CHAPTER III. GEOLOGY.
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CHAPTER III.
GEOLOGY.
HAVING treated, in the preceding part of this report, of the physical features of the districts traversed by me, I shall now proceed to consider the geological relations of the different mountain chains, both in regard to their age and lithological character, and also of the younger formations lying at their bases. In doing this I shall again adopt Lake Roto-iti, where I began my field work, as my starting point. One mile and a-half from this lake, on the northern bank of the stream by which it discharges itself, we observe a conical hill, which stands about 500 feet above the terraced valley at its foot; I could also trace a continuation of the mountain on the southern side of the stream, where also the valley narrowed very much. This conical hill consists of a granitoid rock, a true syenite, but in structure coarser than the syenite of Wakapuaka, near Nelson. On its eastern and western sides metamorphic rocks are exposed, consisting of dioritic schists, with veins of epidosite; on the eastern side of the hill, whence I followed them, to the northern end of the lake, these schists are changed into quartzose schists.
As far as the dense covering of drift formation with which this part of the country is encumbered would permit, I observed that the direction of the plutonic rock was parallel with the high rocky chain at its eastern side, ceasing however before it reached the northern shores of Lake Roto-roa. These eastern chains, which I ascended in Mount Robert, exhibit the character of true sedimentary rocks. More towards the west, the overlying strata consist of very compact arenaceous sandstones of different colours, principally of bluish grey, and occasionally clear and dark olive green, dull reddish brown, and blackish, alternating with a compact conglomerate in a hard siliceous matrix, the latter in general composed of pieces of clay slates, cherts, quartz, hornstones, and epidosites, but without any signs of granites or gneiss amongst them. This conglomerate changes again into a homogeneous sandstone, in which only angular pieces of clay slate of different sizes are sometimes imbedded.
All these beds are very much disturbed, and are intersected by
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slaty cleavage, which passes through them at a considerable angle to the planes of stratification. Their general strike (with a dip of 75 degrees to 85 degrees towards the west) is north and south. More towards the axis of the chain we meet with blackish, blue, and greyish clay slates, which underlie these sandstones and conglomerates. I was unable, notwithstanding the greatest exertions, to find any sign of fossils in these rocks, the sandstones of which have the character of a true greywake, so that it is impossible at present to assign to them their geological age, which, therefore, must be left to the future, when more time can be devoted to a minute search amongst these apparently oldest of the sedimentary rocks.
Leaving Lake Roto-iti, and following its outlet, I observed that the river bed had worked its way through a large accumulation of the great drift, or boulder formation, lying in places 500 feet high above the river bed, and forming occasionally not less than eight terraces of different relative heights, and of which the uppermost is the highest. In these deposits (till) we find the debris of the ranges on both sides, plutonic, eruptive, metamorphic, and sedimentary rocks, imbedded in loam and gravel, and exhibiting now and then somewhat rough lines of stratification. The pieces, some of them of very large size, are partly rounded and partly angular, showing that they have not been transported by rivers, but rather by icebergs or glaciers, according to the now generally received and ingenious theory on this subject. It is striking, indeed, that in the Northern Island of New Zealand we do not meet with this formation, which therefore here, as in South America, is not found to the northward of the 41st degree of south latitude.
Having only time for a cursory examination, I was not so fortunate as to find any fossils, but I am convinced that, upon closer examination, they will (although generally very rare) be found in this large mass of detritus, which extends as far as Nelson, and there forms, at the eastern side of the harbour, hills attaining to an altitude of several hundred feet. The declivities of these hills slope abruptly to the sea, and have, as a base, a greenish clay marl, in which a few small seams of lignite exist.
On examining the country around the lake, I was much struck with a large level opening towards the north, in which, upon a careful view, I found the remains of terraces stretching towards the opening in the eastern chain, which leads to the Wairau, and is commonly known as Cotterell's, or the old pass. I was unable to follow this opening, it not coming within my field of observation.
Upon examining the Roto-iti valley towards the west, where, near the conical hill, it narrows very much, I found that the terraces on the western side of the hill did not at all correspond in appearance with the excavations made by the waters in the
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drift formation on its eastern side. There it appeared that the lake itself had once extended as far as the conical hill, which formed, whilst still connected with its southern continuation, a natural barrier for the waters. It was therefore evident, from these observations, that the outlet of the lake in former times was towards the east, through the Wairau pass, in the deep depression in the eastern chain. It was without doubt in consequence of one of the great changes in the axis of the island, the downward motion of which was towards the west, that the river could no longer find a passage through this now upheaved depression, and that the waters of the lake, after having risen to a considerably higher level, flowed over the barrier formed by the crystalline rocks on its western side, and, uniting with the Howard, took the course of that river, which had probably already forced its way through the Devil's Grip.
I shall have ample opportunity of elucidating the interesting subject of the great changes in the level of the country within (geologically speaking) recent times, of which so many conclusive proofs lie before the eye of the investigator.
The great drift formation is found running from the lake along the banks of its outlet to the junction of the latter with the Howard. Here we enter upon the granitic zone, which extends, with only the interruptions of which I shall speak in the sequel, to Mount Murchison, in which it attains its greatest altitude. This granite is the ordinary compound of quartz, feldspar, and mica, and is generally of a white colour. It is everywhere associated with amphibolites, round which the granite lies, and very striking changes occur at the point of contact of these two rocks, of which an interesting section is found in the perpendicular cliffs on the banks of the Howard, half a mile above the junction of that river with the Roto-iti outlet. About a hundred yards from the line of contact, the granite becomes hornblendic, and much darker, and sends into the amphibolite rock numerous veins, which gradually thin out to the breadth of an inch, and become more finely grained. The green lustry hornblende schist near the contact is without any stratification, the whole rock consisting apparently of interwoven crystals of hornblende. Receding from the granite, these crystals become much smaller, and the feld-spathic parts, with grains of quartz equal to them in quantity, assume an appearance of coarse stratification.
It may be as well here to state that, owing to the great masses of the drift formation, or alluvium, which almost everywhere cover the surface of the ground, added to the luxuriant growth of vegetation, which we find undiminished to the very tops of mountains 3,500 to 4,000 feet high, it is often extremely difficult to obtain a section, except by descending into the gorges of the rivers, which I found rather a difficult task, owing to the deep water and almost vertical banks, or by ascending to the high rocky mountain tops,
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which involved a great loss of time. If, therefore, I have been unable to obtain more extended results than those detailed in this report, it must not be attributed to want of energy or perseverance on my part, but solely to the manifold difficulties under which I laboured, and which prevented me from carrying my researches further than I have.
In the Howard, under the drift formation, small seams of lignite occur, similar to those found near Nelson; I did not see them in situ, but I found large pieces lying upon the shingly banks of that river, evidently brought down during heavy freshes. The granitic zone between the Howard and the western foot of Mount Murchison is ten miles broad, and as I passed across it in its whole breadth, I had an opportunity of obtaining a very good section. The amphibolite occurs, not only in small intercalated masses, but often forms mountains of considerable size, as, for instance, Mount Maclean.
Here and there I found the granite charged with needles of tourmaline, whilst in other places it was intersected by large veins of pegmalite and graphic granite, which latter was of a really beautiful structure. Mount Murchison, which I ascended, has all the characters of a true granitic mountain. On its rounded dome-shaped north-western top, rocking-stones and tables were not wanting, whilst the middle and south-eastern peaks exhibited conspicuously the great power of the atmospherilies in a country where, during part of the year, all the mountains above 4,000 feet in height are regularly covered with snow; this, partly melting during the sunny days, is often re-frozen during the nights, forming ice in the fissures of the rock, which, in their polyhedric clefts, offer ample opportunity for disruption.
The same process continues, although on a smaller scale, in others besides the winter season, the condensation of the clouds and mist creating ice, the greatest agent for the destruction of rocks, during the lower temperature of the nights. Heavy rains bring down the detritus, which has formed, by attrition upon the surface of the rocks, many deep valleys on all sides of the mountain. It is astonishing indeed to see on Mount Murchison how very narrow the ridge is which connects its various peaks, and how abruptly both sides fall for 1,500 to 2,000 feet into deep and narrow gorges, with equally abrupt and steep spurs lying between them. As such narrow ridges become more worn down every year, the formation of pyramidical peaks or needles will be easily understood. I often looked with astonishment at the small rivulets, which have worked deep and broad channels, to which their volume of water bore no apparent proportion, through or parallel with high mountain chains; and in this again we may see a convincing proof that nature, in order to produce great effects, often uses only small but continuously working means, the results obtained being far more extensive than any which would accrue
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from the application, for a short time only, of the greatest hitherto experienced abyssological powers.
The action of the waters is still more visible on Mount Robert, where they have worked three large and deep valleys, running eastward from the central ridge towards the southern extremity of the Roto-iti lake. The valleys are broadest in the middle altitude of the mountain, and narrow, before they reach the lake, to mere gorges. The main ridge itself, sloping abruptly towards the west, is often so sharp as to leave merely space for one's footing. The mountain in many places is only covered with forest to an altitude of 3,000 feet, above which a barren desert begins, strewed with angular pieces of rock of all dimensions. As the agencies which have produced these effects are still carrying on their perpetual work of destruction, it is clear that no vegetation capable of giving firmness to the rubbish can spring up. The whole scenery presents, in fact, an image of utter desolation and waste, and it is only here and there, where comparatively level spots occur upon the ridge, and upon its more moderate declivities, that we find an Alpine vegetation, forming occasionally a lovely carpet of flowers, springing from the desolation around, and contrasting agreeably with the bizarre forms of the rocky points about it.
Here, again, we observe that the two mighty agencies which determine the form of the surface of our globe endeavour to counteract each other, for whilst the abyssological force upheaves or depresses, the atmospheric destroys and levels, both continuing their slow but incessant work for myriads upon myriads of years. The granite near Lake Rotoroa diminishes in extent towards the south, and a change into gneiss-granite and granulites, containing a great quantity of garnets, is traceable, between which amphibolites predominate. In the upper Mangles a vein of almost pure magnetic iron ore appears, which, with a strike nearly north and south, and a dip of 79 degrees towards the east, intersects the granite, and has a breadth of twelve to fourteen feet. At the southern side of the Roto-roa we meet with an assemblage of eruptive rocks, which have broken through the amphibolite, and which appear most conspicuously in the magnificent dome-shaped Mount Hutton, at the southern corner of the lake, composed of a beautiful red augitic porphyry.
