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Volume 6, 1873
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Art. XLVI.—On the Geological Structure of the Thames Gold Fields.

[Read before the Auckland Institute, 9th June, 1873.]

The extraordinary amount of gold that has been obtained from some of the reefs at the Thames gives a great importance to these gold fields, and a correct knowledge of the geological structure of the district cannot fail to be of great interest to science. But at present a considerable difference of opinion on this point exists among geologists, and as it is only by discussion that a more satisfactory state of things can be brought about, no apology is, I think, necessary for bringing the subject before the members of the Auckland Institute.

In order to make clear the points on which different opinions are held, it is necessary, in the first place, to give a short historical summary of our present knowledge of the geology of the district. Professor Hochstetter was the first geologist who visited these gold fields, and he, after a short examination of the country about Coromandel, in 1859, before any auriferous reef had been found, said that “The coast consists of nothing but trachytic breccia and tuff, in the most varying colours, and in the most different states of decomposition, from the hardest rock to a soft clayish mass, and in various places broken through by doleritic and basaltic dykes. Siliceous concretions, in the shape of chalcedony, carnelian, agate, jasper, and the like, are of very frequent occurrence in these tuffs and conglomerates, likewise large blocks of wood silicified and changed into wood opal. By local geologists those trachytic rocks were erroneously taken for granite and porphyry, and, by a gross mistake, the most sanguine hopes were based upon the notion that these siliceous secretions might be auriferous quartz veins. The Coromandel gold originates from quartz reefs of crystalline structure, belonging to a clay-slate palæozoic formation, of which, under the cover of trachytic tuff, and conglomerate, the mountain range of Cape ColvAille Peninsula consists. The mountains are so densely wooded that it is only here and there in the gorges of the streams that sections of these slates may be examined. In these sections the clay-slates are frequently found to resemble Lydian stone. They are arranged more or less vertically, their irregular upturned edges affording the most convenient and abundant pockets for the detention and storage for the

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alluvial gold washed from the higher grounds. * * * On the slope of the hills I saw large blocks of quartz lying, which, from all appearances, originated from reefs or veins that—according to the statement of Mr. Heaphy—protrude on the top of the dividing range in various places, like walls, eight to ten feet high, and ten to twenty feet thick. * * * The Coromandel gold fields—such was my opinion in 1859—bid fair to grow into importance in future years when the auriferous quartz reefs themselves shall have been discovered.” (“New Zealand,” p. 96, et seq.)

In June, 1864, and in February, 1866, Dr. Hector visited Coromandel, and the opinion he then formed of the structure of the country is thus given in his instructions to me in August, 1867 (Geological Report on the Thames Gold Field, by Captain Hutton, September, 1867. Extract from Dr. Hector's instructions, p. 2):—“The range which separates the Thames Valley from the Bay of Plenty I found to consist of a nucleus of aphanite slates, interbedded with green brecciated and greywacke slates, being part of the upper palæozoic series. Flanking and capping this nucleus is a great development of the following members of the tertiary series:—(a.) brown coal formation, very local; (b.) quartzose gravels, cemented so that they break away in large blocks; (c.) Waitemata series (pliocene); (d.) trachytic tuff; (e.) trachytic breccia. The palæozoic rocks are cut by dykes of trachyte (granite of the miners), which is charged with auriferous and cupreous iron pyrites. They, moreover, contain quartz veins, which are also pyritiferous and auriferous. The older rocks decompose very freely to laterite, and the fissures then contain secondary deposits of silica, manganese, etc., especially when near the supposed trachyte dykes, alongside of which, in some cases, there would seem to have been fissures that were only gradually filled up by deposits from thermal waters, giving rise to the banded, irregular, and crystalline structure of the lodes which is so characteristic of Coromandel. * * * A third manner in which quartz occurs in the district is in the trachyte tufas, but it is then more chalcedonic and crystalline, and associated with jasper and chert, and is non-auriferous, as proved by the numerous trials at Keeven's Point, Coromandel.” And further on he instructs me to “search for the grey pyritiferous rock” (i.e., the dykes of trachyte), “in the beds of the streams.”

