Other formats

    TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Report on the Geology & Gold Fields of Otago

Coal

Coal.

Although Otago does not possess any first-class coal suitable for steamers making very long ocean voyages, it is abundantly supplied with very excellent brown coal, lignite, and peat; all of which are very valuable for different purposes.

Peat.—Very good peat is found on many of the mountains, especially on the Rock and Pillar, and Rough Ridge. Its importance in a treeless country cannot be over estimated, and I am of opinion that some steps should be taken to ensure its being worked to the best advantage, for at present a considerable waste is going on by allowing inexperienced cutters to take what they like.

Lignite of pliocene age is found round the margins of nearly all the old lake basins, as at Kyeburn, Idaburn, Hills Creek, Welshman’s Gully, Lawrence, Clyde, Cromwell, Bannockburn, Nevis, Cardrona, and Gibbstown. It no doubt exists in many other places in these basins, but so covered up by gravel as to be difficult to find. The second way in which lignite occurs is in the older river valleys that existed before the time of the great newer pliocene depression. Such are the lignites of the lower Mataura, at Waimea Plains, Waikaka, Mataura Falls, &c.; of the Clutha, at the Teviot; of the Shag River, at Glovers; and of the Waitaki, at Awamoko. In this division we may also place the lignites of Fairfax, and Lovell’s Flat, near Tokomairiro. I have not heard of any page 97having been found up the Waiau, but it probably exists there. In the newer river valleys, such as the Lower Ahuriri, Dunstan and Kawarau Gorges, Oreti and Jacob’s Rivers, it is not likely to occur, as these were not in existence during the great depression when the gradual diminution in the velocity of the rivers favoured the formation of swamps, and consequently the growth and accumulation of vegetable matter on their margins. The lignites near Invercargill, and at Orepuki, may have been formed as estuarine deposits very slightly above the sea level, or they may have been formed by swamps when the land was considerably higher.

These lignites vary very much in quality. Occasionally well preserved trunks of trees are found in them, and generally their vegetable origin is easily recognised by the naked eye; but occasionally they pass into a compact brown mass in which no structure is visible, and which cannot be distinguished from brown coal. They also often contain large lumps of fossil resin, or retinite, which is a proof that they have been formed to a great extent by the decomposition of coniferous trees; but I am not aware that any attempt has yet been made to determine the plants found in them. When this is done we shall be able to form a better opinion as to their origin.

The thickness of these seams is often very great, amounting to 30 feet in some places, but like all lignite beds, they are local and unconnected, and there is no certainty of any of them continuing far.

The following are the only analyses of these lignites that I have been able to find.
—— Totara Creek, Omaru. Watahuna. Tuapeka. Idaburn. Average.*
Water 13.40 11.06 16.80 11.60 13.75
Fixed Carbon 39.10 37.28 34.40 16.97 36.92
Hydro-Carbon 44.70 39.88 44.40 41.85 42.99
Ash 2.80 11.81 4.40 29.58 6.33

These lignites always contain a considerable amount of sulphur, and give out a very unpleasant smell when burning.

Lignites belonging to the Pareora formation are found at Coal Creek in Shag Valley, and in the Pomahaka, and probably that at Kauroo, near Oamaru, is of the same age. I am not sure whether the lignite of Wharie Creek, in the Takatimu Mountains, should not be

* Omitting Idaburn, which must have been a very bad sample.

page 98considered as a brown coal, and I am also uncertain whether it occurs under a pliocene lacrustine deposit, or whether it is covered by rocks belonging to the Pareroa formation.

Brown Coal of eocene age is found in Big Gully Creek in the Waitaki, where it was worked in 1861 by Mr. A. Geddes, and at Green Island and Saddle Hill; at Tokomairiro, Kaitangata, and Coal Point; at Orepuki, and at Taylor’s Creek, Morely Creek, and the Nightcaps in Southland. Thin seams of no value have also been found among the trachytes in Dunedin Harbour. The brown coal at Shag Point is of upper cretaceous age, while that of Preservation Inlet may be either eocene or upper cretaceous, probably the former. These localities include all the important coal fields of Otago, and I shall describe them subsequently. The coal in many of these fields is covered by marine rocks, and in this respect differs from the lignites already mentioned. It differs much in quality, but even the best is not first-class coal, its great fault being that on exposure to the weather it falls to pieces, and therefore it cannot be stored in large quantities, but must be used soon after extraction from the mine. Nevertheless it is a very valuable fuel, and well adapted for all household purposes, stationary engines, and river steamers. In Europe it is also sometimes used for smelting iron and for locomotives. It will also answer equally well for coasting steamers that do not require to lay in a large supply at a time. Its heating power is inferior to that of true coal, but this will be amply compensated for by its greater cheapness, and it will no doubt come largely into use throughout the Province when the railway system is more developed. The most economical way of burning it appears to be in a fire-box with an inclined perforated iron-plate instead of fire bars or a grate. The causes of the defects that I have mentioned are not well understood, although they are known to be connected with the amount of water in combination with the coal. They cannot be due simply to its age, as was formerly the general opinion, for in the valley of Zsil, in Transylvania, beds of real anhydrous coal occur, which are of tertiary age, but which have none of the defects of brown coal. In New Zealand also the eocene coals of the Bay of Islands and Whangarei are much superior to some of the cretaceous coal of Malvern Hills and Shag Point. Disturbance of the beds and subsequent drainage seems to have a very beneficial effect on coal. For instance the coal of Mount Hamilton, which is situated in a disturbed area, and near the top of a mountain, is far superior in quality to that of Shag Paint which has been less disturbed, although both are of the same age. Again the coals of Kaitangata, Tokomairiro, and Morely Creek, which are elevated above the drainage level, are superior to that from Green Island, which is situated below the drainage level. These brown coal deposits differ from the lignites in being more regular in thickness, and in extending over large areas, thus enabling them to be mined page 99with greater advantage. Coal mining in the Province is at present a very simple affair, as the mines have only lately been opened, but every year will increase the difficulty, and it is very important that the mines should now be opened in the best possible manner so as to insure in future years the winning of the greatest amount of coal with the least expense. That some of the coal mines are not now being worked in a satisfactory manner is owing partly to the great expense for carriage, causing a limited local demand only, which would not justify any great expenditure of money in laying a mine out properly; but partly also, in my opinion, to leasing the coal pits for short terms, so that the lessee has no inducement to go to any expense in order to open out his mine properly, for the only effect of this proceeding would be to enable the lessor to raise the rent when the lease fell in; and it is more advantageous for the lessee to limit himself to getting the coal out at once as cheaply as possible, without any reference to the future. The first cause will disappear with the opening of the railways, but the second cause can only be overcome by either selling the coal mines or by leasing them for long terms, such as 99 years. The first method would, in my opinion, be the most advantageous where no monopoly is to be feared, as all along the coast line, or where several pits are situated in the same locality; for when a large sum of money has been sunk in buying a mine, the owners are not likely to let it lie idle. But in the interior of the Province, where there is no firewood, and where there are only one or two mines in the district, the 99 years lease system would probably be the best, as the Government could then regulate the price of the coal at the pit’s mouth.