More towards the west two other mountains of almost equal size appear, formed of diallage and hypersthene rocks, the nearest of which to Mount Hutton I called Mount Playfair, placing the name of the pupil and friend in conjunction with that of his and our great instructor. More towards the south, on the eastern side of the Spencer mountains, we find a whole series of diabasic rocks, diabases, aphanites, Ilerzolithes, porphyries, and amygdaloids, occasionally rising in magnificent cones over the surrounding country, of which Mount Smith, lying at the foot of Mount
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Franklin, and consisting of a compact blackish green augitic porphyry, with crystals of uralite, is the most important.
The Spencer mountains themselves, which tower with their rugged peaks above all these regular cones, consist of the same sedimentary strata as the ranges east of the town of Nelson, with which they stand in the closest connection. These strata become more crystalline towards the east (their axis), exhibiting the same characters as those of the section which I obtained last year when examining the chains from Nelson over the Pelorus to Queen Charlotte Sound. The slates, which on the western side are mere clay slates, there become more crystalline, alternating with dioritic schists, and passing, near the axis, into almost crystalline micaceous clay slates, with quartz layers and veins, and have a remarkably regular strike and dip, as I observed on tracing them from Queen Charlotte Sound towards the south-west to the Kaituna pass.
I am not able at present to say in what connection the two chains on the eastern and western sides of the fissure (where they almost meet) stand to each other, as I did not travel so far; but judging by the boulders in the Maruia, where this river leaves the Cannibal's Gorge, the eruptive rocks must cease there; for, notwithstanding the most minute examination, I was unable to discover any trace of them, finding only the same sandstones and slates as before described, mixed with the detritus of the western chains, which has quite another character, and of which I shall speak in the sequel.
Before leaving the Spencer mountains, I may offer a few remarks upon their origin. We see in the longitudinal fissure, and in the mountains on both sides of it, the effects of a great disturbance in the earth's crust, but we do not see all its causes. These we have principally to seek in the Kaikoras. Unfortunately, when I last year examined the Wairau and Awatere districts, I did not find the necessary time (as the results of my field work had to be embodied in my friend Dr. Hochstetter's lecture, and his stay in New Zealand was necessarily limited) to follow the last river sufficiently far to obtain a clear insight into the structure of that magnificent mountain system; but from the boulders collected in the Awatere, it was certain that it consists partly of eruptive and partly of volcanic rocks. I speak here only of the inland Kaikoras, because, after having seen the seaward Kaikoras, although only from the deck of a steamer, I am satisfied that, like the Spencer mountains, they are composed of sedimentary strata, the chains of which they consist striking northeast and south-west, with large longitudinal valleys between them, and not having in their outline the least appearance of volcanic or eruptive origin.
It is further an important fact, that all the rivers having their sources in that district flow parallel with those chains from south-
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west to north-east, without breaking through them, as is the case with the rivers on the western side of the island. It is therefore certain that all the chains, without exception, on the eastern side of the longitudinal fissure, the Spencer mountains as well as the seaward Kaikoras and their branches, both towards the north and the south, mantle round the inland Kaikoras, by which they have been upheaved; and this event occurred probably at the beginning of the tertiary period, to which I shall refer when treating of the extensive tertiary strata which cover the longitudinal fissure. In what connection the eruptive and volcanic rocks stand to each other can only be determined when these mountains are thoroughly examined by the field geologist.
Returning to the central granitic mountains, we meet, on their eastern side, to the north of the Buller, in the Owen river, a magnificent porphyritic granite, with large rose-coloured orthoklas crystals, which protrudes into the other granite in large veins.
This newer granite forms, more towards the north, low round-topped hills, which lie at the foot of the older white-coloured granites. At the head of the River Owen this porphyritic granite (which played, as I shall hereafter show, such an important part in the formation of the eastern side of this island) strikes towards the north, between the rocky mountain chains on both sides of the low saddle between this river and the Wangapeka.
In the eastern chain, a range of from 5,500 to 0,000 feet high, we not only find large beds of white crystalline granular marble, sometimes sacharoid, alternating with quartz schists, similar to those in the range between the Rivers Takaka and Riwaka (and also observed by Dr. Hochstetter in the gorge of the Wangapeka), but we also meet with some varieties with parallel lying mica, cipollin, and others with hornblende, &c.
Some of the limestone strata do not exhibit the same highly crystalline structure; they are more compact, and show such a variety of colours, that no doubt in future years they will be highly valued as marbles for artistical purposes. Mount Owen itself, now isolated, but in reality the southerly continuation of the dividing range between the Buller and Wangapeka valleys, consists of old clay slates of a bluish grey colour; here and there a rock occurs, a metamorphic rock of the slate formation, which shows, by its dull greenish or brownish colours, and its intricate scaly structure, that it belongs to the cornubianites. Round Mount Owen, and at the western foot of the granitic ranges, we meet with tertiary strata of large extent, forming mountains, which reach in some places to an altitude of 2,500 feet above the level of the sea. On the northern side of the Buller, as well as near Mount Murchison, and near the Tiraumea plains, these strata strike very regularly, nearly north and south, with a dip of from fifty-two to sixty-eight degrees towards the west. The same strike is also observed in the Tutaki, but both there and at its
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junction with the Buller the dip is towards the east, often as great as seventy-five degrees. They consist of regular beds of coarse loose sandstones, coloured by oxide of iron. On the eastern side of the Tiraumea plains, at the Buller, they consist of regular bands, from two to three feet broad, of greyish yellowish clay-marls, alternating with firm grained ferruginous sandstones, under which large pebble-beds appear. Sometimes the sandstones are micaceous, as at the junction of the Maruia with the Buller; often they are black from carbon, when found near shales, which in some places are common. Fossils are very rare, and with the exception of a few specimens of the genus dentalium, two specimens of pecten, identical with those found in the overlying limestones of the brown coal formation at Motupipi, I found only the remains of a bivalve shell, belonging to the freshwater mollusca, and being most nearly allied to Anodonta.
Everywhere small seams of brown coal appear, of which the largest one, which I found in the Mangles, near its junction with the Buller, was three inches thick, but they all, without exception, appeared to thin out and to have been formed by driftwood, imbedded in spots fitted for its reception. In other places small patches of the same coal occur, as if formed by the carbonization of a single trunk.
I observed imbedded in sandstone, in the Buller, to the north of the junction of the Murchison, the trunk of a large tree completely silicified, but still showing distinctly the annual rings. These seams of brown coal are accompanied by many very fine impressions of dicotyledonous leaves in the sandstones and shales, and in some places the leaves are still so well preserved that they can be taken out of the rock in an almost perfect condition.
All the above cited facts prove that these deposits have been accumulated in an estuary, marine and fresh water formations probably alternating, and indicate a frequent oscillation of the level during the period of deposition. The regular succession of gritty sandstones and soft argillaceous marls, alternating with perfect regularity to a considerable altitude, proves further that these beds were deposited under re-occurring circumstances of similar character. It may have been that during heavy freshes in the rivers great quantities of sand and pebbles were brought down, and distributed upon the steep sides of hills lying beneath the waters of the estuary, and that when the freshes subsided, they only brought down mud and fine sand, which were spread over the underlying stratum; or it may have been that the direction of submarine currents was from time to time changed by the successive upheaval and depression of the shores. Whatever the cause, however, the alternation is very striking, and brings vividly before the mind of the explorer the fact that the same agencies which we observe daily in operation, worked without intermission during bygone ages, when the form of this island was entirely different.
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Where the conglomerate occurs, we find that the course of the river is always contracted, because those strata, strongly cemented by an argillo-arenaceous matrix, and consisting principally of rounded masses of chert, hornstone, quartz, quartzite, jasper, sandstone, &c., have resisted much better than the soft sandstones and marls the erosion of the river. In many places we observe striking examples of the degrading force of the river current, and how frequently nature employs only small means to produce great results. In the soft sandstones and clay marls near the bed of the river, large and perfectly circular holes are found, many feet deep, and having frequently a diameter of from five to eight feet, at the bottom of which lie rounded boulders of harder rock. During the freshes these boulders are put in motion by the whirling waters, and by their attrition every year make their abode broader and deeper. This tertiary formation, which is of the same age as the Motupipi brown coal (miocene?) is not only of great extent and elevation, but has also undergone considerable changes in its stratigraphical position; for even admitting that the strata were originally deposited on the slopes of mountains beneath the sea, lying at a considerable angle, this could not have taken place at the angle of seventy-five or eighty degrees which they exhibit in some of the hills in the Tutaki. We may therefore conclude that since their deposition they have not only been upheaved vertically to their present elevation, but that at the same time they were torn and rent in various directions, as exhibited by their present positions. As no tertiary strata appear to the west of the western range, the Lyell and Brunner ranges, &c., it is evident from this and for many other reasons, that the principal waters of the estuary alluded to came from the western side of the island, in which so many interesting changes have taken place, and that it was, in fact, formerly a mere extension of Blind Bay. This tertiary formation stretches as far south as Mount Mantell, where we meet it on both sides of the Matakitaki and the Maruia; it then disappears, and no other deposits cover the outcropping plutonic and metamorphic rocks than drift and alluvium. In the Matakitaki plains we again find tertiary strata covered by a large deposit of boulders, in some places 200 feet thick, below which layers of a greenish blue clay marl are visible, between which here and there occur small seams of lignite, which gradually thin out.
