We owe our first accurate knowledge of the geology of the Taupo volcanic zone to von Hochstetter. Although he spent a comparatively short time in the district, his great geological insight
enabled him to give a remarkably good account of some of its general features, and especially of its wonderful development of geysers and hot springs. He was, however, unable to ascend any of the great volcanic cones in the district, and he states that, although he gazed longingly at the massive outlines of Tongariro and Ruapehu, far to the south of the limit reached by him, neither time nor opportunity offered itself. Moreover, the Maoris had declared the mountain tapu, and would have strenuously opposed any projected ascent.
For many years little was added to our knowledge of the geology of the district, and it was not until after the unlocked-for eruption of Tarawera had recalled attention to the subject in so startling and emphatic a manner that further contributions were published. Mr. Cussen, in a paper read before this Institute last year, gave a general description of Lake Taupo and the country to the west, whilst in the reports on the Tarawera eruption by Mr. Percy Smith and the writer respectively there will be found additional information concerning the Taupo volcanic zone and the cones to the south. Mr. Park and Mr. L. Cussen have also published accounts of the ascent of Ruapehu.
Nevertheless the detailed description of the geology of the great cones to the south of the lake has not yet been given, and there is many a problem connected with the geology of the district which has scarcely been attacked. The present contribution to the subject is founded on observations made during various visits to the Taupo district, more particularly in January, 1888, when the writer passed a few days encamped at the foot of Tongariro. His time was chiefly spent in the examination of Tongariro; and a contemplated visit to Ruapehu was delayed by other work undertaken for the Government. He trusts, however, to have an early opportunity of making a more extensive examination of other parts of this locality.
Perhaps the greatest interest centres round the Ruapehu–Tongariro chain of volcanic mountains, for we have here no fewer than three cones—viz., Ruapehu, Ngauruhoe, and Tongariro—which have shown signs of greater or less volcanic activity within recent years. This chain (see Plate XXVI.), which has a length of fifteen or sixteen miles in a direction 26° east of north, rises up from the centre of a plateau nearly 2,000ft. above the sea-level. The plateau is bounded on the east by the Kaimanawa Range, composed of the ancient Maitai (Carboniferous) slates and sandstones, rising to a height of 5,225ft. in the Umukarikari Mountain. The Ruapehu–Tongariro line is nearly parallel to this range at a distance of some ten miles. To the west the
plateau is bounded by a range of mountains which passes northwards into the main range west of Taupo Lake. On the north the plateau is separated from Lake Taupo by the volcanic mountains Pihanga and Kakaramea (4,266ft.) and the ridge of volcanic rock which connects them. The Waikato River (called in this part of its course by the natives the Tongariro) rises in Ruapehu and flows northwards towards Lake Taupo, receiving many tributaries from the eastern side of the Ruapehu–Tongariro chain and from the western slopes of the Kaimanawa Range, and passes to the east of Pihanga as a broad, shallow, swiftly-flowing river. It enters Lake Taupo at the southern end, about two miles from Tokaanu, and has formed a very considerable delta, composed of the débris of volcanic rock from the western sides of the various cones, and of the slate found in the Kaimanawas. These two materials, of such different origin, are present in approximately equal proportions.
Ruapehu is the highest of the great volcanic cones, rising to 8,878ft. Many observers seem to have suspected that the vapour hanging about the summit on certain occasions, even in fine dry weather, was due to volcanic energy in the mountain; but it was not till April, 1886, when Mr. Cussen ascended the mountain, that the matter was placed beyond doubt.* He found a lake of warm and steaming water occupying a basin about 300ft. below the two chief peaks of the mountain. No steam was rising from the mountain during my visit to the district in 1888; but I was informed that it had been seen a few weeks earlier.
The summits of Ruapehu and Ngauruhoe are distant from one another a little more than eight miles, but they are connected by a ridge of volcanic rock, on top of which lie two lakes known by the name of Nga Puna-a-tama. These lakes appear to lie in craters which mark the sites of former centres of activity along the volcanic line.
