Art. XLII.—Notes on the Physiography and Geology of the King Country.
[Read before the Auckland Institute, 14th November, 1887.]
The district to which these notes refer comprises an area of 4,000 square miles, extending southwards from Alexandra and Kihikihi for nearly 100 miles: bounded by the Waikato River and Lake Taupo on the east, and on the west by the sea, from Kawhia Harbour to the Mokau River. The triangulation of the district was entrusted to me, and I have been for three years engaged in it. This description professes to be simply one of the topography of the country, with such notes on its geology as time and circumstances would allow me to make: it is given in the hope that it may serve to assist the geologist visiting the district in the future, and in the meantime throw some more light on the geology of a country of which very little is known beyond the general descriptions of Dr. von Hochstetter, which, however excellent in themselves, include only a small portion of the area under consideration.
The River Basins.
The three chief river basins by which this country is drained are those of the Whanganui basin, with a drainage area of 960 square miles; the Mokau, 830 square miles; and the Waipa, 820 square miles. Then there are the rivers and streams which flow into the Waikato River from the west, draining 360 square miles; those which empty into Kawhia Harbour and the sea on the West Coast, including Marokopa and Awakino Rivers, draining 720 square miles; and the western side of Taupo Lake, which has a drainage area of 310 square miles.
An idea of the surface configuration may be gleaned from the general sections which accompany this paper. It may be said to be that of a broken, hilly country, with a main range traversing it from south to north. This range is not, however, a continuous unbroken chain: it begins 10 miles to the westward of the Tongariro group; sweeps round 15 miles to the west of Taupo Lake, under the name of the Hauhungaroa Mountain, its height varying from 3,000 feet to 3,780 feet above the sea-level. The Whanganui River takes its rise in the Tongariro Mountain, and flowing westerly cuts through the range immediately to the south of Maungaku, where it traverses a formation of clay marls,
overlain by beds of tuff. Pureora, the most prominent peak on the range, lies 12 miles in a north-west direction from the northern end of Lake Taupo; its height is 3,800 feet above the sea. It rises with a gentle and regular slope from the Maraeroa Plain, which lies 10 miles west of it; its slopes are clothed with forest, which disappears near the top, leaving the summit nearly bare, clothed only with tussock grass and scanty scrub.
Three miles to the northward of Pureora stands the picturesque and interesting mountain peak of Titiraupenga, an isolated volcanic rock formed of augite-andesite. Its position and structure would suggest that it was probably the “plug” or neck of land of an ancient volcano, from which the looser materials forming the cone had been removed by denudation. It stands out a most conspicuous landmark at the end of the range, its bare sides standing perpendicularly 200 feet above the mountain. It is a lonely, isolated column of rock, visible for many miles around, and with its mural sides looks from a distance like the ruins of an ancient castle or “Round Tower.”
Northwards from Titiraupenga the country falls, and there is a saddle or break in the range at the Maraeroa Plains, a rather extensive area of flat and table lands from 1,400 feet to 1,800 feet above sea-level, composed mainly of the tuff beds, to be mentioned again, and covered with a deposit more or less heavy of pumice sands. The land here is of medium quality. Mixed with the pumice deposits is a considerable quantity of organic soil, derived from the marl formations which flank the neighbouring ranges. The open plains are surrounded by forest, which covers the slopes of the hills, and occupies patches on the plains in picturesque clumps and tongues of bush. The Ongarue River, the chief tributary of the Whanganui, rises here, also the main tributaries of the Mokau and Waipa Rivers.
From Maraeroa the land rises again to the northwards, continuing the direction of the main range, under the name of Puke-o-kahu, 2,775 feet; Ranginui, 3,224 feet; Te Ranga, 2,309 feet; and Wharepuhanga, 1,942 feet above the sea. From Wharepuhanga the range falls away to the northwards, and a low valley, 500 feet above the sea and 7 miles wide, separates Wharepuhanga from Maungatautari, an isolated rhyolitic mountain on the line of the main range, 2,623 feet above the sea. The general character of the formations along this main range is tufaceous sandstone, rhyolitic rocks, andesites, and palæozoic slates, the latter appearing in several places along the top of the range, and cut through by the deep-worn water channels on its western slopes.
Parallel with the Hauhungaroa Range, and immediately to the westward of it, lies the Ongarue Valley. The upper basin in the valley is a greatly depressed area, 15 miles in length
from north to south, and from 6 to 8 miles wide. Surrounded by lofty mountains from 2,000 to 3,700 feet above the sea, the valley drops almost suddenly to 600 feet above sea-level. The drainage area of this basin is comparatively small, only about 250 square miles. It is bordered in many places by steep coast-like cliffs of tuffs, andesite, and agglomerates, standing out as greatly denuded fragments of the masses which lie at the back of them. The valley itself is partly filled up by the later-formed deposits of pumice and detritus carried down from the hillsides by the surface waters and running streams.