Dr. Hector's opinion, therefore, agreed with that of Dr. Hochstetter, but he pointed out that the gold reefs (some of which had meanwhile been discovered and were then being worked), were not found in the slates, but in the grey pyritiferous rock, which he took to be dykes of trachyte.

Hitherto gold had only been known at Coromandel, but in August, 1867, it was also found near Shortland; and in September of that year I was sent by Dr. Hector to examine the new discoveries, and I then reported that the country was “almost entirely composed of a huge mass of trachyte tufa

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resting on palæzoic rocks, and cut by numerous dykes of diorite, * but occasionally of trachyte. This tufa appears as a softish, grey, coarse-grained rock, weathering white, and sometimes much stained with peroxide of iron. Where cut by dykes it is hardened for considerable distances, and much altered in appearance. As might be expected, however, from its origin, it varies a good deal in character, often containing rounded blocks of diorite one or two feet in diameter; indeed, in three or four places it passes into a true conglomerate, while occasionally small angular stones are found in it forming a breccia; these latter, however, are very local. The whole of the rock, including some of the dykes, is much impregnated with iron pyrites.” (Geological Report on the Thames Gold Field, 1867. I there state that auriferous quartz veins had been found in eight places in the trachyte tufa, and that I considered that “this tufa is probably of tertiary age, and not older than the Waitemata series.” In this report, therefore, I agree with Dr. Hector that the gold reefs are situated here, as at Coromandel, in “grey pyritiferous rock,” but consider that at Shortland this rock is only a part of the vast overlying accumulations of tertiary trachyte tufa.

In November and December, 1868, I again visited the Shortland and Tapu districts, and in the report I furnished (Second Report on the Thames Gold Fields. Geological Reports, 1868--69, p. 15) I reiterated my former views, adding that the lower part of the tufa formation had been metamorphosed “into a hard, green, pink, or purple felspar-porphyry, or more rarely into a hornblende-porphyry.” I also reported that in the Tapu district some of the lodes were in the older slate formation, which is there nearly vertical, but that most of them were in a trachyte tufa and breccia.

In December, 1869, I visited Coromandel for the first time, and reported, in January, 1870 (Geological Reports, 1870--71, p. 2), that by far the larger part of the country, including the central dividing range, was composed of tertiary trachyte rocks, like those at Shortland, lying unconformably on a basement of older slates, and that all the gold mines were situated in the trachyte tufa. Thus considering the trachyte dykes of Dr. Hector as part of the tufa, and as having nothing to do with the palæozoic slates.

Meanwhile, between 1868 and 1870, Dr. Hector had also several times

[Footnote] * This is a mistake, the dykes are of dolerite, melaphyre, and timazite.

† The map accompanying this report was altered without my knowledge, and it now agrees neither with the report nor with my opinion. [Only the reference to the map was altered without consulting Captain Hutton, as it appeared to be inconsistent with the report as altered by him before it was sent to press. It originally stood thus:—
  • Alluvium. Tertiary Trachytic tufa. Sandstone.

  • Palæozoic? Trachytic tufa.—J. Hector.]

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visited the Thames gold fields, and, in April, 1870, he made a report (Geological Reports, 1871--72, p. 88) in which he somewhat altered his former views, although still not agreeing with me. After giving Professor Hochstetter's opinion, he says (p. 89) that “the gold is not, however, as he (Professor Hochstetter) supposed, derived only from quartz-veins in clay-slates, for, as Captain Hutton very justly points out in his report on this district, the area of these exposed at the surface is very limited. On the other hand Captain Hutton, in the same report, does not distinguish between the comparatively modern breccias and agglomerates, which he describes as containing blocks of variously-coloured scorias and lavas, and the more ancient formation of green tufaceous sandstone and porphyry, in which most of the auriferous lodes occur.” And in the section that accompanies this report he makes his “greenstone tufa” and “greenstone porphyry,” as he here calls them, conformable to the clay-slates and dipping with them at a high angle. But speaking of the Tapu district, he says (l.e., p. 98) that here the reefs occur in “the decomposed slates and bands of greenstone porphyry which intersect them with a prevalent north-east strike.” It therefore appears that more extended observations led Dr. Hector to abandon the idea that the “grey pyritiferous rock,” in which the auriferous veins occur at Coromandel, is a dyke of trachyte, and to suppose now that it is part of a “green tufaceous sandstone and porphyry,” belonging to a formation distinct on the one hand from the older slates, and on the other from the newer trachyte tufa; but he still thinks that at Tapu the reefs are in dykes of “greenstone porphyry” intersecting the slates.

