DR. HECTOR'S REPORT ON THE GEOLOGICAL FEATURES OF SOUTHLAND

Southland Times, Issue 1134, 2 April 1869, Page 2

DR. HECTOR'S REPORT ON THE GEOLOGICAL FEATURES OF SOUTHLAND

The following is published for general information : — Geological Survey, N.Z., Invercargill, March 23, 1869. Sir — I have visited the several districts of this province, respecting the geological indications of which you desired to have my opinion, and I hasten to communicate to your honor the principal results of my examination, in anticipation of a more detailed report. I.— MORLEY CREEK COAL. The determination of the geological position of the coal seam at Morley Creek, in the North-Western District, first occupied my attention, and I was able to satisfy myself that the age previously assigned for this coal, from the inspection of the specimens sent for analysis, is, in the main, correct. The coal seam — for there is only one as yet discovered — occurs towards the base of a group of strata, 200 to 300 feet in thickness, skirting the Southern extremity of the Waireka Hills and Takitimo Mountains. These strata rest unconformably on sandstones and other rocks of Triassic age, and dip to the south, so that they appear to pass under the clay marls and limestones of Sharpe Ridge, and tiie Limestone Gorge Hills, on the Waiau River ; but a few miles of terrace country intervenes, preventing the junction being observed, so that it could not be determined <vbether the coal formation is overlaid conformably by the Tertiary strata. The extent of country occupied by this formation is very largo, and although coal has as yet only been found in a few localities, it prevails most probably throughout the whole area in seams of variable thicknesa. At Morley Creek the acain exposed in a drive close to Messrs lleineeker aud Hunter's homestead, is ten feet in thickness and rests on a floor of tough clay which fills -up the irregularities of the surface of the subjacent rocks, proving a complete break in the geological sequence. It is necessary to point this out, as in a gorge lower down the creek, a clear section is exposed of sandstone, shales, and conglomerates, containing characteristic TjciassijC jOr Lower Secondary fossils, which, from their dipping at the same angle, add in such a direction that they appear to overlie the coal seam,

might lead to the impression that the coal belongs to this ancient geological < period. The coal is overlaid by brown colored clays rend sandy grita, stained by ochre ; and ironstone nodules, derived from these beds, which, lie strewn plentifully on the surface, contain casts of afresh water shell (TTnio) in great abundance. These ferruginous beds form an excellent guide to the position^of, the coal seam, as they impart a red color to the soil and favor the growth of small sized manuka and other scrubby plants, which are readily .distinguished among the prevailing yellow tussock grass. Eossil leaves of plants were found accompanying the coal, apparently of the same kind as those found with the coal on the west coast of Nelson, and this circumstance, together with the other characters of the formation, render it probable that we have here a representation of the same coal formation as that at ths Buller and Grey Rivers in Nelson, Shag Point and Preservation Inlet in Otago, and the Waikato and Bay of Islands in Auckland — all of which belong to a period, which in some respects, must be considered as intermediate between the Chalk and Lower Tertiary formations. It must not, however, on this account be assumed that the coal will necessarily be of an equally valuable description as at some of the above-mentioned localities, or as is frequently asserted in such cases, that the quality will improve as the seam is worked into more deeply. There appears to be great variety in the quality, 01 the coal obtained in this formation due in the first instance to the nature of the vegetation from which it was originally formed, and, secondly, to the conditions of pressure and drainage to which the coal has been subsequently subjected, through dislocations of the strata on which it rests. In the case of the coal at Morley Creek, the presence of large quantities of resinous matter in that portion of the seam excavated for use, renders it highly inflammable, and practically a fuel of great heating power. This may, however, be quite a local peculiarity, and as there is no evidence in the district of such geological disturbances as those which have led to the improvement of the coal over large areas on the west coast of Nelson, it must be borne in mind that the coal in the district under consideration may be foundj to detoriate in value in some parts of the seam. Subsequent survey will, in all probability, prove the coal formation to continne almost uninterruptedly from Preservation Inlet eastward through the province of Southland, and there can be no doubt that it will hereafter be of value in developing the industries of the country. Even at the present time, if it could be obtained of quality equal to the sample at Morley Creek, in an easily-accessible position, the practical experiments of the engineer of the Bluff Railway prove that it might be employed with success as a steam generator. The nearest point to Invercargill where it has yet been found is at the Nightcaps. Eastward of that, it is cut off by the valley of the Aparima, and I am not aware of its having been found at the southern extremity of the Moonlight Ranges, where it may be expected again to crop out. 2.— OTAPIRI RIVER. The discovery by Mr Thompson of a coal seam associated with fossil plants generally considered to be indicative of Upper Meaozoic strata, probably the equivalent of the Oolitic formation in Europe, led me to examine this district, an I found the strata so well displayed that even a very hurried visit was sufficient to enahle me to gather a clear idea of the succession of the formations. The Bection observed in the valley of the Otapiri River is as follows : — At Forest Hill, and extending N.N.W. to Centre Bush Hill, and on to what is known as Castle Rock — though broken through by the Ore ti River with its wide shingle bed — is the Miocene Tertiary limestone, dipping to the South-west, and forming a, succession of bold scarps in the opposite direction, showing, with the underlying clay marls, a thickness exceeding 800 feet; a wide belt of swamp running parallel with the limestone intervenes, and hides the formation of the country between it and the Hokanui Range ; but it is probable that this area is occupied by the Brown Coal formation, which is so largely developed further to the eastward. Indeed, since my return. I have beea informed by the Chief Surveyon that coal of this kind has actually been found at the eastern side of the Win ton Hills. In the upper part of the Ofciipiri Valley, the rocks are very distinctly ex^ posed, and- exhibit the best section oi' the secondary strata which I have observed in New Zealand, showing a vertical thickness of, at the least, 2000 feet. The uppermost beds are the brown clays- 1 ' similar to those overlying the coal at' Morley Creek, but not so highly ferruginous. Beneath these are brown sandstones containing a few marine shells of the same species as those found immediately over the coal at the Brunner mine on the Grey River. This sandstone, the thickness of which is about 150 feut, "was not observed at Morley Creek, but I have reason to believe that the Morley Cruek seam, if present at the Obapiri, will overlie it. Under the Brown Sandstone there occurs a group of soft strata, apparently pipeclays, having a vertical thickness of 100 feet, the exact nature of which I could not observe, as thereis no natural section. It is in this portion of the section that valuable coal seams may be expected, but the search for them would require a more extended examination than I had an opportuuity of making on: this occasion, assisted by a slight amount of labor in excavating and boring. The above soft strata rest on Brown and Gh?ey Sandstone, with local beds of coilglomerate and grit, composed of well-; rolled fragments of granitic and porphyry ro^ks, of a kind not found nearer than the Western Mountains. The sandstones of this group contain fossil ferns, and thin