Tongariro lies close to and abutting on the north-east side of Ngauruhoe, and the latter mountain is often miscalled Tongariro. The two mountains, though so close together, are sufficiently distinct in form and position to call for separate recognition.
Ngauruhoe.—Ngauruhoe is considerably higher than Tongariro. It forms a beautifully regular steep-sided cone, which rises to the height of 7,481ft. The height is greatest to the east, so that viewed from the north the mountain has an obliquely-truncated appearance. From the crater on the top steam constantly issues in considerable volumes, and, driven
[Footnote] * “Thermal Activity in the Ruapehu Crater,” by L. Cussen, “Trans. N.Z. Inst.,” vol. xix., p. 374.
before the wind, forms a long train to leeward. To the north the crater-margin is partly broken down, and the surface of the ground here is rotten and treacherous from the action of the acid vapours. The hydrochloric acid in the vapours reacting on the iron oxides in the scoriæ forms considerable quantities of perchloride, of iron, which stains the ground brilliant shades of yellow and orange, distinctly visible even from the top of Tongariro.
The sides of the cone are for the most part smooth and regular, and are formed of scoriæ and fine ashes, but here and there rugged projecting rocks mark the course of lava-streams. One of these, which descends as far as the south crater on Tongariro, ends there in a steep front of lava, with black scoriaceous surface, about 30ft. in height. This stream is said to be that which flowed from the crater during the eruption of Ngauruhoe in 1869.
It has been reported that Ngauruhoe has shown great signs of activity recently. I am indebted to Mr. Howard Jackson, the engineer in charge of the road-makers near Tongariro, for notes and sketches (see Plate XXVII.) bearing on the subject. Mr. Jackson has been in the district almost continuously for more than twelve months, within full view of the mountain. He states that on the whole the mountain has shown, if anything, rather less activity than usual, but that during bad weather which occurred in April or May, 1888, a gap was formed by the breaking-in of a portion of the crater-wall to the east. As seen either from the east or west, there is now a deep V-shaped notch which must have the depth of about 200ft. (see Plate XXVII., figs. 5 and 6). A comparison of the present outline of the cone with the drawings made by Hochstetter in April, 1859 (figs. 1 and 2) shows that a large amount of change has taken place since that date.
Although Ngauruhoe rises so high above the sea-level, the snow does not lodge long on the cone: the ground in many places is warm, and the internal heat passing slowly outwards, together with the steam rising from the crater, is sufficient to thaw the snow. The steam which rises so constantly from Ngauruhoe comes chiefly from a smaller, deeper crater, with dark and steep sides, lying within the circle of the principal one. From this crater ashes and fragments of scoriæ are frequently ejected.
Tongariro.—Tongariro, the third of the great mountains, is, strictly speaking, composed of a number of distinct cones built up around so many separate points of eruption. These cones, however, are so close together that the mountain forms at its base a single mass. Viewed from a distance the mountain has a broad, flat-topped appearance, but as it is
approached the separate cones of which it is formed at the top become more distinct. (See Plate XXVIII.) The lower slopes of the mountain are composed of lava-streams, which stretch as far as Rotoaira, the lake to the north; indeed, the formation of the lake appears to be due to the blocking of the drainage-channel by a lava-stream. These lower slopes are covered with tussock-grass, which affords a certain amount of support to animal life, so that the slopes of the mountain have been employed as a sheep-run. On the north side there is a large area of forest. Amongst the tussock-grass small herbaceous plants with bright-coloured blossoms grow in far greater abundance than is usually seen in New Zealand except in alpine districts. The summit of the mountain is all but devoid of vegetation.
That portion of Tongariro which lies to the east is often termed by the natives Te Mari, whilst the part to the west is Tongariro proper. The top of the mountain is marked by seven large craters, as well as some smaller ones. The distribution of these will be best gathered from the accompanying sketch-map (see Plate XXIX.), which will, I believe, be found sufficiently accurate for our present purposes.
