It is generally admitted that, omitting cannel coal, &c., all coals have been formed from vegetable matter of initially similar composition. The transformation of this vegetable matter is due to a fractional distillation, and the quality of the resulting coal depends on its relative completeness. The most generally recognized factors controlling this transformation are time, heat, and crustal movements.
It is important to distinguish between the effects of natural distillation and atmospheric weathering. The following table shows the progressive effect of natural distillation.—
|38·26||40·51||20·41||0·82||Charleston||Col. Lab., 29.|
|39·16||40·63||18·46||1·75||Giles Creek||J. Henderson.|
|42·70||41·00||13·70||2·60||Motupipi||Col. Lab., 41.|
|42·13||41·72||10·27||5·88||Golden Ridge||" 41.|
|46·60||43·32||8·87||1·21||Burke's Creek||" 28.|
|90·90||5·10||0·80||5·20||Fox's River||" 35.|
It will be noted that the change from the brown coals to the best of the sub-bituminous is accompanied by a diminution of the water and an increase in both fixed carbon and hydrocarbons. Further change takes the form of an increase of fixed carbon at the expense of the hydrocarbons, the small percentage of water being decreased very slowly. With atmospheric weathering, on the other hand, the percentage of water is increased; but the main change takes the form of a decrease in the hydrocarbons, causing an apparent increase in the fixed-carbons percentage. A comparison of the odd numbers with the next following even numbers in Table B will make this clear as far as sub-bituminous and bituminous coals are concerned.
|1||54·31||33·81||10·46||1·42||Burke's Creek||J. Henderson.|
|2||42·42||49·20||7·28||1·10||Same seam||Col. Lab., 42.|
|5||50·00||38·70||5·80||5·50||Rise, Point Elizabeth||" 38.|
|6||44·08||43·00||5·85||7·07||Dip, Point Elizabeth||" 41.|
A glance at Table C, on page 305, will show that the coals of west Nelson have a very wide range of composition, and it is to this wide range in the qualities of the various coals that the confusion of classification of the beds in the past has been mainly due.
Von Hochstetter * divided the coals of west Nelson into two series—Mesozoic and Tertiary—mainly on account of the difference in composition. Von Haast† does the same. Cox‡ divided the coals between the Lower Greensand and Cretaceo-tertiary on stratigraphical grounds; but, evidently influenced by the difference in composition, Hutton§ placed the coals of Nelson and Motupipi in the Oamaru series, and those of Pakawau, Wangapeka, Westport, Greymouth, and Reefton in the Amuri series, of Cretaceous age. Hector‖ has pointed out the anomalies connected with this classification, and has shown that, as far as west Nelson is concerned, the palaeontological evidence upon which Hutton relied for his classification was very incomplete. Park¶ at first recognized two coal-horizons, but now, as the result of later investigation, places the Wanganui and Inangahua, Westport, and Greymouth coals in the Oamaru series, of Lower Miocene age. Von Ettingshausen, ** from an examination of the fossil plants, considered the strata at Pakawau, Wangapeka, Greymouth, and Reefton of Cretaceous age. Hector†† placed all the coals of west Nelson at the base of his Cretaceo-tertiary, but considered that the coals occur in an upper and a lower horizon. Mackay‡‡ placed the seams in the Cretaceo-tertiary, and did not express any opinion as to their occurrence in different horizons.
From the above it will be seen that very considerable difference of opinion has existed as to age and relationships of these beds.
The writer will attempt to show that the coal-seams may occur in one series of beds. Wherever the basement rock of the coal series is exposed the coal-seams rest either hard on the basement rock or on sandstones and conglomerates immediately overlying it. Such is the case in the Pakawau field, at Motupipi, Nelson, the Owen, Reefton, Charleston, Denniston, and other points. In the Greymouth field the semi-anthracites of Paparoa, the sub-bituminous coals of Blackball, and the brown coals of Moonlight Creek all lie very near the basement slate.
At Point Elizabeth the rocks are downfaulted, and the coal rests upon a considerable thickness of sandstone and shale. These lower beds may possibly represent the coal-measures of Mount Davy. At West Wanganui the coal overlies sandstone, &c., but the basement rock is nowhere visible. These coals have been downfaulted, as is indicated by the difference in strike of the comparatively elevated outliers near Golden Blocks, and there is nothing to show that the West Wanganui coals overlie these or the Pakawau coals.