I have already spoken of Mount Owen and its formation, and shall now proceed to observe upon the central chain of the island, lying on the western side of the longitudinal fissure, between it and the West Coast, and bounded on the north by the Mokihinui river, and on the west by the Grey plains. Although I traversed this chain I have not obtained such sections as I could have desired. In a few places only I found sedimentary rocks in situ, and the scarcity of our provisions just as we reached this central
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part of the island would not permit me to stay there for the time necessary for exploring more closely the structure of these chains. By examining the angular pieces of rock brought down by the mountain torrents, I obtained, however, an insight into the formation of that wild mountain region. This central chain is flanked on its eastern side, as far south as Lake Roto-roa, by granite and gneiss granite, and has there been more denuded than on its western side, where, in the Tasman mountains, the Lyell and Brunner ranges, Mount Alexander on the southern bank of the Inangahua, and Black-hill on the southern bank of the Grey, it forms, as before described, a wall of rocky mountains from 6,000 to 6,500 feet high, striking nearly north and south. The whole chain on its western side, where I ascended Black-hill, consists of gneiss granite. In advance of this chain, and running parallel with it along the line of the Grey plains, we find conical hills and mountains, composed of a red porphyritic granite, which has protruded into the chain, many remarkable examples of which are visible. Some of these mountains reach, as for instance in Mount Gore, to a considerable altitude, and I noticed that where these hills were highest, the gneiss granite chain to the east of them also attained its greatest elevation, and became distinguished by the ruggedness of its outlines and by its needles and peaks. This granite, probably of secondary age, as I shall show when I treat of the Grey and Buller coal-fields, breaks through the central chain in all directions, but without disturbing materially its general strike. It surrounds like a framework the sedimentary rocks, which tower amongst it. in rugged masses, and it is in the lines of this framework that the courses of the principal rivers which traverse the mountain chain occur; and as my route led me mainly along the river courses, it will be seen that, with a very trifling exception, I travelled entirely over granitic ground. The only point on the western side of the chain, north of the Buller, where I saw this gneiss granite in situ, was at the Tata Island, whilst assisting my friend Dr. Hochstetter in collecting details for the geological map prepared by him. I am not able to Bay whether the porphyritic granite also exists near Separation Point, but, from what I have seen of the country, I infer that it will be found at the Wangapeka, in the great break which there occurs in the chain, the hills in the gap very much resembling in form those in the Grey plains. At the head of the River Owen, it is seen protruding through the chain, and continuing in a southerly direction, flanking Mount Murchison on its western side, and is there lost sight of, being for the most part covered by tertiary deposits. On the eastern side of the Lyell range it again rises in large mountain masses, of which Mount Newton, about 4,500 feet high, lying on the right bank of the Matiri, is the highest. Thence, running in a northerly direction, in hills of smaller dimensions, it divides the Lyell range from the Tasman mountains,
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and reaches the coast a few miles north of the Mokihinui river, whence, as before, it flanks the central range as far as Lake Brunner.
Although I have termed this granite porphyritic, its structure is in many places very variegated; often, as in Mount Newton, it becomes finer grained, in other places it is syenitic or hornblendic; sometimes it is dark, from an abundance of mica, and at others it presents the appearance of a true porphyry, the compounds becoming so finely grained as to assume that character, and large rose-coloured orthoklas crystals being richly scattered throughout it. The contrast between this rich coloured rock and the whitish gneiss granite in contact with it, is very striking, and I observed, that where the latter is intersected by veins of the former, the vein granite is more finely grained.
From Mount Newton a branch chain extends down the Buller, separating the stratified rocks of the Lyell and Brunner ranges, which lie on its opposite banks. Another branch chain extends towards the south between the Brunner range and Mount Mantell, and is there associated with trap and amphibolite, the latter of which, at the northern end of the Maruia plains, is of great extent. The granite reaches its greatest elevation in Mount Mueller, which rises from the Maruia plains. It also surrounds the Victoria range, and, at the point where the Ruhu enters the depression, becomes hornblendic. Continuing its course towards the south-south-west, it branches off in different directions, all, however, having a westerly trend, and between which lie the courses of the rivers Inangahua, Waituhu, and Wharau; thence it runs towards the Ahaura.
On the western side of Mount Hochstetter, the Grey leaves the main line of the depression, and follows a smaller branch, which runs north-north-west, thus uniting it with the granite masses on the western side of the range. The same granite is also found in the Grey plains, in conjunction with the metamorphic schists, and in the high granitic mountains near Lake Brunner, preserving its porphyritic structure, as I found from specimens brought to me by Mr. Rochfort. Between this granitic net work and the old gneiss granite on both sides, we find a great variety of metamorphic and sedimentary rocks. On the western side they consist of very quartzose mica schists, granulites, old clay slates, quartz schists, cherts, and very compact sandstones and conglomerates, the latter frequently composed only of angular pieces of old clayslate, cemented together by a semi-crystalline silicious matrix; the whole associated with traps, amphibolites, and dioritic porphyries. The sandstones lie towards the centre of the chain, and I obtained, at the eastern foot of the Victoria range, a good section. The strata in the mountain gorge, in which I examined them, consisted of a hard zoned silicious rock, with small veins of pegmalite. They had a strike nearly north and south, with a dip of
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eighty-two degrees towards the west, and contained near the intersecting veins inchoate hornblende crystals. Higher up in the mountain range, traps and hornblendic granites occurred, as I judged from angular pieces of those rocks brought down by the torrent amongst the sandstones, cherts, and amphibolites, of which its principal gravel consists.
The continuation of Mount Mantell, on the southern banks of the Warwick, consisted, as far as I could judge from the rocks in situ and from pieces brought down by the mountain torrents, of a succession of highly crystalline slates. In the bed of the Maruia, where it breaks through the Cannibals' Gorge, we find, close to the depression, beautiful mica slates, chlorite schists of different colours, quartz schists, old clay slates, cornubianites, and spotted slates (fleckschiefer) with sheaf-like concretions of a blackish green substance (fahlunite?), and also greywake sandstones, green and red Maitai slates, and micaceous clayslates, alternating with small quartz layers, the latter three without doubt brought from the Spencer mountains, and identical with the rocks seen in the sections from Nelson to Queen Charlotte Sound. The rocks in the opening itself, besides granite, are hornblende and quartz schists, exhibiting the same strike and an almost vertical dip. Another interesting fact is the total absence of crystalline limestone on the western side of the central chain, or south of the Buller, on the eastern side; this zone appearing therefore to be confined to the north-eastern side, where it is so largely developed. That the Southern Alps continue to exhibit the same structure as the chains before described, has ben proved to me by the rocks collected by Mr. Rochfort on the saddle between the Hurunui and the Taramakau, and by those rocks taken from the beds of the mountain torrents which fall from Mount Cook, which Mr. J. Mackay had the kindness to bring to me on his return from the lower parts of the West Coast. Mount Cook itself consists of mica slates, chlorite and quartzose schists, as well as of graphite slates and old clayslates.
Amongst the boulders in the Grey we meet with small pieces of nephrite, the greenstone of the Maories, derived from veins in the slaty rocks, but the locality in which it is principally found to the south of the Grey is the Arahaura river. It is also found all along the coast, from Milford Haven to the Punakaiki cliffs, north of the Grey, in the boulders on the sea shore, whence magnificent specimens are obtained. The Maruia plains are of a lacustrine formation, and were evidently the bed of an old lake laid dry, partly by the accumulation of detritus brought down from the mountains, and partly by upheaval, as evidenced by the terraces on both sides of the plains. In some of the vertical sides of these terraces, formed by the action of the rapid river, which has often changed its bed, we observe deposits, consisting, near the margin of the old lake, of sand and gravel, mixed with boulders of every size,
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brought down by adjacent streams, and indicating, by a difference in the size of the particles in the various layers, whether the stream by which they were deposited was at the time swollen or low. At other points, close to the mountain gorges, they exhibit the character of the delta of a torrent, consisting almost exclusively of boulders, which from year to year narrowed by their accumulation that part of the lake, whilst in its central part we meet with freshwater marls of a bluish colour. Here, as at Lake Roto-roa, I observed also that at the northern end, where the waters had to find their outlet, hornblende rocks strike across the valley, which, from their hardness, offered greater resistance to the action of water than the adjoining granitic rocks, which were more easily decomposed.
My endeavours to find fossils in the slaty rocks of this chain were not crowned with success, but, from their lithological character, we may presume that as they resemble closely the rocks of the Cambrian and Silurian age in other parts of the world, they too have been formed in those remote periods. The boulders and angular pieces in the Grey, where it enters the opening, as well as those of the Maruia itself, south of Mount Mueller, show that this depression is the eastern boundary of the granite, which from this point strikes towards the south-south-west. It is without doubt, that the thermal springs discovered by Messrs. Lewis and Maling near the banks of the Maruia, in the Cannibals' Gorge, take their rise at the line of contact of these rocks. I have laid down in the preceding pages what I was able to observe in these central ranges, and I must leave it to other geologists, when this now inhospitable country has been rendered more accessible, and therefore, when they will not labour under the same disadvantages that I did, to fill up the details of the geological map of this province, in which I could only lay down with accuracy that part of the country of which I obtained actual sections, although my successor will, I think, find that the general geological features which I have filled up, partly by induction, and partly by judging from angular pieces of rock brought down by the torrents, will not be far from correct.
On the western bank of the Alexander stream we meet, for the first time, with a sedimentary rock of newer origin than those of the central chain; an arenaceous sandstone of greenish yellow colour, with veins of quartz, striking north and south, with a dip of forty-five degrees towards the west. In a few places casts of small shells are visible, but not sufficiently distinct to enable me to form any conclusion respecting them. This sandstone overlies the gneiss granite encircling Black-hill, and reaches, in the Herschel mountains, a considerable altitude; forming, on the banks of the Grey, hills only a few hundred feet high, it has been altered in many places in the most interesting manner, of which I shall state a few instances. About a mile from the Alexander
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stream, on descending the Grey, I found the sandstone ceased, and a magnificent porphyritic granite appeared, intruding its masses into that rock, affecting however hut little the strike and dip previously observed. The granite itself becomes, where it penetrates the flaggy sandstone beds in tortuous veins, charged with mica, the large feldspar crystals disappearing. The sandstone beds close to the contact are altered to mica schists. Fifty yards from the line of contact I met them again, having succeeded in climbing down to the river's edge in order to obtain a view of the section. The sandstones there had a semi-crystalline appearance, the same strike and dip; the mica had disappeared from the body of the rock itself, but in the numerous joints which intersected the strata, a great quantity of mica scales were to be found, showing in a very instructive manner, that the altering powers of the granitic veins were not strong enough here to effect the same changes as they did near the immediate contact, but that they were still in sufficient vigour, where unopposed by the resistance of the compact rock, to form mica scales. Two miles lower down the Grey we meet with large masses of granulite, with garnet crystals, striking always in the same direction. The granite shows itself also in the Grey plains, forming here and there, in the river bed, low cliffs, often in contact with sedimentary rocks, which are invariably altered into granulites or micaceous schists. The formation of the Grey plains is of a very interesting nature, showing what great changes have occurred in the level of this island in (geologically speaking) recent times. In the northernmost part of these plains low cretaceous hills occur, which slope down to the northern bank of the Buller. From the Buller the whole of the plains as far south as Lake Brunner are covered with drift formation and old alluvium, the only place where cretaceous rocks, a limestone charged with mica, crop out, being in the Inangahua, at its confluence with the Buller, and for eight miles up the former river. More towards the south, in the Grey, the bed of which I examined closely, these older strata cease, and a younger tertiary formation (pliocene?) directly overlies the granite. It consists of a coarse pebble bed, cemented by an argillaceous matrix, on which reposes a stratified bluish or greyish clay, full of silvery mica scales, alternating with small seams of lignite, also full of mica, and with beds of sand. These strata reach, on the eastern side of the plains, an altitude of one hundred feet, and are there overlaid by great quantities of till.