It would seem almost incredible that this great deep valley owes its origin to the ordinary denuding agencies, and it is more probable to be partly the result of a great earth fissure, connected with the volcanic movements to the eastward, and possibly a syncline in connection with the Hauhungaroa Range. It may also be remarked that the Hauhungaroa Range runs parallel with the great fissure of the Taupo volcanic zone, thus apparently showing a sequence of the volcanic operations by which the mountain chains and valleys of other parts of the world are formed.
The Tuhua Mountain is a very prominent and interesting feature in this part of the country. It stands as an isolated mountain on the eastern side of the Ongarue Valley, and 6 miles to the west of the main range. Its height above the sea is 3,425 feet. It is thus quite as high as the main range, which it stands 6 miles away from. On the summit rest large rounded boulders of tufaceous sandstone over 20 feet in diameter. The sides are flanked by a silt-like marl deposit (to be referred to again). On the top is a broad flat platform, over 60 acres in extent. On the south and north the mountain sides are very steep, and in some places quite precipitous, so that vast quantities of material have rolled down into the valleys in extensive landslips, evidences of which both old and recent are to be seen on the southern, western, and northern slopes of the mountain. Hikurangi Mountain is also an interesting feature of the valley, and from a geological point it is of much importance. It is formed of soft tufaceous rock, with hard rhyolitic rock interbedded with it. The sides of the mountain are flanked with brown and blue clay marls, and probably the base is composed of masses of the same rocks, as from near the base, at an altitude of 800 feet above the sea, two dense and powerful mineral springs appear. These are highly charged with salt, and are of considerable importance from their medicinal properties, as disclosed in the analysis by Professor F. D. Brown, and referred to in his paper, “Notes on a Salt Spring in the King Country,” read before the Institute on the 14th of November, 1886. Hikurangi looks at a little distance away from it like a very regular volcanic cone, rising from a
ridge in the valley to a height of 2,530 feet above the sea. Its slopes are clothed with forest. Towards the top its sides are steep, almost precipitous, and it has a flat square-cut table land on top containing 50 acres. Tuhua and Hikurangi Mountains, with their broad platforms on top and steep mural sides, standing out in isolated positions in the valley, ranges which correspond with them in height bordering the valley on either side, are all circumstances suggestive of the fact that these hills formed part of the ancient plateau which occupied the valley.
Probably in no part of New Zealand can be found landscape so varied and picturesque as may be seen in favourable weather from some of the lofty peaks in this part of the district. Viewed under the conditions in which I saw it, it would be difficult to conceive a landscape of greater natural variety and grandeur. It was at sunrise on a clear frosty morning towards the end of May, from the summit of Pureora, 3,800 feet above the sea, overlooking all the surrounding country. The high mountainous district southwards of us was covered with snow; Taupo Lake seemed to be spread out at our feet, its 425 square miles of clear mirror-like surface reflecting the shadows of the eastern hills and promontories of the lake, cast across it by the rising sun. To the south the giant mountains of Tongariro and Ruapehu, the latter mantled in an unbroken sheet of snow, which covered 6,000 feet of its rugged sides; the regular conical outline of the active crater cone at Ngauruhoe, its steep black sides kept bare of snow by the internal heat, and its funnel-shaped crater at the summit,—were distinctly visible to us. Steam ascended slowly from the crater, forming a white cloud-like canopy at the cone.
To the north and east wound the valley of the Waikato River; along its course columns of steam arose from the hundreds of hot springs, fumaroles, and puhias in the “great fissure” of the Taupo volcanic zone. Several of the dilapidated volcanic cones were in view, from whose craters in ages past were vomited forth the streams of lava, mud, ashes, and breccia that are spread far and wide over the surrounding country. Amongst those visible were Pihanga, Kakaramea, and the Tongariro groups—the latter rearing their great crateral cones from 6,500 to 7,480 feet above the sea. To the north of Lake Taupo, Mount Tauhara, described by the Assistant Surveyor-General in his account of the “Taupo volcanic zone,” as “a fine conical hill 3,603 feet high, with a crater on top, now extinct, and clothed at the bottom and western side with a forest of fine trees. The crater rim is very perfect on the north, east, and south sides, but the western, that directed toward the prevalent wind, has broken away. At its base, on the shores of
Taupo Lake, and along the Waikato River, on both sides for miles down, are found innumerable hot springs, all in a state of great activity.” To the south-west, at a distance of 98 miles, the snow-clad sugarloaf peak of Taranaki (Mount Egmont) reared its head high above the surrounding landscape, 8,270 feet above the sea. We had therefore, as it were, spread out before us, and easily recognisable, the great volcanoes of the Taupo volcanic zone, nearly all now extinct, but having close to their base steam rising from the hot springs by which they are surrounded, showing, as Mr. Percy Smith says, “that the volcanic forces have simply changed their position and the character of their activity, and are not dead;” and with these were also within our view some of the grandest variety of lake and mountain scenery in the Colony.
