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On the Coalfields of New South Wales
|Below is an extract from 'On the Coalfields of New South Wales' By Henry T Plews
NEWCASTLE ON THE HUNTER RIVER.
Returning to Sydney, and proceeding from there up the coast, to the North, the Sydney sandstone presenting abrupt bluffs and headlands seaward all the way, we find the coal measures appearing some distance to the south of Reid's Mistake, as the South Headland of Lake Macquarie is named; the coal dips under the entrance to the lake, under sea level, and under the North headland, which is composed of low sandy hills, and continuing, for about four miles further north, to run under these hills, it rises again at Redhead a high sandstone bluff abruptly thrown above the sandhills.
From here the coal seams continue exposed in the face of the cliffs, a distance of six miles further, i.e., to the mouth of the river Hunter, but are much broken up and disturbed by faults and lateral pressures and upheaval, causing a series of undulations, the splitting of seams, and the running together of two or more.
At the highest point, Khanterin or South Shepherd's Hill, about half way, there are five seams exposed.
Standing out some distance from the mainland or Signal Hill, is Nobby Island, and between this and the Signal Hill, used formerly to be an entrance to the river, but the water being shallow and only admitting vessels drawing little water, a breakwater has been thrown across, thus allowing the whole volume of the river to pour through, and scour out the present entrance, between Nobby's and Stockton, or the North Mainland.
Two small seams (the 2 ft. and 4 ft. seams) are found on Nobby Island, and a trap (greenstone) dyke, throwing down these seams to the east three inches, passes through the middle of it in a S. E. and N. W. direction, coking the coal for some yards on each side of it. The top stratum of Nobby Island is the quartzose conglomerate mentioned before, as forming the lower beds of the Sydney sandstone, the two-feet coal lies next, and immediately under it is a very peculiar fine white stone, ribbed or layered almost like a ribbon agate, having a conchoidal fracture, with translucent edges; this, from its great resemblance to "chert," has received that distinctive name, but it is only an argillaceous shale, containing about 2 per cent, of silica, and has probably received its peculiar appearance from having been under water when the latter was highly heated by the eruption of the above and other dykes. This appears to me the most likely and the most feasible way of accounting for it, and the probability of it is strengthened by the fact, that this chert, which for several miles down the coast to the south, as well as for several miles inland to the west, continues to overlie the coal, and to hold the same position immediately inferior to the conglomerate, loses its oherty appearance gradually as it recedes from this dyke.
Crossing now the Hunter river from Nobby Island, we proceed along a coast line of low sandy dunes and ridges, until we reach the south headland of Port Stephen, and here we find the carboniferous beds thrown off to the south by porphyry.
Turning now to the west and proceeding inland towards Raymond Terrace, on the Hunter river, we traverse what we may presume to be the east line of outcrop of the Hunter river coal-field; as at Raymond Terrace we find a sandstone overlying silicious breccia, cropping out from under the bed of the river, presenting the same appearance and nature, and containing many of the same fossils as tho sandstone which underlies the coal at Woolongong.
Further west again, and beyond West Maitland, the same sandstone is found at Harper's Hill, and the coal and it continue to be found at intervals, in the same relative position, all the way up to and on the flanks of the Liverpool Range, or Cordillera.
Most of the faults which dislocate and break up the seams between Nobby Island and Lake Macquarie, and all the greenstone and trachytic dykes from Nobby Island down the coast to Broken Bay, have a general direction of from nearly south-east to north-west, and in this direction the coal measures are, by upheaval, contraction, and lateral pressure together, folded up and down, in long or short undulations, so that the coal beds are alternately elevated and depressed, and the underlying rocks made to partake in similar changes, becoming prominent at points where the strata fold anticlinally, by which the lower beds seem to occupy an apparent position not actually belonging to them; the consequence of this is, that every few yards has its own peculiar dip, and it, therefore, becomes a matter of great difficulty, and, in most cases, with the few data, there exists, at present, from the scarcity of bore-holes, shafts, and wells, of almost utter impossibility to pronounce, with any certainty, the relative position which a seam found inland may hold with the rest in the coal measures. As, for instance, in the section from the coast inland , there are at that particular point of the coast three seams, the top one overlaid by grey post and conglomerate; three-quarters of a mile, in a direct line inland (the section takes it obliquely and makes it a mile,) from the coast, however, all these three seams die out, and when we reach the D, or bore-hole pit, we find a seam totally unlike any of the above three, yet overlaid by the grey post and conglomerate, and no seam below it for a depth of 294 feet; it is, therefore, little more than a mere guess to say that this seam consists of the two lowest seams, the yard seam and the lowest seam in the F pit run together; this seam dips in almost every direction from the shaft, thus :—
At shaft dips 4° S.W. At the northermost part of the workings.. 4° N.W. At the south-west side 12° S.E. And at the north-east side 7° S.E.