The late Mr. E. H. Davis also visited these gold fields in May, 1870, and reported (Geological Reports, 1870–71, p. 56), as far as I can understand him, in favour of two volcanic formations, one of “diorite sandstone,” the other of “tufa”; and these are, I presume, meant to be identical with Dr. Hector's “greenstone tufa” and “trachyte tufa” formations respectively. But he describes Tinker's Gully as “a mass of diorite sandstone with dykes of tufa passing through it” (!) (p. 60), from which I infer that he supposed these two “formations” (?) to be interstratified and elevated on edge; and in other places he seems to think that the “tufa” is only the “diorite sandstone” decomposed. But however this may be, he was, at any rate, of opinion (according to Dr. Hector, l.c., p. 98) that “the Tapu district furnishes very conclusive evidence of two distinct and two widely separated volcanic formations.”

In April, 1872, I again visited Coromandel, in order to examine some coal seams which had been lately discovered, and which I shall presently describe, and I then saw sufficient evidence to confirm me in the views that I had previously expressed.

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I think, therefore, that the following is a fair statement of the present position of the case.

Professor Hochstetter said that there is at Coromandel a tertiary trachytic formation overlying vertical beds of clay-slate and lydian stone of palæozoic age, and he thought that the gold would be found only in the slates and not in the trachytic formation. Dr. Hector also says that there is a trachytic formation overlying slates, but that the gold is principally found in neither one nor the other, but in a distinct volcanic formation which is considerably older than the trachytes, having partaken in the movements and foldings of the clay-slates.

Mr. Davis appears to have agreed with Dr. Hector.

I agree with Professor Hochstetter that there is a tertiary trachytic formation overlying clay-slates, but say that the gold has been almost entirely obtained from the trachytic formation, and not from the slates; and I deny the existence of Dr. Hector's “greenstone tufa” formation as distinct from the tertiary trachytic one.

As it is now an undisputed fact that the principal mines are situated in a felspathic rock, and not in the slates, the question at issue is reduced to this: Is this felspathic rock part of the tertiary trachytic series, or is it part of a distinct formation more closely related in age to the clay-slates than to the trachyte tuffs, which have been deposited unconformably on its upturned edges?

I will, in the first place, examine all the evidence that I can find in Dr. Hector's and Mr. Davis' reports in favour of the distinctness between the “greenstone tufa” and “trachyte tufa” formations, and then I will state the evidence on which I rely for proving that they are one and the same.

1. Lithological Evidence.—Mr. Davis, in his report on the Shortland district (l.c. passim), seems to lay stress on the great diversity of appearance in the rocks found there, as proving that more than one formation must exist; at the same time he makes no attempt to trace out these different formations. Dr. Hector, also, by calling his two formations “greenstone tufa” and “trachyte tufa” would seem to imply that, besides a stratigraphical break, a difference could also be made out in the chemical composition of his two formations. But to show the extreme difficulty that Dr. Hector and Mr. Davis must find in distinguishing between the rocks of their older and younger formations, I may point out that Dr. Hector, in his instructions to me in 1867, states that Keeven's Point, at Coromandel, is composed of non-auriferous trachyte tufa belonging to the younger formation (l.c., p. 2); while, in his report of April, 1870, he calls it a tufaceous porphyry, originally a clay-stone porphyry (l.c., p. 90), and again (p. 92) a grey tufaceous sandstone “like that at Kapanga and Tokatea,” in both cases including it now in his older