I regret very much that I cannot supply anything like detail geological sections—to work out the geology of this large district would be a work of much time and trouble. The superficial deposits of pumice to the westward of Taupo Lake, and all over the valleys of the Tuhua District, render it very difficult to trace the junctions of the beds. I brought with me to Auckland typical specimens of the chief rock formations found in the district; I am indebted to the great kindness of Professor Thomas for their names. At the cost of considerable time and trouble he made numerous microscopic sections of the rocks, and thereby chiefly I am enabled to give such details of the geological formations in the district as this paper may contain.
Rocks of the younger palæozoic formation—fine grained argillaceous slates (the Maitai slates of Hochstetter) appear in at least four places along the top of the main range. At Hauhungaroa they are found at an elevation of 1,950 feet (nearly the summit of the range), over rhyolitic tuff and andesite; they appear lower down on the same part of the range in the deep worn beds of the Pungapunga and Mangakotuketuke Streams, at a height of 800 to 1,000 feet above sea-level. At Puketapu, on the Tuhua and Taupo native track, slates are found at 1,700 feet above the sea.
On the eastern slopes of Ranganui and Rangitoto they are seen to be cut by the mountain streams in at least two places, at an elevation of about 1,300 feet above the sea. On the Mokau-iti River, near its junction with the Mokau, slates form the masses of rock through which the river has scooped out its bed. Hochstetter says that “at the Wairere Falls, on the Mokau, slates of the same description as those at Taupiri form the rock masses over which the river falls.” So far as I have seen, the slates always appear in patches when found at high
elevations on the ranges, but when seen in the deep worn beds of the stream, near the base of the mountains, they are in large compact rock masses.
To this formation, I believe, belong the clays and limestones of the Mokau Valley. Hochstetter says: “The bottom of the Mokau Valley is composed of tertiary clays, impervious to water; while at the slope of the hills tabular limestones protrude, the heights themselves being formed of trachyte tuff.”
These tertiary limestones occupy an area of about 200 square miles in the valleys of the Mokau and Mangapu. In the Mokau Valley they seem to have occupied an area of depression, 25 miles in length from north to south, and about 8 miles in width, surrounded by high hills, 1,200 to 1,800 feet above the sea. The limestone beds have undergone immense denudation; they stand out on the sides of the valleys in picturesque detached masses, resembling the ruins of old castles and fortifications, frequently occupying positions 300 feet above the level of the valley, the peculiar slab-like weathering of the laminated masses making them look like the remains of ruined masonry. The colour of the limestone on the surface is white, probably owing to weathering, but it passes into a compact mass of bluish fine grained rock underneath.
The tertiary limestones occur also on the Marakopa River, at Te Anga, 20 miles inland from the mouth of the river. They are seen here in picturesque columnar rocks, cleft and worn into many shapes and forms, and forming long high walls along the river sides. At Rakau-nui River, Kawhia Harbour, there is also a great development of the limestone formation. Here it forms interesting landscape scenery, in picturesque islands and promontories; portions of the rock masses being dislodged and tilted up in the process of denudation, look as if they were disturbed by violent volcanic action. All this limestone formation is noted for its numerous caves, underground watercourses, funnel-shaped holes, and overhanging rock ledges. The caves were, in former times, no doubt shelter caves and sleeping places of the moa, and in years gone by their bones were found in the caves in large quantities. They have, however, long since disappeared, and we were never fortunate enough to find any relics of the moa in the caves.
Lying to the eastward of the limestone area of the Mokau Valley is an elevated plateau, which rises gradually in an easterly direction, or towards Taupo. The general height of the plateau above the sea is from 1,500 to 2,000 feet; it is divided by deep water-worn gullies into long winding spurs. This plateau extends from the slopes of Rangitoto Mountain southwards for a distance of about 30 miles, and it has an average
width of about 15 miles between the valleys of the Mokau and the Ongarue Rivers. The high ground of the plateau is covered with forest, and the soil is of good quality; the valleys are open—probably the result of continued denudation; their bottoms are for the most part covered with deposits of pumice sands, and consequently poor and unfit for cultivation as a rule. The long sinuous spurs of the plateau are composed of rhyolites, andesites, and tuff, the latter predominating, and with it are interbedded masses of volcanic agglomerate, and enfolding with them rounded pebbles of igneous rocks, and those of slate rocks also. At their base are beds of marl, shale, and sandstone; in what position these beds lie relatively I cannot say with any certainty—I never had the time nor opportunity to trace out their junctions properly. I think the beds of marl and shale rest conformably on the sandstone, as they appear to do in one of the deep-worn banks of the Mapui Stream near the Mokau. The tuff beds overlie the marls, but whether conformably or not I cannot say; the colossal masses of tuff rise up one after another, bordering the valleys; isolated peaks and blocks of it stand out on the ridges, with here and there greatly denuded sides.