Then, again, at a further distance of about a quarter of a mile, or at bore-hole No. 2, this seam thins out to half its thickness; and at borehole No. 1, this and every other seam has totally disappeared. Still, when we reach the banks of the Hunter, we find a seam cropping out, nearly at the level of the river, of the same thickness exactly as this seam at bore-hole No. 2.
There can be no doubt that, after the undulatory movement abovementioned had taken place, the valleys formed by the synclinal curves became large inland lakes, and extensive denudation and future silting up consequently followed. This is the only way in which I can account for the disappearance of the coal on both sides of the D and E Pits, and at other similar places.
This denudation and washing away of the synclinal curve of these undulations will, I am persuaded, be found to be pretty general, especially as we approach the river and the coast, and the most coal will continue to be found in the ridges formed by the anticlinal angle of undulation. This has hitherto been the case, as at Burwood. etc
FOSSIL FLORA OF THE COAL
From the contrarieties which nature in Australia seems to delight in— in its existent botany as well as in almost everything else, its trees and plants being so totally different from those of any other portion of the globe that some of them shed their bark instead of their leaves—one would naturally be led to expect that the Fossil Botany as well would be completely dissimilar to that of other countries. This is, however, so far as the discoveries up to the present time have proved, only the case to the extent of some of the species being different; and although there is an extraordinary paucity of genera, species, and individuals, the sequence with which they appear in the geological formations discovers laws similar to those which regulated the succession of genera and species in other parts of the world.
The "glossopteris browniana" is, of the filices, the one most prevalent in the coal. It is very abundant, and, I believe, it is characteristic of all the coal found in Australasia. The lanceolate leaf or frond of this fern is, at the first glance, so similar to the leaf of the "eucalyptus" or gum tree, that by many it has been taken to be the same, but on closer examination it will be found that the eucalyptus leaf has an intramarginal veinlet running up each side, which the " glossopteris" has not.
Count Strzelecki, in his "Physical Description of New South Wales," in comparing the New South Wales coal fossils to those found in the Burdwan coal field of India, states that the Australian coal fields are entirely deficient in the genera Sigillaria, Lepidodendron, Catamites and Coniferae. This is certainly not the case, particularly as regards the two latter genera, which are comparatively common. The two former, it is true, are exceedingly rare, and I have not myself been able to find any specimens in the coal measures in the immediate neighbourhood of Newcastle that I could pronounce, with any confidence, to belong to those genera; but the Rev. W. B. Clarke states that he has discovered species of sigillaria and lepidodendron on the Paterson river, at Muswellbrook, and at Woolongong, and he further gives the following list of fossil flora :— Pecopteris, neuropteris, odontopteris, cyclopteris, sphenopteris, glossopteris, genus intermediate between taeniopteris and glossopteris, halonia, reed-like stems, calamites, phyllotheca, zeugophyllites, equisetum, lycopodites, new genus of plants with wedge-formed stems, lepidodendron, sometimes lepidostrobi, ulodendron, sigillaria and stigmaria, coniferae: in all about sixty species.
With respect to coniferae, there is at Redhead, six miles from Newcastle, the remains of a fossil forest on a broad bed of shale, uncovered at low water—the roots and trunks of the trees, some of them three feet in diameter, evidently in the situation where grown—completely converted into rich ironstone, partially hydrated by the action of the air and water. Three miles nearer Newcastle, there are a few more trunks; and at eight miles north-west from Singleton are the remains of another fossil forest, imbedded in the same description of rock as those at Redhead.
THE FOSSIL FAUNAE
The fossil zoology of the coal measures of New South Wales is also not very striking in the number of its genera and species. The following have been found in the argillaceous concretionary sandstone underlying the coal:—
At Harper's Hill.—Bellerophon, platyschisma, pleurotomaria, conularia, spirifer, solecurtus, moeonia, nucula, eurydesma, cypricardia, pecten pachydomus, choetites, hemetrypa. At Illarvarra.—Pleurotomaria, natica, platyschisma, theca, lingula, terebratula, productus, spirifer, solecurtus, cardium, pholadomya or allorisma, astarte, astartila, cardinia, nucula, cypricardia, mceonia, eurydesma, avicula, pecten, pterinea, chcetetes, pentadia (crinoidal), conularia, bellerophon, pachydomus, orthenota, notomya, inoceramus, stepopora crinita. At Glendon.—Conularia, spirifer, astarte, pholadomya, cypricardia, fenestella, nuculu, avicula, and encrinital remains of one or two species. In a grey post, about seventy feet below sea level, at Newcastle, a heterocercal ganoidal fish, similar to the one found at Campbelltown, was discovered.
EXTENT OF COAL FIELD.