The ascent of the mountain is most easily accomplished by way of Ketetahi, a hollow at the height of 4,900ft. on the side of the cone which forms the north-west angle of Tongariro. Ketetahi is not, properly speaking, a crater, though explosions of steam seem to have assisted in the excavation of the hollow: it resembles rather the enlarged head of a gully. There are many hot springs and a powerful escape of steam here, whilst the overflow of the water forms a warm stream. The volume of steam rising from Ketetahi is usually very considerable—so copious, indeed, as to render the spot visible for a distance of fifty miles.
Two other places on Tongariro are marked by the escape of steam, which betrays the volcanic forces dormant within the mountain. One of these is on the northern slope of Te Mari, the other is in the Red Crater; but neither shows so much activity as Ketetahi. Above Ketetahi the slope of the cone becomes steeper. It is formed of lava having a comparatively smooth surface of step-like formation, which greatly facilitates the ascent.
On reaching the summit of the cone a remarkable sight presents itself. The top is formed by a circular area half a mile in diameter, which at first strikes one as being perfectly flat. Closer examination, however, shows minor undulations, the surface of the wind-swept ground being strewn with fine sand-like volcanic ashes and lapilli. To the north, and, again, on the opposite side, towards the south, a cliff of lava rising perhaps to 100ft. above the crater-bottom forms the boundary
of the area, whilst elsewhere around the margin the ground dips down abruptly to the steep outer slope of the cone (see Plate XXX). The two lengths of cliff and a few other rocks in the same circle evidently mark the former rim of a crater, which at a subsequent period was filled up by lava, which overflowed the brim and formed streams on the rocky sides of the cone. From its position we may distinguish this filled-up crater as the North Crater of Tongariro. On the west side, and just within its margin, it contains a smaller funnel-like crater of considerable depth. The cliff to the south is chiefly formed of thick horizontal beds of dark lava. At the western angle of the cliff the smooth and slightly-weathered surfaces of the joints in the lava show a remarkable streaky structure, visible from a great distance. This flow-structure, which bears testimony to the unequal movements in the lava at the time of its consolidation, is due to the irregular alternation of a light- and dark-grey material in the ground-mass of the lava. Near this spot the surface of the ground is covered with a layer of an exceedingly light pumice of acid composition, differing greatly from the other rocks found on the mountain. The largest of the fragments was 14in. in its longest diameter. Reference to this and other rocks on Tongariro will be found further on.
Between this North Crater and the rest of the Tongariro system is a dip of 200ft. or 300ft., by which it is marked off from the rest of the mountain-top. To the south, and stretching as far as the slope of Ngauruhoe, is another cone, marked by a very large crater, over half a mile in length. This crater, which we may suitably distinguish as the South Crater, is of a much elongated form, and it is worthy of note that its long axis coincides in direction with the Tongariro–Ruapehu line.
Its walls are very steep, and in many places precipitous. They are highest at the end towards the North Crater, and gradually diminish in height in the direction of Ngauruhoe. The highest part of the crater-wall, which is also the highest point on Tongariro, lies on the western side, and is about 6,450ft. above the sea-level. The crater-flow lies over 800ft. below this point. At times the bottom of the crater must be covered in part by a shallow lake, which discharges at the end towards Ngauruhoe. The water will pass down the watercourse between the latter mountain and Tongariro, and form the beginning of the Mangatepopo Stream, a tributary of the Wanganui River.
Travelling along the ridge which forms the eastern boundary of the South Crater, we pass two large craters on the right. The first of these has high precipitous sides towards the west, whilst to the east its wall is wanting,
and the eye stretches over vast fields of rugged hummocky lava.
The second, or Red Crater, lies further to the north, and is interesting as showing the signs of recent activity. The crater has very steep sides, so that from the west at least it is not possible to descend into it. The upper part of the crater is formed by a great thickness of beds of dark blood-red scoriæ having an extremely close resemblance to the layers of scoriæ of the Tarawera eruption which lie piled up on the borders of the fissure on the Tarawera Mountain. It is said that steam can be frequently seen issuing from this crater; but none was visible on the occasion of my visit. Around the margin of this crater blocks of a dark heavy lava, having the appearance of a basalt, and more basic than the usual lavas of the mountain, are to be found. Across the floor of the crater is a small lava-stream.
