationary engines. When left below ground the slack is very liable to spontaneous combustion, and many mines have been lost from this cause.
In the coal measures are first-class clays and fire-clays; indeed in the matter of materials for making bricks, the locality is thoroughly well supplied.
An excellent sand for building exists in large quantities in the neighbourhood.
The soil of Dunedin and the environs may be said to be generally of an excellent quality; with the exception of the coal-page 88measures, and some few parts where the rachitic roots have unfavourably influenced it, we have usually a rich decomposed basaltic soil, mingled in some places with the debris of limestone, and thus forming a most excellent material for dairy produce. The swampy flats, now drained, of the lower parts of Dunedin, and of the Taieri plains, are wonderfully fertile: the latter being formed in great part of decomposed schist rock, brought down from the interior of the Province, and enriched by the decay of many generations of plants.
The building stones surrounding Dunedin are principally in the volcanic rocks; certainly the Caversham sandstone has been used for this purpose, but without success, as it is incapable of resisting the action of the weather. The following analysis indicates its composition, which is mainly carbonates of lime and magnesia, with a small proportion of silica:—
|Carbonates of lime and magnesia||53.0|
|Oxide of iron||1.4|
|Water and loss||2.1|
On the Peninsula, near the camp, is avery good siliceous sandstone, which has been utilised by the Hon. Mr Larnach in building portions of his residence. From the volcanic series can be obtained not only the hard bluestone or basalt, but also rachitic breccias of considerable beauty and great durability; both are largely used in the public buildings of the city. For purposes of cement manufacture, very good lime stones exist in several parts of Otago; and there is a large deposit of valuable hydraulic limestone in a line extending from Seal Point on the southern side of the Peninsula to Dowling Bay on the northern shore of the lower harbour. This yields an excellent material, but, unfortunately, the cost of cartage is too great to allow of its being extensively utilised.
The necessary mud for the manufacture of Portland cement is obtained from the Dunedin harbourpage 89
Although gold-bearing rocks exist in more than one locality near Dunedin, there has never been that conspicuous success in working them, which the investor has a right to expect.
As has been stated, an instance of an exceedingly interesting auriferous rock occurs at Harbour Cone, near Portobello, on the Peninsula. Although frequently spoken of as a quartz reef, this is in reality a dioritic rock, richly impregnated with iron pyrites, and apparently occurring in very large quantities. Several attempts have been made to work it, but all have, so far, proved ineffectual. Some years ago trial crushings were made in Victoria, which yielded 7 ½ dwts. to half an ounce per ton, and other samples from a portion of the deposit a little lower down the hill gave 8 dwts., 1 oz., 2 dwts., and 6 dwts. per ton respectively. Some doubt was felt as to the probability of finding gold in such a matrix, and other samples were tried in Dunedin, all of which yielded the precious metal. Inlateyears, about 1886, another attempt was made to develope the field, but without success. Professor Ulrich, who has examined the locality, describes the deposit as a peculiar, hard, diorite-like rock, the structure being holo-crystalline and medium coarse-grained; the composition triclinic felspar, hornblende, and some quartz with iron-pyrites finely and uniformly impregnated, more especially through the hornblende part. He considers that a whitish mass overlying the crystalline rock is either a decomposition product or more probably a decomposed rock of genuine rachitic type, which has flowed over the other. This is indicated, he states, by the fact that on its line of continuation, only a few feet distant from the shaft, there exists on the hill-side a massive outcrop of a hard rock, which, according to all appearance, is of rachitic character, consisting of a very fine-grained greyish or yellowish-white base, with impregnated crystals of sanidine-like felspar, hornblende not being observable.
Presuming that the rock exists in quantity, and contains a general average of gold, such as has been mentioned, it seems a matter for regret that steps are not taken to thoroughly determine its value; the prospect of finding rich veins of auriferous quartz, such, as are usual in the Thames district, would lend an additional inducement. In addition to the above instance, there is at Green Island a series of quartz reefs, known as the Saddle page 90Hill Reefs. The stone is of considerable thickness, up to 14 feet, and strikes S. 76 deg. E. It dips northward at an angle of 55deg., and is enclosed in soft phyllite. Several of the reefs carry gold, and some attempts have been made to work, but so far no success has attended the efforts, and the place is now let to a party of miners who are working surface stone.
- Stibnite, Dunedin and Green Island
- Mercury, native.
- Coal (hydrous).
- Brown coal.
- Pitch coal.
- Carbonaceous shale.
- Ambrite (retinite). Barytes.
- Calcite (stalactitic and cry stallised).
- Aragonite (? with strontianite).
- Opal jasper,
- Olivine.page 91
- Seoleeite (?)
- Orthoclase Sanidine.
No account of the geology of this portion of Otago would be complete without a reference to the adjacent coal-fields of Kaitangata and Shag Point.
The former is situated north of the Clutha River, about 40 miles to the south-west of Dunedin, and covers an area of about 40 square miles. The coal measures may be traced to 9 miles north of the Tokomairiro river, where the schist again appears.
The formation consists of conglomerates, sandstones, clays and shales, with coal seams, forming hills 700 feet high in the neighbourhood of Kaitangata, and of less altitude to the north, where they rise against the flanks of Mount Misery, which is upper schist rock. Very good sections would be exposed on the coast, were it not that faults have somewhat obscured the sequence. The seam worked in the Kaitangata Railway and Coal Company's mine, which is at present the only important undertaking on the field, varies in thickness up to over 30 feet. It is, however, somewhat dislocated, and the dip varies from a moderate inclination up to 45 degrees. The coal is a lustrous, black, compact fuel, with a conchoidal fracture. It ignites readily, and forms a first-class household and locomotive coal; but on account of the contained water, which causes it to ultimately break up on exposure, it is not fitted for long sea page 92voyages. The roof is hard coarse conglomerate, 70 feet in thickness, and auriferous, but not sufficiently rich to be workable. The floor is hard clay.
Until quite recently only three workable seams have been known to exist in the field; the uppermost 3ft. 6in, in thickness, and the main seam, 500 feet below, which has been already mentioned. About intermediate between the two is a 9ft. seam, which has not been worked. "Within the last few months, however, a seam 19 feet in thickness has been discovered 150 feet below the main seam.
The average analysis of the Kaitangata coals is as follows:—
The total output from this field to the end of 1888, was 542,123 tons.
The Shag Point coal-field lies to the north, at a distance of about 35 miles, and has been worked for a number of years. The coal seams dip E.S.E. at lodeg. below the sea, where they have been worked to a small extent, and where in all probability a very large deposit exists. Three seams have been worked, the-maximum thickness having been about 12 feet, but they vary considerably both in thickness and in relative position. Quite recently a valuable seam is stated to have been met with, in a borehole, which was put down below what was the lowest known seam. The measures rise into the hills bordering the coast, and form an anticlinal arch, which is terminated to the westward by a syncline, from the base of which they again rise at high angles to Puke Iwitai. Out to landward side of the hills is a recently-established coalmine, from which the excellent fuel usual in the-locality is obtained. According to late investigations by the Geological Survey Department, the field should extend to a considerable distance southward. Recently a number of samples of Shag Point coal have been analysed in the Colonial Laboratory Wellington, with the following results:—page 93
|A. Analysed as received.|
|B. Analysed after exposure.|
The total output from this field is 201,184 tons, to the end of 1888.