It would seem that the plateau was formed of a continuous sheet of the formation of which these blocks and ridges are the remains, and the softer portions being carried away by denudation, the river valleys were formed, and varieties of contour worked out all over the plateau. The beds of tuff contain large quantities of pumice in various sizes, up to a foot in diameter. Some of these are abraded and rounded at the edges, others are sharp and angular. I did not notice any fossils in the beds. These tuff beds much resemble those of the Patetere plateau, to be seen between Waotu and Lichfield; they have a thickness of probably 1,200 feet on the plateau just described.
A reference to the section will show that the plateau rises with a gradual and regular slope of 3 degrees, from the Mokau Valley eastward to a height of 2,300 feet on the Tangitu Range; and then there is a sudden fall of 1,600 feet into the valley of the Ongarue. If the line of slope be still produced in an easterly direction, it will meet the summit of the main range at Hauhungaroa, 12 miles to the west of Lake Taupo, and from there the land slopes into the Taupo basin at an angle of 14 degrees. I have dwelt considerably on the description of this tufa deposit, for the reason that it occupies the largest area of any formation in the district, and is persistently associated with all the other formations, but in what relation to them my investigations were not complete enough to show. I believe that in the Mokau Valley, and in the plateau between it and the Ongarue, the tuff beds lie conformably on the tertiary limestones and calcareous clay marls; but their relation to the marl beds in the Ongarue and Upper Whanganui valleys is, so far as my knowledge goes,
very doubtful. I believe the latter marl beds are of three different ages, and the tufa beds are contemporaneous with those of the middle period, for reasons which will be shown hereafter.
The clay marls, or “blue papa rock,” of the Upper Whanganui belong, I believe, to this formation. These beds occupy a large area, extending inland from Whanganui to the Tuhua. It is a fine-grained calcareous and argillaceous formation of great thickness; it can be traced to an altitude of 2,100 feet above the sea on the Hunua Range, 12 miles south of Taumarunui, where it is exposed in great masses in the numerous landslips on the mountain sides. The beds dip to the south and the south-west at angles varying from 3° to 15°. They sometimes enclose bands of more compact, harder, and more sandy rocks; and in the valley of the Otaunui Stream I noticed at one place large partly rounded boulders of tufaceous sandstone (or possibly septaria), appear, interbedded with the marls. Resting on these marls, and probably conformable to them, are the tuff beds before mentioned as so extensively occupying the higher area of the plateau. They occur here in a precisely similar manner, but not quite so persistently; and here they are associated with lava floes of rhyolites and andesites.
I have said that I considered the marl beds of this part of the district were of three different periods of deposit. I regret that I never met with an opportunity of examining the beds at their junctions. The valleys are here so much covered in with superficial pumice deposits that to obtain sections will be a work of time and trouble. It seemed to me, however, that the tufa beds resting on the lower blue marls lie conformably on that deposit. Resting on the tuffs, again, is another deposit of the same blue-clay marl apparently; this forms the rock masses of the range west of the Ongarue River, known as Tangitu, Maungarahiu, Tapuewahine, etc., through one of the saddles of which (the Paro-o-te-rau) the tunnel on the Main Trunk Railway is now being formed, and the character of rock being driven through is, I believe, all the “blue papa.” Fossils are very scarce in the deposit, and difficult to save; they crumble away on being exposed to the atmosphere. The beds are distinctly stratified, but I have not ascertained their dip. They are greatly subject to atmospheric influences, and, whenever exposed, are quickly channelled by watercourses and removed by denudation.
At the crossing of the Whanganui River, on the native track between Taumarunui and South Taupo, at a place called Taringapupu, the river bed is scooped out in blue marl rock—whether of the older Whanganui series or the later deposit I cannot say; and the relative ages of these beds must remain an open question until their fossils are collected and compared.
Further to the eastward, and within 15 miles of Taupo Lake, in the bottom of the Waipari Stream, these marl beds are also exposed. They appear to dip to the westward at an angle of 10 degrees. They are here interbedded with coarse conglomerates, which enfold rounded slate pebbles and particles of rhyolite and trachyte lavas. I saw no fossils here, but I did not search much for them, and probably a closer examination would disclose some. Where the deposit is crossed by the Taupo track the height is 2,200 feet above the sea. These beds are also exposed four miles higher on the same stream, at an elevation of 2,600 feet, and here they have interbedded with them thin layers of yellowish clay or mud.
