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Volume 21, 1888
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Art. XXXVII.—On the Extent and Duration of Workable Coal in New Zealand.

[Read before the Wellington Philosophical Society, 12th Sept., 1888.]

At the present time, when the Australian Colonies are looking to New Zealand for their supplies of coal, it may be of some interest to consider what position we are in to meet this new demand. I hope to be able to show that, whatever may be said as regards the extent of our metalliferous deposits, we are at least supplied with an abundance of fossil fuel sufficient to meet all our requirements for many years to come.

All the workable coals of this country belong to the Cretaceo-tertiary formation, of the Geological Survey classification, which consists in many places of two distinct groups of beds, differing widely in their mineral characters, in the general sequence of their strata, and in their fossil remains, the one being characterised by a fauna and flora with a distinctly Tertiary facies, the other by forms of an equally-pronounced Secondary type. The relation existing between these two groups of beds has not been very satisfactorily determined; but they are at present supposed by the Survey to be in a manner horizontal equivalents—that is, the result of contemporaneous deposition, the Tertiary strata being taken to represent the shallow-water and the Secondary strata the deepwater conditions of the same period.

How far this theory will meet the stratigraphical and palæontological difficulties of the case, considering that both these groups are sometimes found to exist in the same areas, I do not propose to discuss in this paper; it is of great importance, however, to note that, with one or two exceptions, all our workable coals occur at the base of the group with a Tertiary facies.

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The age of our coal presents a marked distinction from that of the great coal-deposits of England, continental Europe, and North America, which occur in strata lying between the Old and New Red Sandstones. In the early part of this century so imbued were geologists with the idea that true coals were confined to this horizon that the coal-bearing strata received the age-name “Carboniferous,” which is now generally applied to all rocks of this period, whether they contain carbonaceous deposits or not.

The subsequent discoveries of true coal in Lower Secondary strata in New South Wales, in Jurassic strata in India, and Upper Secondary strata in New Zealand, conclusively showed that, given the necessary geological conditions, coal could be formed at any period of the earth's history. Up to the present time no coal-seams have been found in rocks below the Devonian, and from this circumstance it is argued by some scientists that there must have been a scarcity of carbonic acid on the earth's surface prior to this period—too little, in fact, to favour the growth of great forests or dense vegetation of any kind. However true this may be of the Old World, it certainly does not apply to New Zealand. Among the Silurian schists and marbles of Western Otago, which are simply altered sandstones and limestones, there occur layers and nests of graphite under conditions which leave little room for doubt that they are the product of altered carbonaceous matter of vegetable origin.

It is now generally admitted that all coals rest on old soils or land-surfaces, and consist of nothing but vegetable matter. Judging from the leaf-impressions in the coal-shales, it is probable that our coals are principally the result of forest-vegetation of long-continued growth, among which dicotyledons are largely represented, and after these cycads, conifers, and ferns.

With one or two exceptions, the coal-deposits of this country occur near the base of the measures, which generally rest on the basement-rock of the district, showing that the forests grew on a long-persistent and comparatively stationary surface, with perhaps in most cases a tendency to a downward movement. After a period of rest, during which the carbonaceous matter accumulated, the land began to sink, and from the character of the estuarine and marine strata which cover the coal it can be ascertained that these old forests flourished on low-lying areas contiguous to the sea, or in deep estuaries or bays to which the sea had free access. The marginal or littoral character of our coal-areas can be seen at a glance by looking at the mineral map of New Zealand issued with the Geological Reports for 1886–87.

As a result of our coals having been formed on old land-

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surfaces, the seams necessarily partake of all the irregularities of the land, and are consequently subject to great variations of thickness along the line of outcrop. It is also noticeable that where the land is steep the seams thin out rapidly to the dip.

It is a remarkable fact that, although the workable coals of New Zealand are all of the same age, they differ widely in their mineral characters and composition: for example, those of Otago are hydrated brown varieties, sometimes little better than lignites, while those of the west coast of the South Island are anhydrous or bituminous coals, mostly of fine quality, and in some respects superior to the coals from New South Wales. It should be stated, however, that the different varieties shade into each other: thus we have brown coals which exhibit an approach to semi-bituminous coals, which in their turn merge into true bituminous or caking-coals.