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formation; while (at page 91) he says that at Driving Creek “no proper volcanic rocks have ever been met with in the underground workings when they have proved auriferous.” Again, this “grey tufaceous sandstone” of Tokatea is called by Dr. Hector in another place (p. 96) “greenstone tuff,” while Mr. Davis (p. 97) calls the same rock “trachytic tufa.” Again, at Tapu, what Dr. Hector calls (p. 98) “greenstone porphyry,” Mr. Davis calls (p. 99) “older trachytic breccia.” Dr. Hector, also, although he acknowledges (Trans. N.Z. Inst., II., 367) that the bed rock of the auriferous lodes is the same both at Coromandel and the Thames—and therefore, according to him, “greenstone tuff” or “green tufaceous sandstone”—calls (Trans. N.Z. Inst., I., 48) the bed rock from the Golden Crown Claim a “felstone.” It seems, therefore, to me that Dr. Hector has entirely failed to distinguish by litho-logical characters the difference between his two formations; and I may remark that no greenstone tuff, nor any other basic tufaceous rock, has ever yet been brought from the Thames, and that the term “greenstone tuff” is altogether a misnomer. Of course all agglomerates formed by the latest eruptions would be considered by Dr. Hector as belonging to his younger formation, and it is quite true that they never contain gold; but I shall presently show how it is that these superficial accumulations could not be expected to contain gold in any quantity, and that they afford no proof of two formations.

Great stress has also been laid by Dr. Hector on the supposed similarity of the gold-bearing rocks of the Thames with those of Waimungaroa, Batten River, Cape Terawiti (Prog. Report, 1866–67, p. 32), Dun Mountain (Trans. N.Z. Inst., II., p. 365; and III., p. 288), and of Gympie, in Queensland (Trans. N.Z. Inst., II., pp. 366 and 399; also Museum Report, 1870, p. 4). Putting aside the question whether identity of age can in any way be proved by the identity of the rocks from such distant localities, * I must remark that having examined rocks from the Dun Mountain, Cape Terawiti, and Gympie, I can find but a very superficial resemblance between some of them and some of the melaphyres of the Thames, which occur only in dykes, and never contain gold veins. The analyses of the rocks from Gympie—which are from the identical specimens that Dr. Hector says he cannot distinguish from Thames rocks—show that there is a wide difference between them, while the analyses of the Dun Mountain rocks (Lab. Report, 1871, p. 17) show a still greater divergence from those of the Thames; the only one that nearly approaches in composition to the Thames auriferous rock being No. 9—“a fine-grained argillaceous slate, with slaty cleavage.” In fact, the Gympie and Dun Mountain volcanic rocks are true greenstone tuffs, while the Thames rock is a trachyte of a totally different character to the others.

[Footnote] * The Gympie rocks have been shown by Messrs. Daintree and Etheridge to be of Devonian age, while the occurrence of Inoceramus in the Dun Mountain shows that those rocks cannot be older than the Lias.

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The reduction of the rates of the base to the silica in a heterogeneous group of widely dissimilar rocks, as are those given on page 16 of the same report (Lab. Report, 1871), can have no possible scientific value, but even here, if it proves anything, it proves that the Thames rocks are quite different from those of the Dun Mountain and Gympie, as they contain considerably more silica.

Dr. Hector has remarked (Geological Reports, 1870–71, p. 93) that “the country occupied by the volcanic rocks (at Coromandel) has a very distinct appearance from the central portion of this part of the range, which is composed of the tufaceous and porphyritic sandstones and felspathic slates.” In this I quite concur, but it no more proves that the two are distinct formations than does the difference between the central scoria-cone of an active volcano and the lava streams round its base prove that the scoriæ and lava belong to two geological formations; for it is a phenomenon common to all large trachytic districts that the later outbursts are always more basic in character than the earlier ones, and have almost invariably occurred on the flanks of the mountains. We have, in New Zealand, another example of this in the Malvern Hills, in which case the imbedded fossils enable us to prove that the central trachytic mass of Mount Misery belongs to the same formation as the basaltic lava streams of the outskirting Harper Hills.

Both Professor Dana and Mr. Darwin have attempted explanations of these phenomena, but whether their explanations be true or not the fact still remains that large trachytic ranges almost always have a centre of solid felspar rock, and basaltic lavas with scoriaceous agglomerates on their flanks.

2. Mineralogical Evidence.—Dr. Hector has dwelt upon the fact that gold is found in some creeks and not in others; but this, by itself, proves nothing, for not only do all metals occur locally, but gold could not be expected to occur in quantity in those superficial portions of the formation which both Dr. Hector and myself call trachytic agglomerates, for the heat in these portions could not have been sufficiently long continued for the formation of metallic veins, and these rocks are generally so porous that the percolating water would not be compelled to keep in distinct channels.