Again, with the seams at Moonlight, Blackball, and Paparoa, which are taken in ascending order of elevation and carbonization, it is difficult, if not impossible, to account for their relative positions except on the assumption that they all belong to the same horizon and owe their present positions to faulting, and their various compositions to different distillation-conditions. Again, in no section do the brown or sub-bituminous coals actually overlie the bituminous seams, nor do the limestones—which at
[Footnote] * Hochstetter: “New Zealand,” pp. 58, 59, 85.
[Footnote] † Haast: Geology of W. Nelson.
[Footnote] ‡ Cox: Geol. Surv., No. 15, pp. 71–73; No. 16, pp. 5–8.
[Footnote] § Hutton: Trans. N.Z. Inst., vol. 22, p. 387.
[Footnote] ‖ Hector: Geol. Surv., No. 21, p. xxxv.
[Footnote] ¶ Park: “Geology of New Zealand,” 1910, p. 293.
[Footnote] ** Von Ettingshausen: Trans. N.Z. Inst., vol. 23, p. 241.
[Footnote] †† Hector: Geol. Surv., No. 18, p. xxxii et seq.; No. 21, p. xxxv et seq.
[Footnote] ‡‡ Mackay: Geology of S.W. Nelson, pp. 57–61; Papers and Reports relating to Minerals and Mining, 1900, C.-6, p. 4.
many places overlie, perhaps unconformably, the so-called upper seams—at any place overlie the lower bituminous seams. All these things point to the conclusion that the coals of west Nelson belong to one series, of what age is here immaterial. Some other agency than time must, then, be looked for to explain the differences in composition of the coals.
That heat is competent to produce all the changes in coal-composition is well known. Its effects are well shown at Malvern,* where a brown coal has been altered by a volcanic dyke. Such action, however, is purely local, and cannot explain the varieties of coal in west Nelson. Again, the deep burial of coal beneath other rocks, and the consequent increase of temperature, greatly hastens the distillation process. Such a theory is, however, quite inapplicable to west Nelson, where none of the coal-measures are, or appear to have been, deeply buried, and where the occurrence of the more highly carbonized coals on the higher levels seems rather to contradict the theory.
Considering, then, crustal movements: these no doubt have great influence both from the pressure exerted and the heat engendered thereby. Probably the anthracite of Fox's River and the plumbago of Pakawau† have been produced by the action of great faults. But if such be the controlling factor of this problem it is to be expected that the coal near Nelson, which is actually inverted and entirely crushed by the alpine overthrust, would be highly carbonized. It actually contains 53 per cent. of fixed carbon and 10 per cent. of water. Again, the coals of Blackball and Paparoa, separated by a fault, which presumably affected them equally, contain 50 per cent. and 76 per cent. of fixed carbon respectively. Evidently crustal movements are incompetent to account for the variations in composition of the coals.
Time, heat, and pressure have been shown to be inadequate of themselves. Another condition controlling the rate of distillation is the ready escape of the distillation volatile products. These volatile products would have opportunity to escape if the overlying strata were porous or fissured. Porosity in a rock will have little influence where great thicknesses are concerned. Great thicknesses of porous rock do not, however, overlie the coals of west Nelson, the principal overlying rocks being mudstones, and wherever these have been wholly or in part removed the coals are highly carbonized.
Applying this hypothesis to the west Nelson coalfields, we find that in the Whakamarama field the coals of the Pakawau section occur in conglomerates capping the tops of the ranges, the upper more impervious mudstones and limestones, which at one time probably covered them, being removed. The coals are highly carbonized, but on the western dip of the anticline, and to the north where it plunges, the overlying impervious beds are still in existence, and the coals merge into sub-bituminous and even brown coals. The Taitapu field has always been depressed, and the overlying impervious covering is being added to, hence the coals are brown coals. Similarly, in the Whakatu field the main central portion of the field will contain brown coal, although round the edges of the basin coals of all qualities may be found, the degree of carbonization depending on local circumstances. In the Kawatiri field the bulk of the coals will be bituminous, but in parts of the Mangles section and towards the west generally the coals may grade to brown coals beneath the limestones, &c.
[Footnote] * Evans: Trans. N.Z. Inst., vol. 31, p. 557.