In describing the third, or more recent, marl deposit in the Taupo and Tubua Districts, I do so with considerable diffidence, lest my remarks may be misleading.
On the higher elevations, on the ridges and mountain sides west of Taupo Lake, and in the Tuhua District, appear in many places what would seem to be the remains of a light deposit of silt-like clay marl, the greater part of which appears to have been carried away by atmospheric denudation. It consists of a fine-grained clay marl, calcareous and sandy in some places, in others more argillaceous in character. It has its greatest development in the little valleys on the top of the ranges. Lower down, it appears only in places where it may have been carried down by recent landslips. It is found overlying all the other formations. Thus, at Matere, on the Hauhungaroa Range, it is found at an elevation of 3,000 feet above the sea, lying on volcanic tuff and rhyolitic rock. On the Tangitu Range, west of the Ongarue River, it overlies the second marl, or blue papa deposit. In some places the colour is a greyish-brown, in others blueish, and again brown or brick-colour. On Matere Range it flanks the mountain side on the west, occupying the little valleys between the edges in thin, fragmentary deposits. I have constantly met with it in fragmentary slabs all over the Tuhua District, but nowhere else. It is thus only found to the west of Taupo, and at a distance of about 40 miles from the centre of the lake. I saw no fossils amongst it. A sample of the deposit from Hauhungaroa was found to be highly calcareous; whilst another, having the same appearance, and apparently the same kind of deposit, found on Taurewa Mountain, 10 miles west of Tongariro, proved to have no trace of lime in it.
The source of this deposit is an interesting question. It may possibly be laid down in the form of volcanic mud, connected with the outburst of the Taupo volcanoes, when the great masses of pumice which cover the surrounding country were showered out; or it may be a silt-like sediment of mud and clay brought down from the higher elevations by running waters, and deposited at the margin of the sea, at a time when the land
stood 3,000 feet lower than it now stands; in which case it would be of much interest in studying the changes in the level of the country, as connected with the great volcanic movements in the Taupo volcanic zone, and pointing to the existence of much higher land, as occupying what is now the position of Taupo Lake.
The only other formations which remain to be mentioned comprise the recent deposits in the river valleys, consisting of the loose materials brought down from the higher ground by the surface water and running streams, and including the detritus from the various formations mentioned. Of these deposits the most interesting and important developments occur in the valleys of the Upper Whanganui and its tributaries. The mountainous nature of the country gives to the streams the character of mountain torrents. The soft marl formations, loose superficial pumice, and tolerably soft tufaceous rocks of the district are all much subject to the ordinary effects of subaerial denudation, and in consequence of these agencies almost incredible quantities of matter are brought down by the rivers and streams after every fall of rain. The surface configuration of the country is undergoing constant alteration, new landslips constantly appearing on the mountain sides, some of them of great extent, carrying away large forest trees and masses of rock, frequently damming up the courses of streams, and forming temporary small lakes. Unfortunately, the greater portion of the material deposited in the river valleys, and that which occupies the surface, covering up the fertile soil derived from the marl formations, is the superficial pumice deposit to be referred to more particularly later on. Many of the valleys are filled up to a great depth with pumice sands; sometimes in terraces; sometimes in level plains.
In the valley of the Ongarue these pumice beds display a regularly stratified form of deposit, particles and blocks of various sizes up to a foot in diameter lying in alternate layers of finer and coarser fragments. With them are interbedded fine layers of argillaceous strata, and trunks and branches of trees partly changed into charcoal. These facts go to show that the valley was once the bed of a lake; the stratified pumice beds and horizontal terraces could only result from still-water deposit. The valley of the Whanganui, above its great bend at Taumarunui, is also filled up with great beds of loose pumice, in places 2 miles wide and over 100 feet in depth; it seems probable that these were also lake beds at one time. The most noticeable of these is the Rena Plain, 10 miles east of Taumarunui; its width is nearly 2 miles, and its length about 4 miles. The river has cut a deep channel through the pumice beds, which are over 100 feet in depth at the lower end of the plain, where the river course
was probably dammed up, and they gradually thin out towards the head of the valley, where their depth is only 3 feet. The bed rock at the head of this valley is “blue papa.”