To the north of the Red Crater, on the part of the mountain called Te Mari, is an old crater of considerable size. Lying to the east of this, and separated by a comparatively low ridge of rock, is another crater, containing a lake of the most beautiful blue water (see Plate XXXI.); whilst on a ridge between this and the Red Crater are two much smaller lakelets, one of which from its colour has been called Rotopounamu (Greenstone Lake).
On the north-eastern slope of Te Mari is yet another crater of considerable size, and close to this, as already mentioned, there is a large but intermittent escape of steam.
Lake Taupo.—It is not my intention to enter here into a detailed description of Lake Taupo and the surrounding country, but merely to mention such points as we shall have occasion to refer to hereafter or as have not been previously described. The lake has an area of nearly 242 square miles; it is 24⅞ miles in length and 16½ miles in extreme width, and has a shape which has a general resemblance to that of the continent of Africa. In many places the lake is bounded by steep cliffs of lava and associated tuffs. That the lake formerly stood at a higher level is clearly shown by the terraces around it, which are continued up some of the small valleys leading into the lake. One terrace stands at a height of 100ft. above the present water-level of the lake. The accompanying plate (Plate XXXII.) shows this terrace at the south end of the lake, about three miles from Tokaanu, looking in a northerly direction. Another well-marked terrace lies at the height of 300ft. to 400ft. above the lake.
The lavas at the north, east, and part of the western sides are rhyolites; at the south end they are chiefly augite-andesites. The pumice-deposits which form so remarkable a feature of the Taupo district are found to a greater or less
extent all round the lake; they are thickest, however, near the northern end, reaching the thickness of 300ft. in one of the cliffs. Throughout the district, indeed, the pumice is widely spread over the surface of the ground, especially to the east, north-east, and north of the lake. It is found even on the summits of the highest mountains—here, however, merely as a sprinkling, whilst at the bases of the hills and in the valleys it may reach the depth of hundreds of feet. The manner in which the river-valleys have been filled up with pumice, out of which the water has excavated terraces, is sufficient evidence of the influence of running water in distributing the pumice. There are, however, many facts which show that much of the pumice has travelled through the air and fallen in showers in its present position.
Popular belief ascribes this pumice to the great volcanic mountains Ruapehu, Ngauruhoe, and Tongariro, which lie to the south of the lake, the showers of pumice being supposed to have been brought by the prevailing south-west winds. Any one who will examine the distribution of the ash from the Tarawera eruption* will see that the explanation has a primâ facie probability, for in that case the ash was spread out in just such a manner. But the examination of the neighbourhood of Tongariro shows that such an explanation is not applicable to the distribution of the pumice in the Taupo district. The pumice is less abundant in the neighbourhood of the great mountains than elsewhere in the district, and, as will be shown further on, the rocks of Tongariro belong to a different group. The pumice around Lake Taupo contains considerable quantities of rhyolite - fragments other than pumice, especially of the perfectly-laminated variety which has been termed lithoidite. Near Ouaha, on the east side of the lake, I found angular blocks of lithoidite 3ft. and more in diameter, and weighing two tons or more, imbedded in the pumice-deposits. It is obvious that blocks of this size cannot have travelled any great distance, whether by the agency of water or the force of volcanic explosions, but must have been derived from some source near at hand. The only localities near Taupo that I am acquainted with where this variety of rock is found in sitû in lava-streams are at Motutaiko, the island in the lake, and Hamaria, three or four miles distant, on the shores of the lake. We might perhaps not unnaturally look to Tauhara, the volcanic cone at the north end of the lake, as a source of part of the pumice. This mountain has the height of 3,603ft., and the country at its base on both
[Footnote] * A map showing the distribution and depth of the ashes from the Tarawera eruption will be found in the “Report of the Eruption of Tarawera and Rotomahana,” by the writer.
sides of the Waikato River is remarkable for its vast number of hot geysers, &c. The mountain has a large crater on top (Hochstetter stated that there was no crater), and its upper part is thickly covered with forest. I found on top of Tauhara a sprinkling of pumice, with small fragments of lithoidite quite different from the rock of which the mountain consists. The lavas, as far as I observed, are all of acid composition, but contain a rather large amount of hornblende, a mineral not found, so far as I have seen, in the pumice-deposits.
