Art. LIV.—On the Probability of Finding Extensive Coal Deposits within the Province of Wellington.
[Read before the Wellington Philosophical Society, October 4, 1875.]
Where coal is found in New Zealand I believe it invariably underlies the cucullœa beds, not that the sequence is necessary, because of course these beds may rest upon other rocks; but they may, at all events, be considered as an indication, and as showing the possibility of coal being present.
If we strike a line from the Mokau coal seams through the outcrop of the same mineral at Ohura and Tangarakau, the rivers of these names falling into the right bank of the Whanganui, we shall find a line of fracture and of fault, the coal seams dipping from that line, I think, to the south-west, that is to say, as far as an imperfect observation of some of them only enabled me to judge.
No coal as yet has been found on the eastern side of this line.
Now, it is reasonable to suppose that this line of fracture does not mark the eastern limit of the coal seams; but that these may be found to an unlimited distance further east, even up to the flanks of the Ruahine.
The cucullœa beds are found to the eastward of this line in the Whanganui River, and I think the same formation forms the prevailing rock of the Rangitikei, although it is long since I ascended that river, my fossils went astray on account of non-delivery by the Maoris, and, therefore, although the formation is similar in appearance to that on the banks of the Whanganui, I am not prepared to prove that it is of the same age.
We will extend the line above mentioned into the valley of the Waikato, and call it the western line of strike. The coal seams along this line of strike may be held, I think, to dip to the westward, and probably underlie all the country to the westward, although broken through and destroyed by the igneous rocks of Mount Egmont and other volcanic cones.
With regard to the Province of Wellington the problem to be solved is this Does the western line of strike mark the limit of the coal seams towards the east, or do they underlie the cucullœa beds and the tertiaries to a greater or lesser distance further in that direction?
The probabilities are rather greater than less that the coal seams underlie the whole of the country from the western line of strike as far as the
flanks of Ruahine—a great area of, say seventy miles by forty, about one million and three-quarters of acres.
But it is no easy matter to prove whether the coal measures underlie in this area or not. The upper tertiaries attain a great thickness throughout, and as they, doubtless, lie uncomformably on the coal measures, it would be very difficult at any one point to form an estimate of the depth at which the coal seams might be expected.
Possibly a lengthened and steady examination of the country by a competent geologist might show some outcrops of the coal, or, what is just as likely, this might be discovered by accident.
It would undoubtedly be a matter of great importance, now that the country is being opened by railways, to discover an extensive coal field on the West Coast, even should the coal be of medium quality only. It would probably be of similar character to that of the Waikato.
I believe that no outcrop of the coal has been found to the southward of Tangarakau. It would be of importance to find it nearer the coast, even as far west as the line of fault; but the thickness of the upper tertiaries probably presents the same difficulties there as those before mentioned.
It is by no means impossible that coal may be found on the eastern or Wairarapa side of the main range. The cucullœa beds are found in the valley of the Pahaoa River, and I think also in those of the Tauheru and the Whareama; but, certainly in some cases, and possibly in all, these beds rest on rocks older than the coal measures, probably triassic. Still the subject is worth investigation.
The western line of strike and of outcrop from Tangarakau, even as far as the Thames, is certainly a most remarkable fracture as regards its length and exposure of the outcrop of the coal seams. Of course the actual outcrop may lie a few miles either east or west of the line, as a lengthened fracture of rock could hardly be in a strict mathematical straight line; but the actual approximation to straightness of the line is very striking.
One difficulty in the discovery of what exists below is that the rivers lying between the Whanganui and Ruahine Range, viz., the Wangaehu, the Turakina, the Rangitikei and its tributaries, do not cut so deep into the tertiaries as the Whanganui River does, therefore prima facie, an exposure of an outcrop of coal is more likely to be found in the vicinity of the latter river than of those lying further to the east.
But the area is large, besides being densely wooded and difficult to traverse, and local dislocations may possibly be found to expose the coal seams if they exist.
Having now disposed of our local coal question, I will make a venture into theory and into the question of the origin of coal. The generally received
theory on the subject, I take to be, that at the time of the deposition of the coal of the old European carboniferous formation, the earth still retained a great deal more of its original heat than now obtains, that, in consequence, dense vapours and a damp hot atmosphere prevailed, favourable to the growth of an extremely luxuriant vegetation, and from this accumulation of carbonaceous products the coal resulted.