Jasper and chalcedony are said by Dr. Hector (Report on Thames Gold Fields, p. 2), to be characteristic of the younger formation, but they are both found abundantly at Tapu in Dr. Hector's “greenstone porphyry” formation, and occur also at several places in the Shortland district, as for instance the Karaka Creek.

3. Stratigraphical Evidence.—Dr. Hector asserts that his older auriferous formation is only found in narrow belts. He says (Geological Reports, 1870–71, p. 92) that “the shafts and drives on the Tokatea Hill, and also a few of the road cuttings which penetrate the hard rock, show it to be the same

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grey tufaceous sandstone, full of mundic, as at Keeven's Point and Kapanga, thus proving this rock to extend in a narrow belt from the sea level to 1,600 feet altitude.” I do not see how finding a rock at three different places in a line, and at very different altitudes, can prove that it runs in a narrow belt. It seems to me that the absence of the rock on either side of the belt should be first proved; but I have myself traced this rock for several miles in a direction nearly at right angles to the line indicated by Dr. Hector, and it must be remembered that if the gold should be found to run only in a narrow belt it would by no means imply that the bed-rock did the same; for the distribution of a rock is one thing, and the distribution of gold in that rock is quite another thing. He also says (l.c., p. 92) that “the auriferous reefs are generally in the decomposed rock, and, as at Shortland, have a general direction parallel with the boundaries of the formation, or N. 40° E.” But in his map of the Coromandel district he shows his “greenstone tufa” formation running in a nearly north and south direction, and without a single boundary approaching to a N. 40° E. direction. As Dr. Hector has not attempted to map his formations at Shortland, I cannot tell where he supposes the boundaries to lie in that district.

At Tapu, he says (l.c., p. 98) that “the reefs are in bands of greenstone porphyry, which intersect the slates with a prevalent north-east strike.” But most of the claims (except those in the slates) are situated in a brecciated rock, which is certainly not intrusive, and could not intersect slates; neither can it be interbedded with them, for the slates here are nearly vertical, and strike east and west. As Dr. Hector has not mapped any of these bands I do not like to speak positively on the subject, but their occurrence in the way that he describes them appears to me to involve a physical impossibility, and, although I made a careful survey of the district, I saw nothing that would lead me to adopt his opinion.

Mr. Davis asserts (l.c., p. 99) that south of Hastings the older volcanic formation strikes north and south, while the more recent tufas are nearly horizontal; but, from a personal inspection of the locality, I am convinced that Mr. Davis mistook jointing for bedding, there being no planes of stratification visible, while his more recent tufa is but the older one decomposed. I made the same mistake myself in my first report on the Thames. Mr. Davis also appears to think that, as a tufaceous breccia is found at the Thames enclosing fragments of tuff or breccia, it necessarily proves two distinct formations. But this is by no means the case, for it is a common phenomenon in all submarine volcanic districts. When an eruption is over, the vent fills up and consolidates, and a subsequent eruption breaks this up into fragments and scatters them around. An example of this may be seen in the Auckland Domain, in a cutting through basaltic tuff of newer pliocene or pleistocene age.

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Mr. Davis has brought forward two cases of what he supposes to be unconformity between the two formations. One of these is at Omaru Bay, near Coromandel (l.c., p. 97), and the other at Tapu. In both cases a brown tufa or breccia is supposed to lie on a water-worn surface of blue tufa or breccia respectively. With the example at Tapu I am quite familiar; the one at Omaru I have not seen, but my acquaintance with these rocks in other districts leaves no doubt on my mind that the appearance at Omaru is owing to the same cause as the one at Tapu, viz., the decomposition of the upper parts of the beds, a distinct and undulating line often being seen between the decomposed and undecomposed portions of the same rock.