Hochstetter supposed that the formation of Lake Taupo was due to subsidence of its area, a supposition which is strengthened by the abrupt, precipitous character of part of its shores and the fairly uniform depth of its waters. A more popular theory supposes Lake Taupo to have been an immense crater; but evidence of this is wanting, though it is quite conceivable that the sunken area was marked by vents from which perhaps a portion of the pumice which covers the district was derived.
The Waikato leaves Lake Taupo at the north-east end of the lake, and for the first part of its course flows to the north-east in a gorge through rhyolitic tuffs. Three miles from the lake its course is broken by the Huka Rapids and Fall. The tuffs here are locally hardened by the deposit of a siliceous cement from the hot springs, which have attained a considerable development at one time, and are indeed still represented by one or two warm streams close to and above the rapids. The river on reaching the harder rock becomes suddenly narrowed to less than a quarter of its usual breadth, and, confined to a narrow channel with vertical sides and sloping bottom, rushes through it with an arrowy swiftness which the eye can scarcely follow, until at length it plunges downwards some 30ft. into a wide basin of water eaten out of softer rocks. Some of the finer pumiceous sands here contain such an abundant siliceous cement of siliceous sinter that to the naked eye they have the half-glassy appearance of a pitchstone, and have, indeed, been described as lavas.
A few miles further on, however, the Waikato crosses a series of lava-streams, and the harder rocks have led to the formation of the beautiful rapids known as the Aratiatia Falls. The lavas are rhyolites, chiefly of a glassy character, spherulitic obsidians being well developed.
Volcanic Fissures in the Taupo Zone.—We have already seen that the Tongariro–Ruapehu group of volcanoes are arranged in a straight line, which doubtless represents a fissure in the earth's crust by which the molten rocks have forced their way to the surface. It affords, indeed, a remarkable instance of a number of volcanic vents arranged close together on the same fissure. We have the huge mass
of Ruapehu at one end, then the two crater-lakes of Nga Puna-a-tama, the lofty active cone of Ngauruhoe, and then the direct line is continued by the South and North Craters on Tongariro.
If, with a slight deviation in direction, the line be continued to White Island, we find that it passes through a large number of points remarkable for their volcanic activity, including Tarawera and Rotomahana. This line may therefore be justly looked upon as the main line of volcanic activity in the Taupo volcanic zone.
Nor is this the only instance of great fissures connected with the volcanic activity of the district. In the northern part of the Taupo zone we have two lines marked by hot springs as well as by dislocation of the rocks. These lines are parallel to the main line, and probably correspond to great fissures in the rocks. The first of these lines, seven miles from the main line, stretches from Orakeikorako along the east face of the Paeroa Range to Rotoehu, a distance of thirty-seven miles. The second line is eight miles further to the west, and stretches from the hot springs on the Waipapa Creek, near the Waikato, through Rotorua to Rotoiti.
We may reasonably ask whether these lines of fissure are represented further to the south. If they are produced in that direction they will be found to coincide generally with the lie of the shores of Lake Taupo. The main line will correspond with the eastern shores; whilst the Orakeikorako line will correspond with the western shore of the lake from Waihi to the bold and precipitous headland of Karangahape—i.e., south of Western Bay; and the Rotorua line will correspond with the western shores of the broad arm of the lake known as Western Bay.