Bituminous Coal of jurassic age is found in places from the Hokanui Hills to Catlin’s River, and the same formation extends under a considerable part of the plains between the Hokanuis and the sea, but it is here covered up by a thick deposit of gravel and silt. This includes an area of more than 1000 square miles, but over the whole of it no seam thick enough to pay for working on a large scale has as yet been found. Such may, however, exist, but as each year goes by without finding it, the probability of its existence rapidly diminishes, especially as no good seam has been found in this formation in any part of New Zealand.

Up a small creek flowing into the Otapiri, three thin seams have been discovered, none of them however exceeding six inches in thickness. Higher up there exists, according to Mr. J. R. Thomson, a seam of carbonaceous shale about four feet thick, but without any good coal in it. West of the Makarewa, coal has been found in several localities, but here also it does not exceed a foot in thickness. Coal has also been found at the Seaward Downs; and at the Toi-tois, near the mouth of the Mataura, Mr. Brunton, C.E., has been at considerable expense in exploring some outcrops of coal page 100by drives and a shaft. Unfortunately his success has not, as yet, been very great, the coal seams being spoilt by numerous shale partings. The following is the section here as I made it out in January, 1872:—
feet.
Conglomerate 200 +
Green sandstone 200 +
Fire clay, about 6
Coal with shale partings
Dark grey shale 40
Coal (7 thin seams with shale) 6
Green sandstone, about 20
Conglomerate 15 +

Coal has also been found at Waikawa, and in the neighbourhood of Catlin’s River, but always in thin seams.

The following analyses have been made by Dr. Hector and Mr. W. Skey, of coals from this district:—
—— Catlin’s River. Seaward Downs, 2 analyses. Waikawa, average of 4 seams. Average of the whole.
Water 4.20 8.93 8.95 8.23
Fixed Carbon 59.40 55.50 44.43 49.73
Hydro-Carbon 35.00 31.34 37.28 35.27
Ash 1.40 4.22 12.07 8.31

These analyses shew that, even if a thick seam with less ash were found, still the coal would probably be very inferior to that of Newcastle (N.S.W.). At present we cannot include this district among our coal fields.

Mount Hamilton.—Bituminous coal of upper cretaceous age but of good quality is also found at Mount Hamilton, but unfortunately the thinness of the seams, and the small extent of the for matron put the practibility of working it quite out of the question It is situated near the summit of Mount Hamilton, at an elevation of 2,500 feet above the plain of the Oreti. (See Fig 4.) The coal formation is composed of yellow sandstone with thin seams of coal, page 101underlaid by shales with several seams of good black coal, the uppermost and thickest of which is only ten inches in thickness. The upper sandstone is fine grained, but in it I noticed a large angular boulder of dark gray sandstone—(fig. 22)—measuring eight feet by 3 feet in the exposed face, no other pebbles of any kind were near it, and it rested on the top of a thin coal seam. I presume that there can be no doubt but that this boulder has been floated to its present position among the roots of a tree, and that therefore the coal beds are formed partly from drift wood.*

Fig. 22.—Boulder in sandstone: a, sandstone; b, coal; Waipara formation: c, sandstone boulder.

Fig. 22.—Boulder in sandstone: a, sandstone; b, coal; Waipara formation: c, sandstone boulder.

The following is the mean of two analyses of this coal made by Mr. W. Skey:—
Water 1·63
Fixed Carbon 49·04
Hydro-Carbon 38·02
Ash 9·80

A small seam of bituminous coal has also been found on Coal Island, Preservation Inlet, and I shall mention it again when noticing that coal field.

* Dr. Hector reported on this coal to the Provincial Government of Otago in January, 1872, and said that it belonged to the same formation as that of Otapiri and Waikawa. (Provincial Government Gazette, January 17, 1872.)