I obtained here the following sections, taken in ascending order:--
12 feet pebble bed.
3 feet greenish clays, with scales of mica, and pieces of drift-wood changed into lignite.
2 1/2 feet sands, composed of grains of quartz and scales of mica.
2 feet bluish clay marls, changing in their uppermost part into slate clays.
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4 feet lignite, often very fissile.
3 feet slate clays.
3 1/2 feet lignite.
4 1/2 feet clays, gradually becoming more ferruginous, and ultimately changing into loam and drift, which last was at least forty feet thick.
The strike of these beds was from north-north-west to south-south-east, with a dip towards the east-north-east of 28 degrees.
Near the junction of the Mawhera-iti with the Grey, on the southern side of the main river, I obtained another section. Here the banks are almost vertical, exposing a section of nearly 120 feet. In the river itself we find a large stratum of clay marl, in which are also many pieces of driftwood converted into lignite. These beds, of a bluish colour, are nearly horizontal, and at one place the stump of a tree, fifteen inches in diameter, broken off about two feet above the root, stands apparently in situ, the roots still adhering strongly to the clay marls, so as to lead to the inference that it grew upon the spot. These beds, which rise nine feet above the level of the river, were probably deposited in a shallow estuary; they are divided at irregular intervals by horizontally-deposited layers of mica; they change insensibly into loam, which is succeeded by a large accumulation of sand, gravel, and loam, interstratified with layers of boulders, partly angular and partly rounded, and resembling very much the drift formation near Nelson.
At the western side of the Grey plains a succession of ferruginous clays, thirty feet thick, occur, over which again the drift formation appears. The rivers running through them have formed several terraces, which, near the mountains, attain an altitude of 200 feet, whilst in the central part of the plains there is only one terrace from 100 to 120 feet in height, bounding the lower grounds near the river. A glance at the geological map shows that these large tracts of level land were formed before the tertiary period, and that the Buller emptied itself into an arm of the sea, which then occupied the space now included by these plains, into which the Inangahua, the Grey, the Ahaura, &c., also discharged themselves. During the sinking of the country in the glacial period the valley was filled up, and when the island rose again, an insurmountable obstacle was probably presented to the southern flow of the Buller, partly by the accumulation of drift during that period, and partly by the depression caused by the tilting (so to speak) of the whole country from south to north.
A depression in the western chain, worked out either by sea currents or by rivers descending from it, at the same time offered to the Buller its present exit for its waters. This theory is confirmed by the fact that in the Buller valley, between the Paparoha and Papahaua chains, no drift is found having the least resemblance to that seen on the banks of the river flowing through the plains.
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The Paparoha chain, and its continuation to the north of the Buller (the Papahaua range), are of great practical interest, because it is amongst them that we meet with highly valuable and extensive coal-fields, of which the Grey river, shortly before it reaches the sea, has exposed a magnificent section, to the examination of which I devoted some considerable time. The Paparoha central range is surrounded by granites of very various character, and itself consists of gneiss, mica slates, eklogites, quartz schists, and sandstones, which seem to occur in the centre of the chain, the whole cut through or otherwise associated with traps, amphibolites, and dioritic porphyries.
The Davy mountains, of a younger formation, lie on the southwestern side of the granite, which, near the Mico Clifts, reaches the sea; they strike north and south, with a dip towards the east and west, forming a fold, and exhibiting near the Grey, where the range slopes down, a qua-quaversal dip. This range has been denuded in the upper part of the fold, and the strata underlying the coal-bearing formation are exposed, of which, having an opportunity of examining them on the coast, I shall speak in the sequel. It is evident that the disturbing force which threw the strata towards the west, thus forming the arch or fold, has been exercised by the granite on the eastern side of the Davy mountains, as well as by the eruptive rocks which have invaded the strata, as for instance the dioritic porphyry near the Kokiwi river.
An almost complete section of the coal-bearing and overlying strata is exposed on the banks of the Grey, of each of which the following table gives the approximative thickness only, with the exception of the coal seams, which I carefully measured.
FT. IN.
A. Ferruginous sandstone (grindstone) with concretions of clay iron stone...........255 0
B. Clays, brown and dark grey, occasionally bluish........160 0
C. Ferruginous sandstone, like A.......70 0
D. Brownish yellow clays, with mica...............60 0
E. Calcareous sandstones, changing into grits, the latter with smallseams of shale..........120 0
F. Seven small seams of coal, from four inches to sixteen inches, with shales and micaceous grits between.............9 0
G. Micaceous sandstones and grits, with small seams of coal, and trees changed into coal..........21 0
H. Main seam of homogeneous pure coal at the eastern side of the fold...............12 7
And at the western side.....................17 2
I. Grits, often very micaceous, often coloured by carbonic matter............190 0
K. Seam of impure shaly coal........4 0
L. Grits, layers of very micaceous slaty sandstones and coal, the latter probably derived from driftwood..............80 0
M. Coalseam..................1 2
N. Grits and shales, with some layers of mica.............5 0
O. Coalseam...........4 6
P. Grits and shales.................15 0
Q. The last visible seam, very much cracked; thickness as far as I worked into it.........12 1
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The seams F and H, striking north and south on the western side, and with a dip towards the west of 11 degrees, are again visible on the eastern side of the chain, where they have the same strike, but there they dip 13 degrees towards the east. As we near the axis of the chain the dip becomes greater, and increases at the lowest visible seam from 9 degrees to 20 degrees. The main seam contains a magnificent and very compact coal, and evidently has not been so much disturbed as the seams M, O, and Q, which we meet with in the centre of the section, the last three striking east and west, with a dip towards the south of 26 degrees. The seams M and O have been cut through by the river transversely to the flexure (or bend), and their upper parts only, lying sixty feet above the river, are occasionally exposed, the remainder having been washed down, partly by rivulets flowing from the mountain, and partly by the main river during freshes. Amongst the detritus which here covers the sides of the mountains, and which is in fact only the remains of large slips, the seam Q is visible. It is very much cracked, the coal fractured and brittle, and at the same time lighter and more bituminous than that of seam H.
The coal-field, although of a more recent formation, presents exactly the same characters as those belonging to the true carboniferous period; we meet with the same shales, grits, and sandstones, and the coal itself cannot be distinguished from deposits of primary age. The larger seams have their under clays. The grits consist of quartz grains, very little rounded, with occasional crystals, and of mica and feldspar, the whole embedded in a compact silicious matrix, so that, although of pure mechanical origin, they resemble a coarse granite (arkose). In some places the mica predominates, lying in thin horizontal bands in a silicious matter, frequently coloured by carbon, together forming a beautiful rock, the contrast of the black matrix with the silvery mica being very striking. There are also some bands of pure mica, three to four inches think, which by pressure have been formed into a rock, so closely resembling a real mica slate, that it would almost be impossible to detect any distinction between them, unless its origin were known. How various and sudden must have been the alterations in the conditions of the currents of the sea, and in the level of the land, to allow of the deposition of such different strata often within a single inch of each other. The coal itself cannot be distinguished from the Newcastle coal of Australia, its structure, lustre, specific gravity, mineral composition, and combustibility being similar.
I may here give an analysis made by Mr. Tatton, of Nelson, which confirms me in the belief, originally formed on the spot, that this coal-field is of the same age as that of Newcastle namely, the great oolite.
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ANALYSIS OF THE GREY COAL, BY MR. TATTON.
No. 1.
|
|
West Coast.
|
Sydney.
|
| Carbon
|
74-26
|
74.13
|
| Hydrogen, Nitrogen, Oxygen
|
25.74
|
25.87
|
|
|
100.00
|
100.00
|
No. 2.
|
|
Volatile Products
|
Coke
|
Incombustible Ash
|
| 1000 parts; West Coast
|
234
|
766
|
13 1.6
|
| 1000 Parts; 3 Sydney
|
247
|
753
|
15 3/4
|
No. 3.
|
|
Coal.
|
Water.
|
Reduced to
|
|
|
oz.
|
oz.
|
oz. grs.
|
| West Coast
|
10
|
20
|
9 14
|
| Sydney
|
10
|
20
|
9.3/4 8
|
The shales were very rich in impressions of plants, and I collected a great variety, which will enable the paleontologist to determine by what plants these huge coal deposits were principally formed. The greatest part are leaves of dicotyledonous plants, in a high state of preservation, and often several inches long and broad; but I found also portions of plants belonging to the families Cycadeae, Brachyphyllum, Calamites, and Zamites. I only discovered one fern, allied to Pecopteris, and one Equisetum; and we may therefore infer that we have rather to do with the remains of extensive forests than with those of swampy or rushy grounds.
In the shales and sandstones belonging strictly to these coal measures I could not detect any fossils, but in the arenaceous limestone (or rather calcareous sandstones) overlying them I met with some exuviae belonging to the genera Monotis and Serpula. In the clays D, I obtained a better harvest. Here I met with numerous bivalve shells from acephalous mollusks, as well as from brachiopods, amongst which the genera Monotis, Posidonomya, Inoceramus, and Terebratula occur; gastropods are also abundant, amongst which small species of Fusus and Murex were the most numerous. I found, too, a small Echinus, but unfortunately in a very fragmentary condition. The absence of cephalopods is very striking, and we may conclude from it that these strata were deposited in shallow or muddy waters not frequented by belemnites, ammonites, &c. As soon as the specimens collected by me have been described by an able paleontologist, I will make the result of this investigation known to you.