It will be noticed, however, that the coincidence is not exact, the lines showing a tendency to converge as we approach the south, the point of convergence being Ruapehu. There can be no doubt that Ruapehu marks the position of an important centre with reference to the broader structural features of the North Island. It is here, or near here, that the line of elevation marked by the northern peninsula joins the main axis or backbone of both islands. The line of the western coast of the peninsula north of Auckland, if produced, will be found to pass approximately through Ruapehu. In other words, the structural axes and dislocations of the country radiate from near Ruapehu, and the manifestation of volcanic forces here is determined by its position with reference to the great flexures of the earth's crust.
The examination of these radiating lines cannot fail to remind the geological reader of the system of cracks obtained by Daubrée during experiments on the fractures produced in
homogeneous substances, such as ice, by torsion.* Such systems of fractures may also be readily obtained in glass. Daubrée obtained systems of fractures crossing one another at angles approaching a right angle. In each system some of the fractures were arranged in a radiating or fan-shaped manner, others were parallel to each other. Great caution, of course, is needful in applying such results to the explanation of natural dislocations, for it is uncertain how far we are justified in supposing the earth's crust would behave as a homogeneous mass, and the conditions of the experiment are no doubt very different. Still, the resemblance of the arrangement of these natural dislocations to that of the artificial fractures is sufficiently close to lend interest to the comparison.
Many instances of transverse dislocations might be cited in the Taupo zone. The line of volcanic mountains at the south end of Lake Taupo may be due to such a fracture. We have here the two mountains Kakaramea (4,266ft.) and Pihanga, of about the same height. They are connected by a high ridge of volcanic rock, and the long axis of the group lies at right angles to the line of the Ruapehu–Tongariro fissure. Pihanga has a crater showing to the north, and Kakaramea is said to have traces of a crater. The rocks of which they are composed are generally similar to those of Tongariro. Where the slopes of these mountains come down to the south shore of Lake Taupo we find the hot springs and geysers of Tokaanu, as well as various springs at intervals as far as Waihi.
Rocks of Tongariro.—Specimens of rocks from Ngauruhoe and Ruapehu were collected by Mr. Cussen in 1887, and were described by me in the “Transactions” of the Institute.† Since then I have had the opportunity of making very extensive collections of rocks on Tongariro and in its neighbourhood, and am able to add to what was then written. I am indebted also to the great kindness of Mr. J. A. Pond, Colonial Analyst, for a valuable and extensive series of analyses of rocks from Tongariro, Ngauruhoe, and Lake Taupo. These furnish an important and welcome supplement to the results obtained by microscopical examination.
Hochstetter has stated that all the rocks collected by him in the Taupo volcanic zone belonged to the family of acid volcanic rocks known as rhyolites. In the paper referred to it was shown that the intermediate (or slightly basic) group of lavas known as the augite-andesites were largely represented in the district. This result is amply confirmed by Mr. Pond's
[Footnote] * Daubrée, “Etudes Synthétiques de Géologie Expérimentales,” tome i., p. 307.
[Footnote] † “Trans. N.Z. Inst.,” vol. xx., p. 306.