Dr. Hector also mentions what he considers a case of unconformity in the Ohinemuri district. He says (Geological Reports, 1870–71, p. 102) “the sudden alteration in the form of the hills, and the marked change in the mineral composition of the rock, and other circumstances, indicate that b is unconformably superimposed on d, and that the two formations are distinct.” On this I would remark that “the alteration in the form of the hills” is probably caused by the “marked change in the mineral composition of the rock,” so that the evidence of unconformity is simply the alteration in the mineral composition of the rock “and other circumstances” which are not specified, and I submit that no geologist would consider this unconformity as proved. But apart from this, unconformity among volcanic rocks can by no means be taken as a proof of two formations, for the products of a volcanic eruption generally lie more or less unconformably on those of the last. I think, therefore, that the evidence adduced in favour of two distinct volcanic formations at the Thames completely breaks down on every point.

Having at last, “Deo juvante,” finished my criticisms, I now enter on the more pleasing task of giving the evidence on which I rely for proving that Dr. Hector's “greenstone tufa” formation is simply the older and central portion of the “trachytic tufa” formation, of which the coast accumulations of scoriaceous agglomerates are but the last dying efforts. I ought, however, first to define that I mean by a volcanic formation, or period, the whole length of time from the first volcanic outbreaks in a district to their final extinction, provided that the series of outbursts are not interrupted by a period of repose so great in extent as to be comparable in duration with a geological formation or period.

1. Lithological Evidence.—The rocks at the Thames are, as I have already said, very variable in appearance, not more so, however, than is usual in trachytic districts. The chemical composition also remains essentially the same in all the varieties, and corresponds with the same class of trachytic rocks in other parts of the world, such as Hungary, Styria, Teneriffe, the Siebengebirge Mountains, etc., as the following table will show:--

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A B C D E F
Max. Min. Mean
Silica 64. 50. 56.5 68.38 66.39 62.83 57.17 53.85
Alumina 24. 13. 17.86 13.92 17.74 21.25 16.9 17.95
Oxide of iron 13. 2. 9.10 2.8 4.97 4.11 8.5 6.94
Lime 10. .6 3.77 .84 .53 .72 6.3 8.33
Magnesia 3.6 .6 1.49 2.20 .47 .42 1.9 6.47
Alkalies 6. 2.4 3.76 7.53 4.99 6.37 4.9 3.25
Water 7.(?) 3.44 5.63 4.64 4.89 4.15 3.38 2.55
Specific gravity 2.68 2.29 2.45 2.57 2.64
  • A Bed-rock of auriferous veins at the Thames and Coromandel. The mean is from six analyses, which are all that are published.

  • B Trachyte porphyry lava from Monte Guardia, Lipari. Resembles a compact clay-stone, and often contains imbedded fragments of augite rock (Bischof).

  • C Trachytic conglomerate of the Ofenkuhlen. Homogeneous, white, and thinly stratified (Bischof).

  • D The same (Bischof).

  • E Trachyte from Gleichenberg, Styria. Resembles felstone porphyry, compact, and of a greyish-green colour with a few felspar crystals (Bischof).

  • F Trachyte. Hrad Mountain, Hungary. Matrix fine-grained, grey, rather porous, and very hard. There are a few very small laminæ of felspar and hornblende in it (Bischof).

The chemical composition of these rocks, it will be noticed, is similar; E especially is remarkably like the Thames rock, both chemically and physically. They are all called trachytes, and are all characterized by containing a large amount of water of constitution; at the same time they differ among one another quite as much as do the most different varieties of the tuff rocks from the Thames that have, as yet, been analyzed. Of course the dyke rocks must not be compared with these; they are more basic, and do not contain auriferous veins at the Thames. Hitherto I have called the bed-rock at the Thames a tuff, or tufa, and in this I have been followed by Dr. Hector and Mr. Davis; but a recent examination of the Malvern Hills has led me to doubt the propriety of the name. This rock has undoubtedly not been ejected in the fragmental state that is implied by the word “tufa,” neither has it flowed over as a lava in the ordinary sense of the term, but appears to have welled up in a manner different from anything that has been observed on the surface of the earth. On the whole I believe it to be more nearly allied to a lava than to a tuff, and I consequently prefer the word trachyte to that of trachyte tufa.