It would be difficult to define a dynamic agency competent to produce the metamorphism of the coals of the Grey and Buller coalfields, and at the same time so exclusive as to restrict its operations to these areas.

When this interesting question receives more attention, it will, I think, be found that the quality of the coal is largely influenced by the character of the enclosing strata: thus, when the measures are loose and porous the decomposition of the vegetable matter will probably result in the formation of lignites or hydrous brown coals, such as those of Otago; when greensands of a less pervious nature, a better class of coal will be formed, of which examples may be found at the Mokau, Waipu, Whangarei, and Kawakawa coalfields; and when heavy deposits of impervious fireclays, the result will be bituminous coals.

As the result of a large number of analyses in the Colonial Laboratory, Dr. Hector in 1872 classified the coals of New Zealand as follows:—

I. Hydrous (coal containing from 6 to 20 per cent. of permanent water)—

a.

Lignite.

b.

Brown coal.

c.

Pitch-coal.

II. Anhydrous (coal containing less than 6 per cent. of water)—

a.

Glance-coal.

b.

Semi-bituminous coal.

c.

Bituminous coal.

The workable coals come under three principal divisions—namely, (1) brown coal, (2) pitch-coal, and (3) bituminous coal. These varieties are distributed in what may be termed

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geographical areas of deposition; thus, the brown coals occur principally in the eastern portion of the great axial division of the South Island, the bituminous coals in the west coast district of Nelson, and the pitch-coals in the North Island.

In preparing the following estimate of workable coal I have not included lignites, nor coals of any kind where the thickness of the seams is less than 2ft. These will no doubt be of great importance when the coalfields become exhausted; but until then they will have no market-value, and will probably be little sought for except for purely local consumption where other kinds of fuel are scarce:—

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Brown Coal (Table No. 1).
Name of Coalfield. Author of Estimate. Amount of Coal in Tons.
Waikato Hutton 140,000,000
Drury From surveys by Cox 8,000,000
Waipa Park 10,000,000
Kawhia From surveys by McKay 4,000,000
Malvern Hills* Lindop 17,089,000
Kakahu Park 3,500,000
Oamaru-Waitaki From surveys by McKay 2,000,000
Shag Point v. Haast 1,000,000
Green Island and Saddle Hill Denniston 74,700,000
Clutha-Tokomairiro Hector 140,000,000
Wairaki Hutton 100,000,000
Orepuki " 5,000,000
Hokonui Park 1,000,000
Total 506,289,000

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Pitch-coal (Table No. 2).
Name of Coalfield. Author of Estimate. Amount of Coal in Tons.
Whangarei-Hikurangi From surveys by Cox 20,000,000
Waipa " 5,000,000
Mangawai " 6,000,000
Mokauiti Park 5,500,000
Mokau-Awakino " 210,000,000
Upper Wanganui " 50,000,000
West Wanganui Hector 96,000,000
Tadmor and Hope Park 10,000,000
Owen " 2,500,000
Inangahua From surveys by McKay 100,000,000
Maruia " Cox 20,000,000
Total 525,000,000

[Footnote] *The altered brown coals are included in this estimate.

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Bituminous Coal (Table No. 3).
Name of Coalfield. Author of Estimate. Amount of Coal in Tons.
Kawakawa From surveys by McKay 2,500,000
Pakawau " Cox 2,500,000
Collingwood " " 1,500,000
Mokihinui Hector 3,000,000
Buller Cox and Denniston 140,000,000
Grey Hector 37,500,000
Total 187,000,000

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Total amount of coal— Tons.
Brown coal 506,289,000
Pitch-coal 525,000,000
Bituminous coal 187,000,000
Total 1,218,289,000

After all necessary deductions for losses in working, &c., the total quantity of available coal may be set down at a thousand millions of tons, which at the present market-value of coal would represent about £750,000,000.

Until actual surveys are made, the above figures must only be looked upon as approximate estimates, but in most cases they are well within the mark. Most of the coal is level-free, and only such seams as are workable at the present time have been included in these returns. Ample allowances have also been made for areas of coal removed by denudation.

In addition to the coalfields mentioned in the above tables, small patches of coal-bearing measures occur at Takaka, Baton, Tiraumea, Karamea, and Lyell Mountains, in Nelson; at Waihaoa and Waipara, in Canterbury; and at Preservation Inlet in Otago.