[Footnote] † Cox: Geol. Surv., No. 15, p. 71.
The coals of the eastern Oweka field are more elevated than those in the central section, and the upper beds of the measures have been removed. Thus the coals are superior to those of the central portion, which in turn grade from sub-bituminous on the east to brown coals on the west. In the Greymouth field the central elevated ridge carries semi-anthracites to bituminous, the degree of carbonization decreasing towards the south as the elevation decreases. South of the Tyneside the coals will probably be sub-bituminous, as are those of Blackball and Point Elizabeth. In the latter place mudstones and limestone overlie the coal. Similarly, in the Westport field the coals of the Mount Rochfort plateau decrease in carbonization toward the north, where they will be overlaid in depth by mudstones and become sub-bituminous or brown coals. The seams of the coastal section are likely also to be brown coals, while those of the Orikaka section will grade from sub-bituminous to bituminous according to local conditions. The anthracites of Fox's River, which may be included in the coastal section, are probably of purely local occurrence. Only brown coals have hitherto been reported from the Karamea field, but it seems feasible to suppose that bituminous coals may occur on some elevated ledge.
From the above considerations, and from the analyses shown in Table C, the following table may be prepared :—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Field.||Section.||Analyses.||Quality of Coal.|
|Wanganui||1, 2, and 3||Sub-bituminous to brown.|
|Whakamarama||Pakawau||4 and 5||Bituminous to sub-bituminous.|
|Taitapu||‥||6 and 7||Brown.|
|Whakatu||‥||8, 9, and 10||"|
|Kawatiri||Mangles||11||Bituminous to sub-bituminous.|
|Glenroy||12 and 13||Bituminous to brown.|
|Eastern||14, 15, and 16||Sub-bituminous.|
|Oweka||Central||17, 18, 22, 23, and 24||Sub-bituminous to brown.|
|Greymouth||Brunner||26, 27, 28, and 30||Semi-anthracitous to bituminous.|
|Orikaka||19, 20, and 21||Sub-bituminous to brown.|
|Westport||Mount Rochfort||35, 36, and 37||Bituminous.|
|Coastal||31, 32, 33, and 34||Brown.|
Not even the roughest estimate of the quantity of coal available in west Nelson can be given. This is due in part to lack of data, but principally to the irregular distribution of the seams or lenses of coal throughout the measures and the rapidity with which the thickness of the seams vary. At Denniston only one-tenth of the area of coal-measures contains workable coal. There is, however, little doubt but that many hundreds of
millions of tons are available. The bulk of this will consist of brown coals in no wise superior to the brown coals of the rest of New Zealand. Of the remainder the greater proportion will be sub-bituminous in quality. The comparatively small areas containing bituminous and anthracitous coals are elevated and geologically accessible, and because of this the quantities of coal they contain are approximately known.
|1||35·76||43·63||15·40||4·21||3·86||Turimawiwi||Col. Lab., 25.|
|8||53·59||33·80||10·20||2·41||‥||Enner Glynn||" 30.|
|16||56·18||32·24||9·61||1·97||‥||Murray Creek||" 22.|
|17||42·42||49·20||7·28||1·10||3·89||Burke's Creek||" 42.|
|21||64·06||11·59||10·14||14·21||‥||Hawk's Crag||Col. Lab., 29.|
|22||39·16||40·63||18·46||1·75||0·41||Giles Creek||J. Henderson.|
|23||41·58||35·79||20·21||2·42||‥||Little Grey||Col. Lab., 29.|
|24||39·23||30·30||20·06||10·41||‥||Slaty Creek||" 29.|
|27†||59·23||33·33||2·11||5·33||3·36||North Brunner||J. Henderson.|
|28||58·00||37·83||0·37||3·80||1·96||Tyneside||Col. Lab., 38.|
|29‡||50·79||38·23||7·90||3·08||0·44||Point Elizabeth||" 38.|
|30||59·27||35·34||2·34||3·05||0·28||Mount Davy||" 41.|
|31||90·90||5·10||0·80||5·20||‥||Fox's River||" 35.|
|34||26·83||35·31||18·24||19·62||‥||Cape Foulwind||" 29.|
[Footnote] * Mean of five.
[Footnote] † Mean of seven.
[Footnote] ‡ Mean of eleven.
[Footnote] § Mean of sixteen.