The coal-bearing strata are covered for seven miles near the coast line by cretaceous rocks, of which I shall speak after having completed the description of the former. Where these cretaceous
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rocks terminate, the formation for half a mile is obscure the country consisting of low wooded hills, covered with boulders and till. We then again reach the Davy mountains, the declivities of which slope abruptly to the sea shore, and the following strata are exposed, taken in descending order, and travelling northward: a sandy clay, whitish yellow, with concretions of clay iron-stone and iron pyrites, assuming as we advanced more the character of an argillaceous sandstone, of a reddish colour, resembling a Keuper sandstone, and being from fifty to sixty feet thick. In this sandstone I found the cast of a monotis. This was succeeded by a very compact greenish baggy limestone, from 100 to 120 feet thick, sometimes silicious, with remains of crinoideans. Then a band of carbonaceous shale, often assuming the character of an impure coal, and about four feet thick. Below this seam lay a conglomerate, or rather a pebble bed, at least 400 feet thick, consisting of well rounded pebbles, seldom larger than the list, of a green compact sandstone, which constituted the greater part of the conglomerate, with occasionally pieces of quartz and jasper, but without any traces of plutonic or eruptive rocks. These pebbles were embedded in a white quartzose sand, the particles of which, although not cemented, cohered strongly, and formed here and there, where free from pebbles, bands from three to four feet of a white sandstone, consisting purely of small crystals or grains of quartz. All these beds have been very much disturbed, exhibiting various large faults, by which the strata have been dislocated vertically for at least 200 feet.
At the mouth of the Wai-a-whenua this conglomerate predominates. In the bed of the river I found grit, shale, and pieces of coal, which, on ascending the river for some distance, became more frequent and angular. It was evident that the coal-bearing strata, which near the sea had been denuded, still existed here, forming, about a mile up the river, where the mountain began to rise considerably, very steep walls; but my time was too limited to permit of my ascending further to examine them.
From this point to Maukurinui, lying in the centre of the Davy mountains, a distance of three to four miles, the strata were very much disturbed, exhibiting numerous flexures and faults, so that it required the greatest care to reduce into order their apparently confused condition. The strata below the above described conglomerate, consisted of a sandstone, in stripes, from a foot to sixteen inches thick, of a greenish and bluish grey colour, the green being arenaceous and the others argillaceous, giving to the rock a ribbon-like appearance. Amongst these bands appeared now and then small beds of conglomerate, with pieces of driftwood changed into coal; total thickness, several hundred feet.
A bluish clay slate, with slaty cleavage, twenty-five feet, underlaid by a very compact sandstone, which, in its upper part, is of a more greenish colour. Towards the centre of the chain
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this sandstone becomes more silicious, and of a blackish blue colour, and is in some places flaggy, whilst in others the stratification is only indicated by the different colours of the deposits. All these rocks are intersected by joints. At the lowest visible part of these strata (at Maukurinui point), the sandstone is traversed by large quartz veins. It has been described by a former explorer as basalt, he mistaking the joints, coupled with the great dip of the strata, for a columnar structure.
At this bold point, whilst climbing up the steep side of the cliff, I found impressions of fossil fishes in the rock, but as the native urged me not to lose a minute, the tide then flowing fast, and being likely to jam us for the night amongst these vertical cliffs, I was obliged, to my great annoyance, to leave the spot, which would, had I found time to secure some of these rare fossil remains, have thrown much light upon this interesting formation. It would be needless to give the strike and dip of all the strata, as they change very often, sometimes becoming almost vertical, and sometimes appearing to have been overthrown, but I shall give a general outline of them.
At the Wai-a-whenua, south of the Maukurinui point, they strike from south-east to north-west, with a dip towards the south-west of 11 degrees, rising to 45 degrees near Kararoa. Near the Maukurinui they strike north-east and south-west, with a dip of 53 degrees towards the north-west. Half a mile north of the last-named promontory the mountains recede, and I did not again obtain a section until near the Kokiwi, where I met with masses of dioritic porphyry protruding through the chain, which, with the lateral dislocation, accounts for all these disturbances. Half a mile east of the mouth of the Grey, we meet with a succession of small limestone hills, which run nearly parallel with the coast, gradually approaching the sea, which they reach in the wild headland called Komatiki-tawhao. The strata in these hills, with a strike nearly north and south, and a dip of 35 degrees towards the west, exhibit all the characters of true upper cretaceous rocks, although we do not meet with any deposits resembling chalk.
Commencing in descending order, we first meet with flaggy beds, greensands, full of long rounded branching concretions, sometimes flattening towards the end, and often entangled with each other, and which without doubt are casts of fucoids (kelp). Besides Cardium and a Terebratula, I found a large Pecten and a Cyprina, and many Echinoderms, amongst which I collected a magnificent Cidaris, and several specimens allied to Spatangus and Toxaster. This greensand is underlaid by bluish grey chalk marls, 250 feet thick, with bands of compact pale yellowish limestone, which, being harder than the chalk marl, has better resisted the action of the waves, and everywhere projects from the body of the rock. For the first 120 feet, in a descending order, these
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limestone bands are only one to one and a-half feet thick whilst the intervening chalk marl strata have an average breadth of six feet; but in the next 130 feet the bands become broader and the chalk marl less wide, the limestone constituting below these the whole rock. The limestone too becomes more sandy, and is sometimes charged with small fragments of coal, probably derived from the detritus of the Davy mountains. The same fossils as before enumerated continue to be numerous, as well as the remains of a crustacean (a crab), pieces of coral, a large Lima, a large fossil allied to Exogyra, another allied to Crania, and numerous Serpulae.
This formation, which ceases when the Davy mountains, upon which it lies unconformably, approach the coast, again appears at the Punakaiki cliffs. Here it consists of greensands full of glauconitic grains, but without any signs of stratification, the colours, greenish and darkish yellow, being the only lines of demarcation between the various deposits. These greensands are from 100 to 120 feet thick, and are underlaid by arenaceous flaggy limestones, with a regular strike from west-north-west to east-south-east, and a dip of 17 degrees towards the west-south-west. Below these again, on the southern bank of the Pororari, a white crystalline tabular limestone appears, consisting of small fragments of shells, corals, and Foraminiferae, with glauconitic grains, forming castlelike walls, arches, and towers, and presenting magnificent scenery. Below these we meet with dark marls, often very argillaceous, between which small flaggy beds of the above-described limestone lie. Here I first found large fossil oysters and Scalariae, as well, as the greater part of the fossils before enumerated.
These cretaceous beds continue for five miles, but for a long distance I had no opportunity of examining the rocks near the sea, having to scramble through the dense vegetation at the summit of the perpendicular cliffs. In the gullies, however, although I observed that nearly the same strike and dip continued, the underlying strata were sometimes of a nearly earthy texture, in which concretions of chalcedony were imbedded; in other places they were more argillaceous, and of a yellowish colour, with concretions of iron pyrites, but at intervals of fifteen to twenty feet small layers of white crystalline limestone occurred. Scalariae here abounded, and I observed that the exuviae were most numerous where the change took place from crystalline limestone to clay marl, the upper surface of the clay marl bed being literally strewed with fossil shells, amongst which large pectens predominated.
Half a mile north of the Mico Cliffs the granite begins, upon which the above formation, which is at least 1,500 feet thick, rests. As these beds lie everywhere along the West Coast directly upon the granite, we may infer that the protrusion of the latter occurred either during the cretaceous period (all the older cretaceous strata with belemnites and ammonites of the Kawhia
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harbour being missing), or between the Cretaceous and Jurassic periods. The granite is here of a dark colour, and always contains large feldspar crystals, composing at times the greater portion of the rock. It continues for one mile towards the north, where this porphyritic structure disappears.
Two miles south of the Potikohua stream, the cretaceous formation again begins, but is here of a more argillaceous nature; brownish and bluish black interstratified clays, often from eighty to 100 feet thick, alternating with marls, and containing the same fossils as before, accompanied by impressions of leaves and fossil shells allied to Cyrena. Included in these beds were here and there rounded pebbles of an older limestone formation, in all probability underlying the coal formation, and full of fossils, of which I collected a good quantity. The strike of these extensive secondary strata is again nearly north and south, with a dip towards the west, varying between 0 degrees and 13 degrees. The large cave Te-ana-o-Matuku is hollowed out of granitic breccia, consisting partly of angular and partly of rounded boulders of granite, strongly cemented by granitic rubbish.
This granitic breccia projects like a wall far into the sea, becoming towards the east smaller and lower, and whilst the cretaceous rocks on its western side have been washed away, on its eastern they still exist, and amongst them we lose all traces of it. In this breccia a rough stratification is visible, striking from north-west towards south-east, with a dip of 17 degrees towards the north-east. The cave itself runs from north to south, is seventy-five paces long, ten paces broad, and thirty feet high; from the middle another smaller branch runs towards the west, which is thirty-five paces long, and the entrance of which is washed by the sea.
There is only one explanation to be given of this remarkable phenomenon, namely, that the detritus brought down by a torrent descending from a granitic mountain in the west, which has now disappeared, here entered a cretaceous sea, either in a bay or in the main sea, in which latter case the mountain probably stood on an island. The cave was afterwards excavated by the sea, which, during the slow upheaval of the present land, dashed, as it now does, furiously against the cliffs on its margin. In the bed of the river flowing near this cave, a great part of the boulders consists besides of granite gneiss and metamorphic rocks, of eklogite, a magnificent rock (consisting of grass-green smaragdite, red garnets, and silvery mica), and of grits, shales, and sandstones resembling those of the Davy mountains.