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2. Stratigraphical Evidence.—The different varieties of rock pass gradually one into the other, no line of division extending beyond a few yards having been found, and no sequence of the different varieties can be traced, as can always be done among stratified rocks. I think, therefore, that all varieties of trachyte, porphyry, and breccia must be considered as belonging to one formation. Now this formation is spread over the greater part of the peninsula, and attains a height of 2,600 feet above the sea; and a closer examination of the district shows that the porphyries and other more metamorphosed varieties of the trachyte are found only towards its lower part, and do not extend up into the hills, as may be easily seen by examining the mines at various levels in the Karaka and Tararu Creeks, where meta-morphism has been most active. This cannot be due to decomposition of the upper portions, for near the base of the formation the rocks are quite hard in places where they have been exposed to the atmosphere for a long time, while higher up long drives into the hills show that the rock there has never been changed into a porphyry, for it contains no crystals nor crystal cavities. It is impossible to account for this fact on the supposition that the porphyries are older rocks tilted up, but it follows naturally from the supposition that the whole mass is of volcanic formation, which was ejected in a heated state; for then the lower portions must have retained their heat longer than the upper, and, as the felspathic nature of the rock renders it very liable to meta-morphism, there is nothing extraordinary in finding the lower parts changed into a porphyry.

Those districts which are most metamorphosed are also most brecciated, such as the Hape, Karaka, and Tararu Creeks, the beach north of the Opitomoko, and the country about Tapu. This probably shows that the more metamorphosed districts were nearer to the volcanic vents. The absence of scoriæ is no argument against this view, for, as I have already said, the trachyte is more nearly allied to lava than to tuff, and it was certainly ejected below the sea, while scoriæ can only be formed in the air. I have already mentioned the great extent of country which this formation covers. This alone proves either that it is nearly horizontal, or that it is thrown into undulating curves. But the mines at the Thames have shown that it is crossed in all directions by nearly vertical dykes, which have no particular direction of dip in different localities, which would certainly be the case if the formation was thrown into undulating curves. At Coromandel seams of coal have been found at Sykes Gully, in the Kapanga township, and in the Hinau Creek, a small tributary of the Matawai. * In the first case the coal dips N.N.W. 15°, and in the second it is almost horizontal. In both places it is overlaid by

[Footnote] * Both these beds of coal occur in the district marked as “greenstone tufa formation” in Dr. Hector's map of Coromandel. (Geo. Rep., 1870–71, p. 98.)

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trachyte and trachytic agglomerate, which, in Sykes Creek, contains gold. At this place the coal is underlaid by a grey fire-clay, which rests unconformably on slates, while at the Hinau the coal rests upon trachytic agglomerate, which again rests unconformably on slates. The coals from the two places are similar to one another (for analysis see Geological Reports, 1870–71, p. 175). These facts therefore prove that the trachyte formation lies in a nearly horizontal position, or in the position in which it was originally formed; and, as the underlying slate rocks are always highly inclined, it necessarily follows that the trachytic formation lies unconformably on their upturned edges, and this can be distinctly seen at the point north of Tararu, at Tapu Creek, on the coast between the Mata and the Waikawhau, and at the Waiau, Coromandel.

I have now, I think, proved—

1.

That no line of separation can be drawn showing the existence of two volcanic formations separated from each other by a long period of time.

2.

That the rock in which the auriferous veins are found does not run in nearly vertical bands, but is lying in its original (nearly horizontal) position.

3.

That all the phenomena are consistent with the idea that the formation is one, the older portions forming the centre, and the younger the outskirts.

With regard to the age of the older part of the formation, we have no palæontological evidence, but there appears to be no reason for separating it from the trachytes of the Great Barrier to the north, nor those of the Aroha to the south, which are undoubtedly tertiary, as they are connected with still existing craters. We have also no evidence of the occurrence of any volcanic rocks in this part of New Zealand before the deposition of the Waitemata series, which I consider to belong to the oligocene period. The rocks themselves, both the trachytes and the dykes, closely resemble those of the gold-bearing rocks of Hungary, which have been lately proved by the Austrian survey to belong to the miocene period, as was indeed long ago pointed out by Professor v. Pettko (Q.J.G.S., 1848, ap. 61); and although I cannot attach so much importance to the nature of volcanic rocks in determining their age as is done by most German geologists, still in this case the two kinds of evidence point the same way. I therefore think that the gold-bearing trachytes of the Thames belong to the oligocene period, a period when volcanic action was active not only near Auckland, but also in the South Island and in the Chatham Islands.