The deep-channelled bed of the Whanganui River between Taumarunui and the sea, with its perpendicular banks, often over 200 feet high, carved out of the blue marl beds, the absence of river terraces or anything approaching a wide, extensive river valley all bear evidence of the rapid upheaval of the land in the interior. The distance from Taumarunui to Whanganui by the river is about 130 miles. No road nor native track, so far as I know, traverses the country, which is accessible only by the river. The journey from Taumarunui to the coast in a canoe takes four days. The whole course of the river is a succession of magnificent gorge scenery, a luxuriant, tropical-like growth of fern-trees and variously-tinted undergrowth, fringing the banks and mixed among stately forest trees, which overhang the river high above. There are several extensive rapids, which require skill and care to pilot the canoe over. Picturesque buttress-like headlands have been rounded out on the marl beds by the current of the river. They look like the prow of some great vessel; their sides are marked by rows of holes, one tier above another, formed by the poles used to propel canoes up the stream.
I have frequently referred to the superficial pumice deposit in the district. It is found most persistently in all the valleys, terraces, and flats within a radius of 40 miles round, from the south-west to the north-east, from the centre of Taupo Lake, sometimes covering the surface with a deep deposit of large and small particles, and rendering land which otherwise would be fertile quite arid and useless. It is particularly detrimental in the valleys of the Tuhua District and Upper Whanganui, where large areas of level land along the river sides and on the terraces are rendered unfit for cultivation. This pumice deposit appears everywhere more or less within the limits mentioned above; but on the mountain tops and down their slopes it is seen for the most part merely as a thin sprinkling, and accumulates more in the little valleys and flats on the hillsides, frequently to a depth of 25 to 50 feet, carried there, no doubt, by the surface waters and the wind. In the Tuhua District, a few miles to the south-east of Taumarunui, a fine tract of about 10,000 acres of good soil, derived from the blue papa rock, is partly spoiled by the pumice filling the valleys and covering the rolling country on the slopes of the hills. I have carefully studied this pumice deposit with a view to ascertaining the mode of distribution. Between Taupo Lake and the valley of the Ongarue, a distance of
25 miles, the pumice is most persistent, and does not appear to alter much in the form of distribution or in the size of the particles. In the valley of the Pungapunga, 15 miles west of Taupo Lake, blocks of pumice measuring over 2 feet in diameter are found, and all the valleys and flats are covered deeply with the deposit. As we recede from Taupo the pumice deposit thins out gradually, and the particles decrease in size, until at a distance of 50 miles from the centre of the lake very little is seen, and that in very small particles. I have frequently seen pumice mixed with the earth brought up by the roots of trees which were blown over by the wind. I have also found it, at a depth of 3 feet or more, when constructing trigonometrical stations on the mountain tops, but only in such manner as I think may be accounted for by its becoming mixed up with the earth in the ordinary course of surface-soil formation, its movements by the roots of trees and other vegetation and by the action of earth-worms. The pumice must, I think, be regarded as a superficial deposit, and probably the product of some of the latest volcanic efforts. I saw no trace of the remains of a crater within the district in which it is found although having, in the course of my duty, visited nearly all the higher mountains. Round Tongariro and Ruapehu very little pumice is found in comparison with other localities; whilst at the Waimarino Plains and westward of Ruapehu scarcely any is seen. The Assistant Surveyor-General, in his “Geology of the Northern Portion of Hawke's Bay,” read before the Auckland Institute, 27th November, 1876,* mentions the pumice as occurring in that district under somewhat similar conditions. He says: “Towards the east the deposit gradually thins out, until, approaching the vicinity of Poverty Bay, very little is seen.” The portion of Poverty Bay referred to by Mr. Percy Smith would probably be about 50 miles from Taupo Lake; thus the limit of the pumice deposit in that direction corresponds with that on the west of the lake.
Mr. Percy Smith further states: “The general opinion appears to be that this pumice was ejected from Tongariro and adjacent volcanoes, and was spread over the surface of the country by the wind. There are certain considerations which favour this view, such for instance as finding the greatest thickness of the sand on the lee sides of high ranges, where it would naturally accumulate, and also from the fact that the size of the particles appears to diminish as we recede from the supposed centre of distribution. The only other hypothesis which would account for the presence of pumice over such an extent of country is, that it has been carried into its present position by water.” This hypothesis seems to me quite out of
[Footnote] * “Trans. N.Z. Inst.,” vol. ix., p. 565.
the question as applied to the King Country. The deep beds of pumice in the valleys of the Whanganui, Pungapunga, and elsewhere, are unquestionably alluvial deposits, the blocks of pumice and loose sand being washed off the hill-sides, and deposited partly, it may be, in the still waters of long narrow lakes, which might be formed by the river beds being temporarily choked up by trunks of trees and detritus carried down in heavy floods.