irregular seams of coal. The seam discovered by Mr Thompson in a tributary; creek of the Otapiri, is no doubt one of these, and several of like description found iv this formation, both in other parts of New Zealand and in Australia, have invariably proved to be so impure > : and irregular as not to be worth working • The strata, as seen in the Otapiri Valley, are certainly much more regular and more free from the " false bedding," which indicates sudden changes in the currents during their deposit, than is usual in this formation; but notwithstanding this favorable point, I am not sanguine that any workable deposit of coal will be found in the series below what are known as the Taenioptr.m.s sandstone, from a fossil fern which they contain abundantly. This formation corresponds in age with the upper part of I the carbonaceous series of Australia, and below it, in regular sequence, the Otapiri section shows an immense thickness of I lower secondary strata, comprising shales, sandstones, and grits, from which I obtained a variety of secondary fossils. I observed no indication of the occurrence of coal-bearing strata in this series, which, I suspect, corresponds with the middle coal series of Australia, or upperof the two formations worked for coal in New South Wales ; but the question is of such importance that it will require a prolonged examination to determine this definitely. The above strata, which form the greater part of the Hokanui mountains, are rarely inclined at angles exceedingils ° to 20°, the prevailing direction or trend being N."W. and S,E. r , in which direction they have now. been traced from Mount Hamilton, at the north extremity of the Takitimos to Waikawa, on the East Coast. It is only in the upper part of this formation that we any expect valuable coal seams to be discovered ; and, as the upper beds have been denuded for large areas, it must not be assumed that the sandstone rocks of themselves indicate the existence of coalbeds, as might be done with safety if the carbonaceous layers occurred at the base of the formation. At Morley Creek the coal, as I have already observed, rests uncomformably on sandstone, and then on the lower beds of the above section, the whole of the intermediate plant-beds being absent. This circumstance, coupled with the fact that the Morley Creek formation follows to a great extent the bolder surface features of the country at the present time, whereas the whole of the plant-beds and sandstones of the above formation strike right through the countryrand form part of the mountain ranges, is sufficient to prove, even without fossils, the greater antiquity of the Otapiri beds. The . structure of this district is therefore highly interesting, and I will take immediate steps for its thorough investigation, which will be greatly facilitated by the communication ' of any information respecting localities where coal seams or fossiliferous beds have been observed within the district, which may be in the possession of settlers. The structure of the country, even so far as at present known, . cannot be described clearly without the assistance of sections and sketch map, and these I will have prepared as soon as . possible. The sketch map of the geology of the province which I formerly forwarded, I find will require a good deal of modification, as the topographical map has been greatly improved since the geological features were first sketched in. 3.— GOLDFIELD OP THE LONGWOOD RANGE. At the southern extremity of the Takitimo Mountains, I found a largo. area. of hilly country, composed of Breeciated rocks, ' with beds of Diorite, or greenstone, and felspatbic rocks, that at first sight resemble Trachytes, but which I believe to be of upper Paleozoic age. The same rocks form the greater part of the Longwood Range, from Raymond's G-ap south to Foveaux Straits; but on the east they are broken through by the tertiary volcanic rocks (basalts) of Mount Pleasant, and on the north are ' supported against a nucleus of syenitic aqd crystalline hornblendic rocks. "We have, therefore, in this district a repetition of the leading features of the Coromandel Peninsula, where the Thames diggings are situated. The analogy is further supported, as, for the last three years, gold-digging has been carried on in various parts of the Longwood Range with tolerable success, considering the great difficulties to be overcome, arising from a deficient supply of water, and the dense growth of fores b. The principal diggings in the district are at Orepuki, but there the gold is, I understand, obtained from » beach-drift, having, most probably, been brought down the Waiau River, as, along with the gold, platina is obtained in small scales, which is also the case at the Waiau diggings, a few miles below where the river leaves the lakes. The only localities I was able to visit in the Longwood Range are out lie eastern slope, and on tributaries of the Purakino, or the west branch of the Aparima. What are termed the Purakino diggings, are the farthest from Riverton, and are about two miles ba< k in x the bush from the farthest point which 'can be reached with a loaded boat. The lower part Jof the river channel, exeopt at the Narrows, where a barrier of the Aphanite Breccia bounds both sides, is cut through blue clay marls containing pliocene fossils. As the hills ate approached, these marls pass gradually into a, deposit of red gravel of 8 1 n all- s i zed a tones — being the local ■ variety of the deposit of gravel * wliich underlies so much of Southland. This gravel follows up into the valleys of ~ the range to a height of 200 feet above the sea, whmi it is frequently covered by a very coar.ie original wash of boulders and anguliu- gravel, derived from the rocks of the adjacent hills. From this lasjt-mentioned wash, when it overlaps. „ 'the underlying fine gravels, most of tne ; gold has. hitherto been obtained. The"' waJsh consists of huge boulders of 'the" r Aphanite rock, formed by its decomposition in situ, and intermixed with uige