Several thin seams of bituminous coal, ranging from a few inches to 15in. in thickness, occur in Jurassic rocks at the Hokonui Range, Waikawa, and Mataura. At the two former places prospecting operations were at one time undertaken to prove the extent of the coal, and quite recently a bore 131ft. deep has been put down at Rocklands, near Fortrose, with the same object. The seams in this formation are everywhere too thin to work, and, judging from the rapidly-alternating character of the strata, the land-movements at this period were too frequent to permit of the accumulation of large carbonaceous deposits, and for this cause little hope can be held out of thicker seams being found at these places.

Having determined the probable quantity of coal contained in our coalfields, we now arrive at the important problem,

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How long may such quantity of coal be reasonably expected to last?

If we turn to the records of our output since 1878, we shall find that the total quantity of coal raised in the colony in that year was 162,218 tons, and in 1881 337,262 tons, which is equal to an increase of 100 per cent. in three years. During the last seven years the growth of the output has been slower, and shows something of a geometrical or proportional rate of increase.

The latest and highest recorded output—that of 1887, amounting to 558,620 tons—bears but a small proportion to our vast stores of coal, which in fact contain the former 2,181 times. But a little consideration will show that it would be absurd to speak as if we had enough coal to last for more than two thousand years, since the present rate of consumption is not a fixed but a growing rate.

The production of coal in Great Britain in 1887, according to the reports of the Inspectors of Mines, was 162,119,812 tons, which is contained in the total estimated available quantity of coal—amounting to 146,480,285,398 tons, as determined by the Royal Coal Commission in 1866—no less than 903 times. With this material it has been estimated by various authorities that the coal of Great Britain must be exhausted at periods ranging from eighty to one hundred and fifty years from the present time.

In New Zealand the coal industry is still in its infancy, and it is in consequence impossible at the present stage of our output to determine whether the rate of our increase is geometrical or arithmetical.

It is obvious that the output is governed by two causes—first, the natural increase of our population, and, second, the growth of capital applied to the development and extension of our manufactures.

Starting from the actual output in 1887, the output at intervals of ten years up to 1957 would be as follows—assuming that the increase of output continues uniform with the average yearly increase for the last seven years, which is sufficiently near for our purpose:—

Tons.
1887 (actual output) 558, 620
1897 (calculated output) 910,000
1907 " 1,471,500
1917 " 2,397,000
1927 " 3,905,000
1937 " 6,503,000
1947 " 10,593,000
1957 " 17,250,000

On the assumption that the facts of the past seven years supply adequate indications of the law of consumption of the

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future, we arrive at the conclusion that in 1957—that is, sixty-nine years hence—our output would be thirty times its present amount, and that something like a third of the coal existing on these islands would by that time have been consumed; and it should be noted that this third would represent the most available, the most easily-worked, and the most valuable of our coal. By this computation our coal would be exhausted about the year 1978, or in ninety years from the present time. In 1978 the calculated output would be 48,100,000 tons; but it would be misleading to imply that this enormous output will come suddenly to an absolute stop. It is quite obvious that at some point in the interval the output must reach a maximum, after which, by the operation of certain causes which must act on the output, the production will gradually diminish till the inevitable point of exhaustion is reached.

Assuming that the low rate of increase indicated above continues in the immediate future, the choicest and most available half of our coal would be consumed in 1964, the total estimated output at that date amounting to 502,775,000 tons. In that year the output would be 24,275,000 tons; and, assuming that at this point the maximum output is reached, and that a geometrical decrease backwards commences from that year, the date of exhaustion would be somewhere about the year 2053—that is, 165 years from the present time.

It should always be remembered that the total quantity of available coal in New Zealand is only equal to about five years' output of Great Britain, and is, in fact, contained twice over in the coalfields of the county of Cumberland alone. It is only when our annual output is placed against the total amount that our stores of coal can be spoken of as vast or extensive.

Steam is the great motive-power of the period; consequently the growth and development of our manufactures and shipping must always be relative to, and in many cases caused by, the production of coal. As, therefore, the inevitable must come, whether in ninety years or 165 years, it is necessary that our coal-deposits should be worked with economy and with a regard to the future as well as the present prospects of this important industry.