The large area of country, and great height at which pumice is found, precludes the idea that it is due to the action of water, and all the evidence so far goes to show that it was spread in the air, and, I think points to the region of Lake Taupo as the centre of distribution. It is true that many of the particles, even amongst those found on the mountain tops, have an abraded appearance; but would not this be the natural result of their mode of ejection from the crater, being carried by the violent tornado of escaping gases high up into the air, and for miles in any direction that the prevailing wind may take them, their attrition in the air rounding off their angles?
In the month of January, 1886, I made a hasty hydrographic survey of Taupo Lake, in connection with the topographical survey of the country. I may here give briefly the results of my observations, as they are intimately associated with some interesting questions referred to, and may contribute to forming a correct theory as to the origin of the basin. The lake covers an area of 154,680 acres, being 24 miles 70 chains in length, and 16 ½ miles wide; the mean depth is 65 fathoms or 390 feet; the greatest depth, at a point nearly in the middle, being 89 fathoms or 534 feet. Shallow water was found only at one place in the lake, that is on the Haromatangi Reef, which lies nearly half-way between Motutaiko Island and Karaka Point. Here the rock is covered by only 7 feet of water. The northern and western shores of the lake are formed of steep rugged cliffs of rhyolites and augite-andesites, with great compact masses of tuff. The height of the cliffs in the western bay varies from 100 feet to 800 feet, with deep water close alongside them, from 40 to 50 fathoms being found with our boat made fast to the cliff. At Karangahape Point the cliff is 1,500 feet in height, from its base at the bottom of the lake to the trig, station on its summit. As will be seen from the section, (Plate XIX.), it rises perpendicularly from the floor of the lake, almost overhanging the water; thus, in taking soundings, with our boat's stern touching the cliff, the lead-line showed a depth of 390 feet, and a portion of the cliff 600 feet above our heads seemed to overhang the boat. Behind these cliffs on the western side of the lake, and between the
lake and the Hauhungaroa Range, are immense beds of white pumice sands, through which the rivers and streams have worn out for themselves deep channelled courses, with frequently wide canons at the bottom. Part of the northern shore and a great deal of the eastern shore of the lake is formed by cliffs of pure pumice, in one place 300 feet above the lake, and spreading thence southerly and easterly to the slopes of the Kaimanawa Range, covering the country with a deep deposit of over 100 feet in thickness. In a north and north-east direction it extends for many miles, covering the Kaingaroa Plains, and leaving deep deposits over all the valleys from Taupo to Atiamuri and Whakamaru.
Motutaiko is an interesting feature of the lake. It is formed of a column of rhyolitic lava ascending perpendicularly from the floor of the lake to a height of 600 feet, half of it being above and half below the water. On the north side of the island are masses of coarse conglomerate, consisting of glassy varieties of rhyolite, obsidian, etc., and interbedded with tuff and pumice sands. The rhyolite on Motutaiko is a very remarkable kind of lava, and I have seen nothing like it elsewhere in the district. It has a decidedly lamellar or stratified structure; the thin sheets of stone lie one over the other, and would seem to have been of a very viscid character, cooling as the thin liquid sheets moved over one another, and assuming very ropy, twisted, and contorted forms. This is doubtless the same class of rock discovered by Dr. Hochstetter, which, he says, the Natives called taupo, and from which the lake is said to have its name. In his description, he says:—
“It consists of an extremely remarkable kind of rock, which has attracted the attention of every stranger travelling along the lake. It is a volcanic rock of very striking lamellar structure; like the leaves of a book, sometimes of microscopic fineness, the thin lamellar sheets of stone lie one above another. Greyish black layers, resembling silicious schist of various lighter and darker shades, alternate with pearl-grey, violet, flesh-coloured, sometimes even with brick-coloured layers, so that the streaky mass reminds the observer of agate. From the numerous white transparent quartz grains, and small yellowish-white felspar crystals (sanidine) enclosed, it moreover has a porphyritic structure, while in smaller or larger vesicular spaces light-brown mica appears. There can be no doubt of the genuine lava character of the rock. As by the stretching and pulling of a mass composed of mixed fusions, artificially streaked glass is produced, so this rock is likely to have originated from a volcanic magma composed of various stone fusions.”
Richthofen has, in 1860, described quite a similar kind of rock from the vicinity of Tokay, Saraspatak, etc., in Hungary,
under the name of lithaidit; while Dr. J. Ratti has named a similar lava upon the Liparian Islands, liparite.
A specimen of the lava from Motutaiko, which I brought down with me, exhibits the peculiar curved and ropy forms in which it cooled, the thin lamellar sheets conforming regularly to the curve of the stone. This island rock is possibly an old volcanic mountain, the lower materials which formed the cone being dispersed by the action of the water and the ordinary subaerial denudation, and now covering the floor of the lake. A distinctly marked terrace extends right round Taupo Lake, 100 feet higher than the present level of the water, indicating that for a long period the lake stood 100 feet higher than it now does; the fall is probably due to the lowering of its only outlet, the Waikato River, where it probably broke through a barrier, about one mile from the point where it leaves the lake. Te Heuheu points out a flat rock, on the edge of the lake at Waihi, which he says his ancestors used for a sacrificial altar shortly after their first arrival in Taupo. This shows that the lake has not altered its level within 400 years.