boulders of quartz. The workings havt been very partial, and have been confined to paddocks and creek sluicing. The resemblance of the stuff thrown out oi the paddocks to. the same material at Coromandel, is very marked, the only rock absent being the quartz cement, and quartzites indicative of the action of hot springs which are so plentiful there. The gold is also very different from that obtained at the Thames and Coromandel. being almost entirely free from silver. Though obtained principally from the coarse upper wash, gold also is found throughout the finer gravel, but in neither case in large quantities. The workings at this locality have been almost abandoned, but a company has been recently formed for the purpose of bringing in a race to be led iound the heads of the gullies, bo that they can be readily sluiced. It is hoped that in the cutting of this race the auriferous leaders will be discovered from which the gold is derived, and the scheme appears a promising one. That the gold is very local in its origin, there can be no doubt, as it is very patchy and variable in the size of the grains within short distances. The amount of quartz in some of the gullies is very great, but it is chiefly of the kind termed by the miners "barren quartz," and from the great rarity of sulphurets of iron, I am inclined to think that the gold matrix will be found in very insignificant lodes in the decomposed rock, and marked at out-crop only by a thin seam of iron ore, Obtaining a small quantity of friable quartz. This indication should be carefully attended to by the miners. At the only other diggings which I visited, known as the old Longwood diggings, about four miles from Riverton, the gold occurs much in the same manner as the: above, but the wash has been followed down the creeks to the sea level, and, irom the description given by intelligent miners, the gold is confined to the streams flowing from one particular spur, and is not found in quantity in any other place in the immediate neighborhood. — This has led them to cross-cut the spur in search of a Teef. The work, which consists of a trench 12 chains long, and having an average depth of 12 feet, requires only a short distance to complete it, and has been effected by the work of two men for less than a month, but as yet they have not been successful in cutting what they consider an auriferous leader. They have to excavate to the above depth on account! of the immerse deposit of fine c!ay, which envelopes the spurs, separated from the surface of the decomposed rock only by athinsparselayer of quartz pebbles,from whichasmall quantity of gold can be everywhere obtained. In one gully alone of the above spur, I observed any chemical change in the rock which is peculiarly indicative 'of a metalliferous lode. Eor a few feet in thickness, the hard Diorite was converted into, a soft red and green chloritic substance, and contiguous to this the rock passes into a substance like tale slate, and contains a series of quartz-leaders, backed by the usual rotten Aphanite rock which the: miners call pipeclay. Such circumstances are favorable, if not for the actual occurrence of gold, at least as guides in discovering the position and direction of the dislocations of the rock with which . the auriferous lodes are probably associated. The main |features I wish to impress arerflst. That the gold does not occur in the Longwood Bange under the same circumstances as in Otago, but rather like the gold at the Thames, and at Gipps Land and Gympie Creek in Australia. 2nd. In these cases the amount of alluvial gold is a very small proportion of the total yield. 3rd. That it invariably appears capricious and patchy to the first discoverers, and until the true order of the formation has been discovered by ex tensive diggings. And, lastly, that the gold is always associated with rock containing sulphurets of iron and other metals, and that the yield falls off as the lodes are followed down into the hard, undecoinposed rock. , - I have the honor to be, Sir, ■j' 1 Tours most obediently, - ; James Hectoe.