Now, if this theory be correct, we should expect to find the same conditions extending over both hemispheres, and I think also we might expect the greatest coal formations in the tropical regions. What we actually find is as follows:—Large areas of carboniferous palæozoic coal in the Northern Hemisphere, in Europe, in America, and in China; I think I may say no palæozoic coal within the tropics, and possibly none in the Southern Hemisphere.
In Africa, so far as I know, no coal is found; in Australia, the coal is claimed by the Rev. W. B. Clarke to belong to the palæozoic era, while Professor M'Coy asserts that it is of triassic age. As Professor M'Coy is a palæontologist, and as the question is one of palæontology, prima facie we may assume that his view is the correct one.
In New Zealand the coal is of upper mesozoie age, probably lower cretaceous, and in South America all the coal that has been found is, I believe, of tertiary age, although, from the absence of books of reference, I am not able to speak positively as to its exact place.
This, however, is comparatively immaterial. The question of the existence of true palæozoic coal in the southern hemisphere lies between the arguments of Professor M'Coy and of the Rev. W. B. Clarke as to the age of the Australian coal.
But if we require a high temperature for the earth to form the palæozic coal, shall we not also require high temperatures for the formation of the secondary and tertiary coals?
We have a school of geologists who are very strong in argument in the present day as to the effects of cold glaciers and ice sheets covering, the surface, not only in temperate regions, but even within the tropics.
Now these changes of temperature from hot to cold and then to increased warmth are inconsistent with a theory of changes from secular cooling, which ought to be constant in one direction, viz., from heat to cold, and ought not to show capricious action from heat to cold and then to heat again.
But is there not something weak in the original theory, viz., that the origin of the old carboniferous coal was caused by the dense vegetation of a period when the interior heat created dense vapours and a warm moist atmosphere, and thus produced an excess of vegetation.
Let us think over the matter. To produce a dense atmosphere from interior heat, it seems to me that we must go the length of supposing the waters of the ocean to be raised to the boiling point. Unless raised to that point, I do not see that the evaporation would be much greater than at present. Then, with the water at the boiling point, how are we to manage enough cold for precipitation?
With water at the boiling point, how are we to account for the existence of the corals, of the crustaceans, of the molluscs, etc., of the coal measures? how account for the similar organisms, including fish, of the old red sandstone and of the silurian rocks? These old molluscs and fish may have been good eating when boiled, but could not very well live and reproduce their species in boiling water.
Again, with the ocean at the boiling point, what would be the effect of the internal heat on the land? I would suggest that it would be to deprive the surface of moisture, which would be driven off in a state of steam, and would therefore render it unfit to support vegetation, instead of, as is supposed, to maintain that of a rank and luxuriant nature. A moderate increase of external heat, that is to say, of heat derived from the sun, would produce great changes, whether favourable to the production of a dense vegetation or not, might depend upon circumstances, but a vast increase of internal heat must be assumed to make any perceptible change in the climate as regards the growth of plants.
I may, however, be combating a shadow, because we have only to refer to Lyell to find an opinion of primary authority as to the origin of coal. He states as follows—“So long as the botanist taught that a tropical climate was implied by the carboniferous flora, geologists might well be at a loss to reconcile the preservation of so much vegetable matter with a high temperature, for heat hastens the decomposition of fallen leaves and trunks of trees, whether in the atmosphere or in water. It is well known that peat, so abundant in the bogs of high latitudes, ceases to grow in the swamps of warmer regions. It seems, however, to have become a more and more received opinion that the coal plants do not on the whole indicate a climate resembling that now enjoyed in the equatorial zone. The ferns range as far as the southern part of New Zealand, and Araucaria pines occur in Norfolk Island. A great predominence of ferns and lycopodians indicate moisture, equability of temperature, and freedom from frost, rather than intense heat, and we know too little of the segillariæ, calamites, asterophyllites, and other peculiar forms of the carboniferous period to be able to speculate with confidence on the kind of climate they may have required.”
The above quotation fully disposes of the question of the formation of coal from an increase of vegetation caused by a supposed greater internal heat at the time of deposition, on the theory of secular cooking of the earth.
I should not have entered upon the question at all, had I not found, in the course of conversation, that many persons were fully persuaded of, and held as an article of faith, the idea of the growth of the coal plants from the effect of internal heat.
I hope that I have succeeded in showing the fallacy of that view, and that we must fall back upon the rays of the sun as the true producers of the coal vegetation. The existence of animal life in the palæozoic ocean proves that changes in the temperature of the ocean since that time, if any, must have been confined within narrow limits.