A mile further on, the hypogene rocks again reach the coast; the granites become more trappean, and have everywhere intruded into metamorphic schistose micaceous rocks (granulites and mica schists), either in large masses or in tortuous veins, affording to the observer ample opportunities for the study of the
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action of intruding granite upon sedimentary strata. In many places blackish schistose concretions of various forms are included in the granite near the lines of contact, and exactly resembling the rocks of the metamorphic strata, leading to the belief that they were merely fragments of sedimentary rock embedded in the granite whilst in a soft state. These rocks form, as far as Cape Foulwind, the walls against which the sea breaks where no sand beaches occur. They are bounded towards the east, as I noticed when travelling over the Tuhinu hills, by a whitish and more finely grained granite, which, near the mouth of the Waitakere, has a gneiss-like appearance.
North of the last-named stream, some magnificent cliffs are visible, consisting of the same micaceous schist, striking from north-east to south-west, with a dip of 16 degrees towards the south-east. Through this bed run large veins of a beautiful granite, ramifying in all directions, and varying in breadth from several feet to a few inches, and consisting of very large yellowish crystals of feldspar, large hexagonal tables of silvery mica, and of translucent quartz of a vivid bluish tint. These veins are themselves intersected by straight dikes of greenstone, the whole forming the prototype of a geological section. Near Cape Foulwind, the main mass of the granite is porphyritic, and on its western side is bounded by the same micaceous schists, here almost vertical, followed near the sea by semi-crystalline sandstones. The transition from the granite to the metamorphic schists is sometimes almost imperceptible, it first becoming finely grained, and then passing by degrees into the schistose rock, so that it is at times difficult to find the line of demarcation.
Near the Potikohua river, the cretaceous formation leaves the coast, and runs towards the Paparoha chain. Between it and the crystalline rocks at the coast line a large deposit of sands occur, partly white, partly ferruginous, finely stratified, and covering the sides of low granitic hills, whose tops are seen rising above it. In these beds, in the neighbourhood of the Waitakere, deposits of lignite occur, which by former explorers have been described as coal seams.
North of Cape Foulwind, and lying upon the granite rocks, the cretaceous formation again appears, consisting of greensand, stratified clays, and limestones. Amongst the uppermost visible strata is a limestone of whitish blue colour, consisting almost entirely of corals. Between the Waitakere and the Waimangaroha lies the delta of the Buller, the changes in the course of the river as it ran through it at different periods being indicated by numerous terraces.
For several miles before reaching the Buller from Omu Point, proofs of a gradual sinking of the surface are visible, even to the superficial observer. Not only have all the trees on the margin of the forest, which here reaches the sea coast, died, but in the
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tide-way itself, and even for three feet below high-water mark, we find the whole line a succession of huge dead trees, still rooted to the bottom, and presenting with their naked branches an image of utter destruction. It is impossible that they could have stood for any great length of time in such a position without having been uprooted, many of them in fact being already prostrated on the sand; and we may therefore infer that as some spots on the eastern side of this island have been raised several feet during the last few years by earthquakes, doubtless proceeding from submarine volcanic eruptions, so on the western side sudden depressions have occurred so as to submerge those forests.
It would lead me too far were I here to cite all the different data I possess concerning volcanic action and earthquakes, and the consequent change in the level of these islands, and I must therefore keep them for future publication; but I may add, that although everything I observed proves that whilst the eastern side of the island rises, the western falls, there have been occasional oscillations, as observable, for instance, between the Rivers Wanganui and Karamea. Here we find sand beds twelve feet high, with small layers of magnetic iron-sand between, upon which the sea is now continually encroaching. That these banks are not formed of drift sand is not only shown by ripple marks, and by the presence of numerous shells similar to those now existing on the sea beach, amongst which Venus intermedia in perfect condition is most frequent, but also by that of a large drift tree, now rotten, which lay amongst these beds. Prom these facts we may conclude that but a few years have elapsed since they were upraised.
In ascending the Buller, we observe, before reaching. the gorge where the river enters the mountains, a succession of apparently young strata, a blue clay marl overlaid by a soft ferruginous sandstone. At the western foot of the chain, north of the Buller, the same formation also occurs; but here, between the blue clay marls and the sandstone, lie about twenty feet in thickness of black marls with mica scales. The general strike of these beds, which are 250 feet above the level of the sea, is from north-northeast to south-south-west, with a dip towards the west-north-west of 17 degrees. Above them the alluvial deposits begin. The gorge itself exhibits a syenitic granite of an even structure for several miles, through which the porphyritic granite is protruding in large veins. Greenstone dikes occur also in some spots.
This granite extends north of the Buller in a wedge-like form into the Papahaua chain, forming hills of 500 to 1,000 feet high, and ceasing in the centre of the chain. On both its sides, uniting in the centre, lie the coal-bearing strata, striking, at Mount Rochfort, regularly east and west, with a dip of 5 degrees towards the north. These strata, although of the same age as the Grey coal-fields, and consisting of the same gritty and shaly beds,
[Image of page 113]
exhibit in some respect a different aspect, the soft sandy clays not occurring, and being replaced by large accumulations of conglomerates. A very striking feature is that these beds lie almost horizontally, forming near the sides of the granite wedge, vertical, and even in some places overhanging walls, from 500 to 1,500 feet in height.
These strata do not appear to have been upheaved by granite when in a soft state, but when this rock was in a solid condition, a proof of which we find near the sea, north of the Ngakuwaho stream. There largo masses of porphyritic granite occur, resembling that which has protruded through the older syenitic granite. Near it we meet with a granitic breccia, consisting of large angular pieces of syenitic granite and micaceous rocks, embedded in a trappean granitic matrix, which could not have been formed by the rolling of the waves on the shore or by submarine currents, but only by friction, when the intruding masses upheaved the syenitic granite, and with it the coal-bearing strata, so that the pieces which were detached by this vertical movement became embedded in the granitic matrix. In some other places, as, for example, to the north of Mount Frederic, the porphyritic granite has itself intruded into the coal-bearing strata, and many highly interesting rocks have been formed by alteration of the grits, coals, and shales.
From the top of Mount Rochfort to the course of the Waimangaroha, which separates the last-named mountain from Mount Frederic, the general strike of the carboniferous rocks, which attain here a thickness of 3,500 feet, is from east to west, with a dip from 4 degrees to 7 degrees north. Having examined these strata somewhat closely, I am enabled to give the following table of succession, in descending order, beginning at the top of Mount Rochfort.
|
|
FEET
|
| Coarse grits, with small beds of fine grained sandstones
|
80
|
| Conglomerate, in a matrix of white quartzose sand, consisting of rounded pebbles of quartz and quartzite
|
30
|
| Ferruginous sandstone (grindstone)
|
6
|
| Conglomerate
|
40
|
| Sandstone, arenaceous, whitish green
|
8
|
| Conglomerate, alternating with smaller beds of sandstone
|
200
|
| Very micaceous grits
|
120
|
| Shales
|
15
|
For the next 500 feet, the same alternating succession of grits and shales continues, as observed in the gullies round Mount Rochfort, but without any apparent indications of coal. The shales, where not too micaceous, are replete with impressions of plants, all specifically the same as those in the Grey coal-field, of which a Voltzia is here the most conspicuous. Below them again, for several hundred feet, slaty sandstones occur, succeeded by the grits and shales, amongst which I discovered a coal-seam.
[Image of page 114]
In descending from Mount Rochfort, and crossing in an easterly direction the mountain plateau, intersected by innumerable streamlets, the rocky walls of which everywhere offer good sections, we again find ourselves, after having passed over all the strata before enumerated, amongst grits and shales.
Having examined at least eighty feet of these sandstones, the following strata, in descending order, are met with.
|
|
FT. IN.
|
| Tabular arenaceous sandstones, occasionally containing concretions clay ironstone
|
of
30 0
|
| Coal seam
|
8 2
|
| Shales, with small coal-seams
|
20 O
|
| Grits, sometimes very micaceous, sometimes coloured by carbon.
|
70 0
|
The coal seam strikes regularly from north-north-east to south-south-west, with a dip of 6 degrees west-north-west. I traced it on both sides of the valley, and followed it also in other directions, where transverse valleys have been cut through it. It has the same character as the Grey coal, as shown by the following analysis, made by Mr. Tatton at my request.
ANALYSIS.
|
|
Buller,
|
Sydney.
|
| Carbon
|
74.24.
|
74.13
|
| Hydrogen, Nitrogen, Oxygen
|
25.76
|
25.87
|
|
|
100.00
|
100.00
|
It may be as well here to add that the analysis of both coals was made upon surface specimens, which had been exposed for probably an indefinite period, both to the action of the atmosphere and to that of water, so that we may expect, on getting into the seam, that the amount of carbon will be found to be larger.
The valley where this coal-seam occurs lies 1,914 feet above the level of the sea, and as it would be almost impossible to find it without some mark, I have placed on the nearest hill, 200 feet above it, a flagstaff, which will be visible far over the plateau.
From this flagstaff I took the following bearings:--
|
|
D.
|
M.
|
| Centre of highest summit of Mount Rochfort
|
236
|
25
|
| Summit of Mount Frederic
|
11
|
10
|
| Lowest saddle between the plateau and the Orikaka.
|
61
|
40
|
| Highest point of Mount William
|
95
|
45
|
| Junction of the two streams which form the Waimangaroha
|
24
|
30
|
| Valley where the coal-seam is exposed, about 400 yards distant.
|
268
|
20
|
I placed also another smaller flagstaff in the valley itself where the seam occurs. I met with a seam, which is probably the same, about 150 to 200 feet above the sea, in the gorge of the Whareatea
[Image of page 115]
river; but it was so much washed out and covered by the detritus from above, that I could not take any measurements although I observed that the same succession of strata occurred there; the strike was from south-west to north-east, with a dip towards the north-west of 31 degrees. Perhaps its position here is due to a merely local disturbance. The granite in the wedge is overlaid by a green arenaceous sandstone, the lowermost rock of the carboniferous strata.
On the eastern side of the Papahaua chain, in Mount William, I again found the same strata striking regularly north-east and south-west, with a dip of 6 degrees towards the south-east.
At the western foot of Mount Rochfort no granite is to be found, but proceeding towards the north, we find it lying at the base of Mount Frederic. Here the strata overlying the granite dip towards the south-east. These carboniferous rocks cover an area of 120 square miles, and will one day be highly valuable, although at present the better position of the Grey coal-field, the thickness of its seams, and the facilities it, offers for working them, more strongly recommend it to the notice of the capitalist.