At Karaka Point, a promontory of the lake 4 miles from Tapuaeharuru, are some interesting caves, worn by the action of the water of the lake on the rhyolite tuff, at a time when the water was considerably above its present level. There is also an interesting natural archway, cut out of the tufaceous rock, and instructive rounded, waterworn, and conical-shaped pillars of the same formation.
It has frequently been suggested that the only one outlet of the lake—the Waikato River—does not seem to carry off as much water as the numerous tributaries supply, and that consequently there may be subterranean passages through which the water flowed to supply the volcanic foci of the district.
Whilst surveying the lake I took the opportunity to measure the volume of its rivers and streams. I append a table showing the results of the measurements, which were very carefully taken. They show that practically the same amount of water is discharged as flows into the lake, the inflow being 16,483 gallons per second, and outflow 16,230 gallons per second.
As to the origin of the lake basin, I think there can be little doubt, from the facts, that volcanic action has played a large part in its formation. It is next to incredible that the waters of the lake could have worked out such varieties of shape and contour as are seen on the great coast-like cliffs of the western shores of the lake. Their whole appearance bears evidence, I think, of violent agencies, either of eruption or subsidence. The immense pumice beds surrounding the lake, which gradually diminish in bulk and size of the particles as they recede from it, are all circumstances which point towards the present site occupied by Lake Taupo as being once the scene of great volcanic activity.
In summarising, it becomes apparent that the character of the land and the quality of the soil derived from these various rock formations must be very variable. Where the soil is derived from the marls, shales, or limestone, it is rich and fertile. Some of the soil of volcanic derivation is also good, but unfortunately a great deal of the flat land and valleys are nearly rendered useless by the surface covering of pumice sands. There is, nevertheless, a large area of good settlement land amongst plateaus and hill slopes, and extensive flats suitable for agricultural purposes in the basins of the Waipa and Mokau.
In reference to the mineral character and construction of the main range, as affecting the probability of gold being found there, I trust that I may not appear as propounding a theory or offering an opinion in discussing this large and most interesting branch of the subject.
I have called that the main range which sweeps round to the west of Taupo Lake and reaches the Maungatautari, including Rangitoto and Tuhua Mountains, for these reasons: (1.) That the slates are found along it in many places; (2.) If the line of the range be produced north and south, it will be seen to pass northwards through the Thames, Te Aroha, and Cape Colville, and the Great Barrier Island, and southwards through Tongariro group to the southern side of the Kaimanawa Range; (3.) All along this line are found in greater or less quantity the same class of slates, pointing I think to the probability that this may be at least a branch of the old palæozoic mountain range which, commencing in the south-east of Otago, sweeps round the West Coast to Cook Strait, and thence is said to be continued through the North Island in a north-easterly direction to the East Cape.
It is along this line that the gold deposits at the Thames and Coromandel have been found, and also the reported gold discoveries made at various times at Maungatautari, Tuhua, and Rangitoto.
Mr. Herbert Cox, in his “Report on the Goldfields of the Cape Colville Peninsula, 1882,” says:—“The Cape Colville peninsula consists of a hill of slates, overlaid by various volcanic formations. The slates crop out in the lower parts of the gullies cut by the creeks which fall into Cabbage Bay. And although at some places the auriferous reefs are in country which does not appear to have any direct connection with the slate formation, yet at no great depths these rocks are certain to occur.” Slates appear along the Hauhungaroa Range in a precisely similar way as they do at the Thames and Coromandel. They are also the basement rock of the district, as appears from their being found at Wairere on the Mokau, north
of Maungatautari, in the gorge of the Waikato River. So far as my limited experience goes, it has appeared to me that the general characters of the formations on the main range of the King Country correspond to a considerable extent with those described by Mr. Cox as containing the auriferous series at the Thames. The tufaceous rocks are described as a composition of igneous fragments, and are considered to be auriferous, especially when in the vicinity of slates, and when the tufaceous character is due to the decomposition of highly indurated and metamorphosed masses. It is probable that portions of the masses forming the Hauhungaroa Range are allied portions of the rocks mentioned above, and products of the great volcanic action of the Taupo zone, and appear very like some of the formations of the Thames Goldfields described by Mr. Cox.
In collecting together these notes, gathered rather promiscuously, I have endeavoured to do so in as concise a manner as possible, and in such a form as to be useful in future investigations; but they have run into greater length than I intended.
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