The plane of dip of the coal measures in the Hunter River district is, as I stated before, exceedingly variable, from one degree to sixteen degrees and more, to nearly every point of the compass, caused by the local dislocations, and the undulations mentioned above; but there can be little doubt, after a due examination, that the general and mean dip of the coal field is to the south, or perhaps a little to the east of south. The dip at Raymond Terrace and at Nobby Island being to the south; the dip on the coast between Sydney and Broken Bay being westward, and the dip of the seams at Illawarra being north west, the longitudinal axis of this basin is, therefore, most probably in a direction nearly north east and south west, that is, nearly parallel with the escarpment of the overlying Sydney sandstone, at the back of Woolongong; the Illawarra beds being on its eastern and the Newcastle beds on its western side.
The transverse axis in this case will ran, as it ought according to all the physical conditions of the country, nearly through the middle of the County of Cumberland, and the trough or depression filled by the upper shales and sandstones that is between the Hawkesbury river and the Port Jackson estuary, the former occupying a line of faults continued also up the course of the Colo. The Hunter river also occupies a similar line of faults parallel to this axis.
The strike being thus north east, it is evident that there must be a considerable tract of coal measures washed away to the eastward of Newcastle, or sunk by a downcast fault below the ocean.
The seams at Illawarra, dipping four degrees, must be the anticlinal curve of one of the before-named undulations, as if we take a dip of only one degree from Newcastle to the south, and from Illawarra to the north, the synclinal curve will meet at Broken Bay, which is exactly half way, at a depth of about 5000 feet, the depth of the seams if continuous at that angle. Now, as the coal shales, as stated before, are discovered cropping out in the deep valleys at the foot of the immense escarpment caused by the upheaval of the Blue Mountains, near Penrith, and as coal apparently identical has been discovered at the foot of Mount York, in the vale of Clwydd, 2100 feet above the sea, and as the computed greatest thickness of the Sydney sandstone, (which is, so far as has been yet discovered, immediately superimposed on and conformable with the coal measures,) measured at right angles to its dip, is 1200 to 1400 feet, it is apparent that a fault of great magnitude must exist, throwing the seams up to within 1500 or 1600 feet of the surface at Broken Bay.
Further discoveries of course may very much modify this view, or indeed prove it to be altogether erroneous, but in the meanwhile taking it to be correct, the full area of this coal-field lying to the east of the dividing range or Cordillera, will be about 15,000 or 16,000 square miles, i.e., the whole of this area, except where the igneous rocks have burst through, is, in my opinion, occupied by the one contemporaneous series of carboniferous formation, the whole of it of course not being available.
Up to the present time, although boreholes have been put down to a depth of four hundred and fifty feet below high water mark at Newcastle, the number of consecutive seams found there at any one place has never exceeded six, giving an aggregate of about 19 feet, thus the depth of the coal measures is by no means great. At Mount Wingan, near Murrurrundi, on the eastern flank of the Liverpool range, where the coal cropping out, has at sometime spontaneously taken fire and is still burning, the fossiliferous sandstone of Harper's Hill, Raymond Terrace, and Woolongong, interpolate seams of coal, so that it is not improbable that seams may be found throughout the coal-field intercalated with or overlaid by this sandstone, and if this coal-field belongs to the same era as the independent coal measures of this country, which, in my opinion, there exists no sufficient reason to doubt, these lower seams will be identical with the seams of our mountain limestone.
If the depth of the Hunter river coal-field, however, is so slight in comparison with the coal-fields of other countries, the extent of country lying between the several extreme points where coal has been discovered, north, south, east, and west, and where the Sydney sandstone, existing as the surface stratum, it is not improbable the coal seams may be also persistent throughout, is so enormous as to be second, I believe, only to the coal-fields of North America.
I will enumerate a few of the localities where coal has been found; first, in the Hunter river district; some distance south of Lake Macquarie, at Lake Macquarie, Morpeth, Four Mile Creek, Hexham, Sugar Loaf Ranges, Wallis's Creek, Stoney Creek, Anvil Creek, Black Creek, Purrendurra, Glendon Brook, Tolga Creek, on the Paterson, Wollombi Brook, Leamington, Wollon, Jerry's Plains, Sadleir's Creek, Junction of Foybrook and Falbiook, Ravensworth, Maidwater Creek, Muswellbrook (where the old slate rocks buret through the coal), Edenglassie, Piercefield, Bengala, Page's River, Murrurrundi, Kingdon Ponds, Mount Wingan Wailand's Range, Scone, junction of Goulburn and Hunter Rivers, and in many other localities. From Newcastle to the north, as far as the Mackenzie river, in 23° south latitude (where coal, identical in its features with the Hunter river coal was found by Dr. Leickhardt), the coal has been found at many intermediate points, as at Port Macquarie, and on the banks of the Brisbane river, Moreton Bay, where it is worked to a limited extent, and as a sandstone similar to the Sydney sandstone exists at the Gulf of Carpentaria, in Torres' Straits, it is not improbable that the measures continue further north still.
Transactions of the North of England Institute of Mining Engineers Vol VI, 1858 ,'On the Coalfield of New South Wales' by Henry T. Plews