A mile north of the Ngakuwaho stream the granite again makes its appearance, overlaid by young tertiary deposits. It is here syenitic, and more towards the north trappean, and changes in some places insensibly into felsite porphyry, as for instance near Kongahu Island. In other places the mica is replaced by chlorite, and large veins of the same mineral are visible, intersecting the rock. We meet also besides with many porphyritic granites containing large orthoklas crystals, and with others containing albite in greater proportion. Some of the trappean granites are full of rounded concretions of a blackish colour, and of a micaceous structure, exactly resembling the metamorphic schists which we observe near the hypogene rocks. These metamorphic rocks consist partly of micaceous schists and partly of granulites, and are intersected by occasional quartz veins. Their strike and dip are generally very irregular, although in some places, where I obtained good sections, their general strike was north and south, with a dip of sixty to eighty degrees towards the west. The cretaceous rocks which we meet with at the junction of the Inangahua and Buller extend to the coast, which they reach to the north of the Mokihinui stream. Here they consist of a flaggy, bluish, and somewhat argillaceous limestone, intersected by large veins of calespar, and in some places full of casts of fucoids. This rock extends, with only occasional intervals, where granitoid rock crops out, to the mouth of the Wanganui, sometimes assuming the character of a chalk-marl, and sometimes that of a tabular whitish crystalline limestone, full of large oysters, pectens, terebratulas, and pieces of corals, in connection with the same fossils which I observed in the rocks between the Buller and Grey; but here also as in the latter rocks, I observed no signs of belemnites or ammonites.
[Image of page 116]
Some of these cretaceous cliffs, which are nearly perpendicular as at the Otahu cliffs, are from 700 to 800 feet high, and consist of bluish chalk marls, from eight to twelve feet thick, with bands from two to three feet thick, of a white crystalline limestone, occasionally assuming a pale yellow colour, giving to the whole a ribbon-like appearance. In those sections where the limestone is found upon the granite, we observe very interesting phenomena, indicating that during the deposition of the limestone, great disturbances had taken place, either by earthquakes or by changes in the direction of great submarine currents. In many places the cretaceous rocks, limestones, chalk marls, or greensands, are seen lying directly upon the surface of the granite, which is sometimes smooth, as if by attrition, and sometimes rugged, as if acted upon by waves or by submarine currents. These sedimentary rocks frequently contain no fragments of hypogene rock, and so continue for twenty or thirty feet, when they assume what at first blush would appear to be quite a granitoid appearance. In a calcareous base are disseminated quartz grains, crystals of feldspar, and scales of mica, in such abundance, that it is necessary to examine it closely in order to convince oneself of its mechanical origin. In other places the granite is overlaid by a breccia consisting of large angular pieces of granite and mica schists, imbedded in a base of a green semi-crystalline limestone; which is besides full of quartz grains and mica. The greater the interval between the sedimentary deposits and the granite the smaller are the angular pieces, the limestone itself becoming more whitish. At a distance of thirty or forty feet from the granite, the angular pieces altogether cease, and mica scales are only occasionally found. The limestone itself begins to assume an earthy texture, and eighty feet from the granite changes into a chalk-marl. In other places, above twenty or thirty feet of quietly deposited matter, fresh revolutions appear to have taken place, and large beds of breccia, similar to those before described, again make their appearance.
As far as I could judge from the boulders brought down by the streamlets, the granite is associated with hornblendic rocks and traps; and the existence of sandstones, like those underlying the Grey and Buller formation, induced me to believe that the same rocks, as well as several metamorphic rocks, existed here, overlying the granite, which had protruded through them. The cretaceous formation leaves the coast at the mouth of the Wanganui, and strikes in a north-easterly direction towards the Tasman mountains, the detritus of which, gneiss granite and metamorphic schists, is again brought to the sea by the larger streams. As described in the foregoing part, between the Tasman mountains and the sea, we again meet with the great drift formation through which all the different rivers have worked their way. At the northeastern end of the Karamea plains, sandstone ranges are found,
[Image of page 117]
probably of the same age as the Davy mountains. Where the Karamea breaks through the western side of the central chain the porphyritic granite seems also to exist, but as the state of the weather was such that it necessitated the greatest possible speed, I was unable to ascend any of those streams, which in summer time would have been an easy task.
Four miles north of the Oparara the granite again reaches the coast, and continues, with only little interruption, to the Ihua Tuaroa point. North of the Oparara, and at the mouth of the Rivers Haihai and Whakapoai, we find the remains of what was probably once an extensive secondary formation, which at the mouth of both rivers forms very bold headlands. At the first-named place it consists of a shelly tabular limestone, of a greenish yellow colour, formed principally of fragments of shells of the fragile genera Pecten and Lima, but having imbedded amongst them large oysters, which have better resisted the action of the waves.
These limestone beds are, as far as I could judge, 200 feet thick, and are underlaid by greensands and loose ferruginous sandstones, which rest directly on the granite. At the Haihai these strata are confined to the coast, but in the valley of the Whakapoai they stretch from two to three miles inland. It was here, in climbing round the sea cliffs at the northern side of the Whakapoai river, that Mr. S. Mackay found a seam of coal, which is of the same age as the coal of Pakawau. Enormous masses of boulders, sometimes of great size, are brought down by the torrents, and are carried far into the sea, forming tongue-like beds, jutting beyond the lines of the rocky cliffs.
In the same places we also meet with tertiary deposits, probably of pliocene age, and consisting of loose conglomerates, ferruginous sand, and beds of lignite. The granites here found are of various kinds, sometimes porphyritic passing into felsite porphyry, and sometimes trappean or syenitic, and here and there intersected by straight greenstone dikes. In other places veins of apatite, fluorspar, and pistazite occur. They have sometimes a tabular, and at others a nearly cuboidal parallelopipedoic or polyhedric structure. To some angular pieces of these porphyritic rocks, I found adhering a coat of silicate of copper. In other places grains of oxyde of tin were met with, and had I had time to examine this wild region, I am convinced that the discovery of valuable ores would have rewarded my researches. The rivers traversing this granitic zone bring down, besides granite of all descriptions, great quantities of amphibolites and dioritic porphyries, with pieces of mica schists, quartzites, and other metamorphic rocks of an indistinct character.
The Kaurangi point projects from the last granitic mountain near the coast, which thence strikes inland in a north-easterly direction, towards the Aorere valley, low cretaceous hills then
[Image of page 118]
forming the sea coast to Cape Farewell. The rocks at Ihua Tuaroa, the first promontory north of the Kaurangi point, are greensand, the stratification only discernible by changes in their colour. Whilst the strike and dip of the cretaceous rocks south of Taura-te-Weka, following as they do the undulations of the granite, vary so considerably that it would be impossible to lay down any general rule as to their position in that respect; the general strike of the strata thence to Cape Farewell is from southwest to north-east, with a dip of six to eleven degrees towards the north-west. Behind and between their various masses lie large deposits of drift-sand.
It is evident that the boulders in the rivers along this part of the coast are derived from the northernmost continuation of the gneiss granite zone, which ends where the granite crosses the Aorere river at the Haidinger peak. The cretaceous formation, which above Ihua Tuaroa consists only of hills a few hundred feet high, and changes from greensand into chalk marl and limestone exactly resembling those all along the coast, and with the same fossils, begins to rise at the northern end of the Wanganui inlet, and presents us with high rocky walls, 800 to 1,000 feet high. It was under these hills that I had to seek for the Pakawau coalfield, to the examination of which I devoted several days. Below the greensand, 200 feet thick, I found in descending order the following strata.
|
|
FT. IN.
|
| Limestone of an earthy texture
|
150 0
|
| Tabular semi-crystalline limestone
|
80 0
|
| Chalk marls, with bands of limestone
|
70 O
|
| Clay marls, changing into greenish clays
|
18 O
|
| Ferruginous sands
|
10 0
|
| Conglomerates, alternating with unstratified quartzose sands
|
40 0
|
| Gritty soft sandstone
|
10 0
|
| Conglomerate, with pieces of coal, and containing the detritus
of the rocks which at present form the boulders
of the rivers in the neighbourhood
|
80 0
|
| Blue marly clays
|
4 0
|
| Gritty sandstone
|
12 0
|
| Very ferruginous sand, with concretions of clay ironstone,
which also often fill large fissures
|
30 0
|
| Soft grits
|
25 0
|
| Coal
|
0 6
|
| Clay marl, with impressions of plants
|
3 0
|
| Fine stratified sand
|
10 0
|
| Coal
|
0 5
|
This succession continues downwards for several hundred feet, with small seams of coal, ferruginous sand, grits, and clay marls; and at last we reach the main seam, which, in the Wanganui harbour, is exposed in several places, and is of an average thickness of two feet three inches.
As my friend, Dr. F. Hochstetter, has already treated of the Pakawau coal-field, I think it unnecessary to speak of it; al-
[Image of page 119]
though I feel it my duty to add, that, from observations I made I am convinced that the seams that are seen in Pakawau are the same as those exposed in the Wanganui inlet, and below which, for several hundred feet, the strata, consisting of sand grits and pebble beds, are visible.
The strata in this extensive zone strike and dip in several directions, forming arcs, of which the upper parts have been denuded. Two of these foldings were examined by me; one at the West Coast, north of the mouth of the Wanganui inlet, following the strata in both directions; and the other south of the hill which forms Cape Farewell; besides which I obtained numerous sections in the Wanganui harbour itself, the results of which are noted in the foregoing table. The greatest dip of these strata does not exceed 10 degrees.
The few impressions of fossil flora which I observed in the Pakawau coal-bearing strata were all specifically, and probably also generically, different from those of the Grey and Buller coalfields, and no fossil shells rewarded the most minute research. I found near the pah at West Wanganui a piece of silicified wood, derived from a large tree; the annual rings were still distinctly visible, and it was probably the cast of a portion of some coniferous tree.
All these strata exhibit a character which leads to the conclusion that they originally formed either the delta of a large river or an estuary; and it would therefore be interesting to discover fossils, which would at once give us an insight into the true nature of these deposits. The cretaceous limestones and greensands not only lie conformably upon the coal-bearing strata, but their succession in sequence is perfectly visible; so that there can be no doubt that these coal-bearing strata are of the same formation as the overlying rocks.