analyses. My own collections show that the typical rocks of Tongariro are augite-andesites, though more basic rocks, which may be regarded as members of the basalt group, are also represented. Perhaps the commonest variety of these augite-andesites is one which is found, amongst other places, on the slope of the North Crater of Tongariro, where it forms the lava-streams above Papakai. Similar lavas descend as far as the shores of Rotoaira. The rock is a porphyritic one, of medium grain and dark colour, showing when quite fresh a slight resinous lustre. The porphyritic crystals are numerous, but none of them reach a length of over 3mm., and they are usually much smaller. Examination with the microscope shows that the porphyritic crystals consist of felspar and augite in about equal proportions and in well-formed crystals; there are a few smaller magnetites in irregular crystals. The felspars are almost all striated, and many of them are crowded with inclusions of glass, which is sometimes brownish and pure, at other times is colourless, but containing globulites and dark granules. The felspars also contain inclusions of augite and apatite. The augites are in eight-sided prisms; in thin section they are yellowish and only feebly pleochroic, the range of tints being from greenish-yellow to brownish-yellow. A few of the largest augites are completely honeycombed by groundmass. The groundmass is hyalopilitic—i.e., it consists of a felt of crystallites united by rather abundant colourless glass. It contains a considerable number of black granules of magnetite. The crystallites include great numbers of long, slender, non-polarising longulites, and very few of these incipient forms are sufficiently developed to polarise. Olivine is altogether wanting in the rock. The augite and felspar not unfrequently occur in nests composed of great numbers of crystals, which only show their proper form on the exterior of the groups. The analysis of this rock is shown as No. 1 in the following table. Its specific gravity is 2·76:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|—||Rotoaira.||Otouku||Red Crater.||Ash.||Ngauruhoe.||North Crater.||Motutaiko.||Hamaria.|
|No. 1.||No. 2.||No. 3.||No. 4.||No. 5.||No. 6.||No. 7.||No. 8.|
|Iron peroxide and alumina||30·55||29·55||29·45||31·95||32·55||17·2||19·15||17·2|
Nos. 1–4 are from Tongariro: No. 1, an augite-andesite from the lava-stream wich reaches Rotoaira; No. 2, ditto from Otouku; No. 3,
ditto from Red Crater; No. 4, ash on slope of Tongariro; No. 5, augite-andesite, the lava of 1869, Ngauruhoe; No. 6, pumice from North Crater, Tongariro; No. 7, rhyolite lava from Motutaiko; No. 8, lithoidite from Hamaria, Lake Taupo.
Other varieties of augite-andesite from Tongariro differ chiefly in the degree of devitrification of the glass, the porphyritic crystals being essentially the same. Specimens may be obtained in which the groundmass is quite free from crystallites, and consists of a pure glass of a rich brown colour. That this was the original condition of the ground-mass in the other varieties may be seen from the inclusions of brown glass in some of their larger felspars. Here we must include specimens of brownish pumice, which contains augite and plagioclase in crystals and groups of crystals—evidently only a similar rock distended with very numerous minute vapour-cavities.
On the side of Ngauruhoe which slopes down to the South Crater on Tongariro, is a lava-stream which is said to have flowed from Ngauruhoe in 1869. This stream ends in an irregular front just where the slope of the cone joins the flat bottom of the crater on Tongariro. The lava is much fissured, and the smooth surfaces of the blocks have a strong glassy lustre. This lava is essentially like those on Tongariro, except that the groundmass is rather more glassy, and is perhaps richer in iron-oxides. The proportion of silica, however (see analysis No. 5), is rather lower, being only 57·0. Its specific gravity is 2·82.
Amongst other lavas of Tongariro may be mentioned that which forms the blocks of lava at the bighest point of Tongariro. This contains a few crystals of olivine. The same mineral occurs in crystals visible to the naked eye in the lava which reaches Otouku. This rock is shown by its analysis (see No. 2) to differ but slightly in chemical composition from the commoner type of Tongariro lava. Its structure, however, is very distinct. The porphyritic crystals are not numerous, but include plagioclase, augite, and olivine. The groundmass is almost entirely crystalline, and shows a pronounced fluidal structure. It consists of small augite prisms and minute laths of felspar, with scanty magnetite granules. In parts of the rock a little glass may be traced; in others it appears holocrystalline. The fluidal structure is due partly to the parallel grouping of the felspar laths, but more especially to the arrangement of the augite and felspar in such a way that along certain lines the minute augite crystals largely predominate, along others the felspar laths. The specific gravity is 2·83, being doubtless a little higher on account of the crystalline character of the groundmass.
A rock which occurs in blocks around the Red Crater is
much more basic than any of the preceding rocks. It is heavy and black, with all the appearance of a basalt, and shows microscopic olivine. It contains, however, felspar and augite crystals like the porphyritic crystals in the augite andesites previously described. They are not very abundant, and to them are added olivine in numerous crystals, and felspar laths. The groundmass is in the main crystallitic, but shows far more crystalline particles between crossed nicols, and glass is scarcely visible. The rock thus shows a relationship to the augite andesites, but at the same time approximates to the basalts. Another variety collected by Mr. Cussen contains chiefly olivine among the larger crystals, though a few smaller augites are present. The groundmass is partly crystallitic, but contains very numerous felspar laths.