At Cape Farewell we meet with the same rocks as on the western side of the Wanganui harbour, striking from south-east to northwest, with a dip of 22 degrees towards the north-east. Here, also, at the southern side of the promontory, small seams of coal are visible, under shingle beds derived from the waste of the crystalline chains.
Before arriving at West Wanganui I found in several of the rivers, as far as their mouths, and coming from the Whakamarama range, micaceous shales and grits exactly like those of the Grey and Buller coal-fields, interspersed amongst granitic and metamorphic boulders; and in the Pakawau river itself I was not a little astonished at finding pieces of the same description. As they are in character quite different from those of the West Wanganui coal-field, I was led to believe that the Pakawau river flowed through strata belonging to the same age as the Grey and Buller coal-field.
Looking from the hills south of Collingwood on the right bank
[Image of page 120]
of the Aorere, over the Whakamarama range, a great difference in the character of various parts of the chain was clearly visible. The chain divided by the Kaituna exhibited at its southern extremity quite the same aspect as the granitic mountains observed all over the country; whilst north of that river a stratification, with rugged, precipitous sides of the hills, precisely like that seen in the Papahaua chain, was visible; and it appeared to me that some of those strata consisted of conglomerates.
Knowing that small beds of an earthy graphite had been found in the northern part of the chain, and judging from the boulders which I had an opportunity of examining in different rivers flowing from this chain, I came to the conclusion that a coal-field would be found here; the importance of which, considering its vicinity to Nelson and to the Strait, could not, if it were discovered, be over-rated. Not having time to search the chain in order to satisfy myself of the justness of my deductions, I hoped that I would obtain some information respecting it from persons in Nelson who had visited it; and I was not a little delighted at being shown by Mr. H. Curtis, to whom I mentioned my notions on the subject, a piece of beautiful black coal, which he had himself found near one of the rivulets flowing into the Aorere river; so that the existence of valuable coal in the Whakamarama range is so far proved.
GOLD.
I have omitted in the foregoing geological notes to treat of the gold-bearing deposits, and of the localities in which this precious metal is to be found, preferring to treat the subject separately, and I shall now proceed to offer some remarks on this important matter.
When Sir Roderick Murchison examined, in 1844, the specimens of rocks brought to England by Count Strzelecki from the eastern chain of Australia, he observed a striking resemblance between them and the rocks in the auriferous Ural mountains in Russia, whence he had shortly before returned. Not only was their mineral character the same, namely, silurian rocks, partly metamorphic and associated with younger plutonic rocks, but the chains also corresponded in their meridional strike, and in their altitude. This great geologist came to the conclusion that the Australian mountains, and their detritus accumulated in their depressions and upon their slopes, would, like the Ural chain, contain gold; and he did not hesitate in recommending to the miners in England, of whom many were then without occupation, to emigrate to Australia, and dig for this precious metal.
A few years afterwards gold was found in Australia; but it was
[Image of page 121]
not until 1851 that gold-mining began on a large scale in that colony, the effect of which has been to convert a country then inhabited only by a few farmers, squatters, and convicts, into an emporium of wealth, and to raise it in a few years to a position without precedent in history.
Had Sir Roderick Murchison received the specimens which lie examined from the eastern meridional central chain of this Island, instead of from Australia, he would have pronounced the same opinion, based upon his deep knowledge of geology, which, although the newest, is perhaps, in many respects, one of the most important sciences; a knowledge of which is requisite for the sound development of the natural wealth of nations. It is true we have not yet found any fossils in these slaty rocks, but I am convinced that they will be found, and will demonstrate that these strata are of the same geological age as those of the Ural mountains and the Australian Alps. Here, as there, their meridional strike is very little disturbed by eruptive rocks; here, as there, the altitude of the mountains is between 6,000 and 7,000 feet; here, as there, large quartz veins traverse the slaty rocks; and nature has here, perhaps, to a greater extent even than in Australia, done the work of crushing, in the process of denuding the uppermost parts of the lofty ridges where the vein stones were probably most highly charged with gold.
The luxuriant growth of the New Zealand vegetation, together with the absence of roads, has at present confined the work of gold mining to the boundaries of the gold-fields; but it has been proved by a few adventurous men, who have penetrated more deeply into the now inhospitable mountain regions, that the detritus becomes richer in gold, which itself becomes coarser, as we ascend the river courses. We may therefore infer that, were the rich tracts lying in the depressions or on the slopes of the chains, and which cover hundreds of square miles, opened to enterprise, a rich reward would be the result.
As in Australia, the gold is found here in a loose conglomerate or drift, with small seams of lignite (as, for instance, in Golden Gully, Massacre Bay), of two different tertiary ages; and also in the recent alluvial drift, which, however, is generally poorer in metal.
In order to ascertain the existence of gold in the country traversed by me, I first washed in the Rivers Roto-iti and Roto-roa, before they unite; and although we only collected sand from under and between the huge boulders in the river bed, we found nearly in every dish a few specks of very fine scaly gold. This fact was interesting indeed, because, until then, it had been thought that the gold in New Zealand was derived only from metamorphic or silurian slaty rocks; but here we had a proof that it was also diffused in plutonic rocks. It could, in fact, only have been derived from the decomposition of granite, because
[Image of page 122]
both rivers traverse mountains composed solely of that rock, associated with hornblende rocks.
I have since heard that the gold collected in the Batten river, near the Whangapeka, by a small party of diggers there, is found upon the surface of decomposing granite. The gold itself is like gunpowder, and, from the description of the rock given to me by one of the men, I believe it to be hornblendic or syenitic. As the granite between the Roto-iti and Roto-roa rivers is also often hornblendic, may we not infer that the gold in those rivers is also derived from this peculiar rock?
We also tried the river Owen, but were not very successful, one single speck only proving its existence in that river.
Throughout the course of the Buller gold was found in its bed and on its banks; and I collected some handsome specimens of heavy rolled gold in the rents of the tertiary rocks near the junction of the Mangles with the Buller, sometimes taking it out with the knife. I am unable to state whether the gold there is derived from the older tertiary rocks, or whether it has been washed down out of the overlying drifts of probably postpliocene age.
Two of my party went up the Matiri, and tried there, but without any success, although they were experienced hands, and washed through the greater part of the day.
South of the Buller, in the Maruia, as well as in the whole course of the Grey and its tributaries, rarely leaving untried any spot which seemed likely, we searched in vain, unable to detect the least sign of the precious metal.
So that, judging from ascertained facts, I may reasonably conclude that the gold in the Buller is derived from the waste of the chains north of it, and that it is there that we shall find the quartz reefs which contained the gold in their now denuded parts, and the debris of which lie scattered all over the country. Again, in the gorge of the Buller, a few miles from its mouth, small scales of gold were found on washing the detritus.
From what I have seen, I feel confident that the drift accumulated in the Matakitaki plains, as well as near the junction of the Inangahua, where large flats also occur, will one day afford to the digger sufficient reward for his labours; but I would warn him not to venture into that solitary country before roads are made, so that easy communication be maintained with the settlement, in order that he need not lose half of his time in swagging provisions to the scene of his operations.
CONCLUDING REMARKS.
Having given, in the foregoing part, my observations made upon the spot during the course of my journey, I will now finally draw from them some conclusions as to the earlier form of this island,
[Image of page 123]
and as to the periods when the different changes before referred to took place.
The first great change which I was able to trace occurred when the porphyritic granite was protruded, and which, at the western side of the island, appears in such predominant masses. Before this event, which changed the whole form of the country, it appears that large tracts of level land existed, as indicated by extensive coal-fields, and thick deposits of grits and shales, in the formation of which large rivers had a considerable share.
That huge masses of granite had previously existed is evident, seeing that in some places a granitic rock of greater age is traversed in tortuous veins by the porphyritic granite, and seeing also that the grits and shales of the coal formation are principally derived from the decomposition of that older rock. After this period of great disturbance, in the progress of which the mountains of the central chain took nearly their present forms, a long space of time evidently elapsed without any further great disturbance, as proved by the thick cretaceous deposits on the West Coast. These deposits were probably formed in a bay of considerable size, or between extensive islands, as shown by the breccia at Te-ana-o-Matuku and the remains of old sedimentary rocks near Cape Foulwind.
During that time the whole eastern side of the island was probably also in the condition of dry land, no signs of cretaceous rocks having as yet been found there. But the quiet state of the island which immediately succeeded the formation of the porphyritic granites was at length again invaded. Eruptions of plutonic rocks took place, and whilst the western side of the island was raised high above the sea level, its centre became submerged. It was then that in all probability the eruptive masses of the Kaikoras made their appearance.
Another long interval of repose succeeded, during which the large accumulations of the tertiary (miocene?) period, which we find in the longitudinal fissure, and round Blind Bay, took place. These deposits were again upheaved by a fresh protrusion of eruptive rocks, diabases and porphyries, near Nelson and all along the western side of the Spencer mountains. It was then that the island assumed nearly its present physical character.
We also have evidence of another disturbance which took place in the glacial period, during which the country sunk, as in other countries in the same latitudes, and afterwards, rising again, re-assumed its present position.
After the last-mentioned eruption of crystalline rocks, volcanic agencies began to work on the eastern side of the island, and thence an important change took place in the level of the country. These agencies are still at work, upheaving young tertiary strata and alluvial deposits, the former of which have already been raised to an altitude of nearly 2,000 feet, and the latter to
[Image of page 124]
the height of 400 feet along the eastern coast, whilst the western side appears to he sinking in an equal ratio.
I have only to express my regret that I have not yet been able to obtain the highly interesting geological data which are to be got by examining the eastern side of this island, and which would no doubt give me a clearer insight into the wonderful operations of nature, and would have enabled me to lay before you a more complete exhibition of facts.
And so the work of the Creator never ceases, and whilst we see before our eyes men, animals, and plants appear, so are new continents and islands continually formed, although to the human eye in an almost imperceptible manner; and as we see things living on our planet, and existing from it, become buried in it, their bodies resolved into the elements from which they were built up and sustained, so the eternal sea forms the great sepulchre for continents and islands, which have served their time, and want repose.