The analysis of the former variety by Mr. Pond shows (see No. 3) that it contains 52·1 per cent. of silica—an amount considerably higher than that usually present in rocks recognised as augite-andesites. Its specific gravity is 2·94, which also indicates a rock more basic than ordinary augite-andesites. The magnesia and lime are also present in much higher proportion than in the other rocks of Tongariro. Rosenbusch, in his last edition,* states that the augite-andesites seldom contain less than 56 per cent. of silica, whilst some basalts attain the same percentage. Teall quotes twenty-three analyses of porphyrites and andesites as varying between 66·75 and 54·73 per cent, of silica, whilst the same number of basalts showed percentages of silica varying between 53·73 and 42·65.† Under the circumstances, we shall probably be justified in terming the rock a basalt, though we must admit that it is closely related to the augite-andesites. The system of classification of rocks is necessarily more or less arbitrary, and the present rock is one which lies near the line of division between the augite-andesites and the basalts.
Pumice on Tongariro.—We have referred above to the popular idea that the great quantities of pumice around Lake Taupo are derived from Tongariro and Ruapehu, and have stated that the examination of the country to the south of Lake Taupo lends little support to any such theory. Pumice is indeed found on Tongariro, but in comparatively small quantity. A small amount of pumice of an acid character, containing 75·25 per cent. of silica (see analysis No. 6), was found in the North Crater of Tongariro, and one or two small fragments of rhyolites were found elsewhere on the mountain-top. But with these exceptions all the rocks high up on the mountain were of a more basic character. On the lower
[Footnote] * “Physiographic der Massigen Gesteine,” p. 702.
[Footnote] † “British Petrography,” p. 49.
slopes of Tongariro, to the north, on the other hand, pumice is found to a depth which may reach 2ft. or 3ft, but often much less, or may be altogether wanting. The deposit was probably thicker once, as some of the streams show small terraces of pumice. This pumice is, however, only a superficial coating: below we find ashes or lavas of more basic composition. On the lower slope of Tongariro, above Papakai, such sections as the following may be observed in recent watercourses:—
|1. Black soil||½–1|
|2. Subsoil, white and pumiceous||½–1½|
|3. White pumice||½–2|
|4. Layer of loam, denoting an old soil and subsoil, the upper 6in. darker||2|
|5. Volcanic ash, fragments weathered brownish, dark-grey when broken open: towards bottom in finer and coarser layers||6–10|
|6. Brown loam, sharply marked off from the former: may represent an old land-surface||1–4|
|7. Breccia of fragments of lava, apparently augite-andesite.|
Examination of the deeper layers Nos. 5 and 7 shows that they are composed of rocks related to the augite-andesites. An analysis of the ash from layer No. 5, by Mr. Pond, shows 57·9 per cent. of silica. (See analysis No. 4.) The specific gravity was 2·68, affording a similar indication.
We may conclude, therefore, that the eruptions of Tongariro for a considerable period have yielded lavas of intermediate (and basic) composition only. The pumice of acid composition which lies on the surface must have been derived from some more distant source, and of course at a date subsequent to the great eruption of Tongariro which produced the ash of layer No. 5. Seeing how widely the pumice has been distributed in the district, we need not hesitate to make such a supposition; and I may quote as a parallel case the ashes from the eruption of Tarawera which were deposited on the cone of Mount Edgecumbe, at a distance of fifteen miles, to the thickness of 14in.
In concluding, I desire to express my indebtedness to Mr. Percy Smith, Surveyor-General, for topographical information relating to the Taupo district, and to Mrs. J. McCosh Clark for the artistic sketches illustrating some of the geological features of the district which is the subject of the present paper.