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The Pamphlet Collection of Sir Robert Stout: Volume 67

Geology

Geology.

The only published account of the geology of the district is the survey of Dr. P. von Hochstetter, made in 1859 for the Provincial Government of Auckland. We have found this account to be very accurate, and have merely added to it a few additional observations of our own.

Volcanic Rocks.—The central portion of the district is composed of rhyolitic rocks, which are bounded on the south-east by the Whakatane Mountains, a high range lying on the east bank of the Rangitaiki River, composed of Palaeozoic sandstones belonging to the Maitai system. To the west and north they are page break
Plate 1.

Plate 1.

Summit of Mt Tarawera from Paeroa. 2ND July 1886.

page 5 bounded by the trachytic tuffs and breccias of the Patetere plateau and of the hills between the north side of Rotoiti and Matata. Dr. Hochstetter considered that the trachytic rocks of the Patetere plateau were an older formation than the rhyolites of the Taupo zone.

The rhyolites form all the hills from the south side of Rotoiti and Ngongotahi to Mount Tarawera. They surround Lakes Rotokakahi, Tarawera, and Rotomahana, and stretch across the Kaingaroa Plains to Galatea and south-west to Ateamuri. No other volcanic rocks were previously known in the neighbourhood of the recent eruptions; but one of the Okaro craters shows in its walls a dark augite-andesite, and has thus got the name of the Black Crater. The rhyolites are partly the stony variety with grains and crystals of quartz known as liparite or quartz-trachyte, and partly the vitreous variety known as pitchstone—the vitrophyre of Vogelsang. This pitchstone varies much in character, and is often spherulitic; but we have seen no true obsidian in the district.

Mount Edgecombe on the north-east, and Tauhara on the south-west, near Lake Taupo, are characteristic volcanic cones, both marked by Von Hochstetter as trachytic; but in the rhyolitic district there is no true cone unless it may be Haroharo, near Rotoehu.

Mount Tarawera.—Mount Tarawera is a flat-topped ridge about three miles long and half a mile to a quarter of a mile broad, running in a north-east and south-west direction. It rises precipitously from an elevated plateau deeply cut into hills and valleys by the rain (see Frontispiece). On the eastern side a long spur, covered with forest, runs out towards Mount Edgecombe, and on the south-west side another shorter spur stretches to the south. The ridge itself is divided by a saddle, estimated by Mr. Percy Smith at 500ft. deep, which divides a smaller north-west portion, called Wahanga, from the chief part of the mountain. The highest peak is called Ruawahia, and is situated just south of the saddle, while the southern peak of the ridge is called Tarawera by the Maoris. By Europeans, however, the name Tarawera is generally applied to the whole mountain, including Wahanga. It bears no resemblance to a volcanic cone, whether composed of tuft or of viscid lava; and Mr. Percy Smith, who ascended it three times previous to the eruption, states that there was no appearance of a crater on the top. On a point of so much importance we may be permitted to quote Mr. Smith's own words, as he is the only competent observer page 6 who has visited the mountain. He says, "Prior to the eruption the two mountains of Wahanga and Ruawahia (for Tarawera is only a local name on the south end of Ruawahia) formed two high table-lands of about three miles in total length by about half a mile in width, divided by the saddle before referred to, the top of which was covered with large angular fragments of trachyte, which had the appearance of having been shivered into pieces by frost; and the top was further divided into hillocks by deep crevasses running irregularly in all directions. The edge of this table-land has steep, precipitous, rocky sides, falling off into gentle slopes all round, on which were several forests of considerable size—now, alas, all destroyed."*

Lake Rotomahana.—Before the eruption Rotomahana was a shallow lake, with patches of raupo (Typha latifolia) of irregular form, less than a mile in length by a quarter of a mile in breadth, lying in a north-and-south direction. The northern and southern ends were low and swampy; the sides, both east and west, were higher and rocky, but the rocks were decomposed into soft fumarole-clays, red, white, and grey in colour. On the eastern side the hills rose rather abruptly to a height of about 200ft. above the lake, while on the west they sloped up, more gently, to what is called by Hochstetter the Papawera plateau, about 750ft. or 800ft. above the lake. South-west of the lake and about a mile distant is the hill Hape o Toroa, 2,300ft. (Hector) above the sea, and between 900ft. and 1,000ft. above the former level of the lake. Between this hill and the lake is another smaller one called Oruakorako. All these hills were covered with fern and tea-tree (Leptospermum).

Old Lake-basins and Sinter Deposits.—Extensive old lacustrine beds are found on the south side of Rotorua, forming the flat between the lake and "Whakarewarewa, and they extend along each side to Awahou and Te Ngae. These beds are horizontally stratified, but often current-bedded, and consist of sand and pumice-gravel with occasional beds of fine pumice-dust, evidently an old volcanic ash. At the base of the cliff's near Te Ngae we found, in one locality, a bed of angular rhyolitic gravel, the stones being of all sizes up to 3in. or 4in. in diameter. These beds are covered unconformably by a younger set of rhyolite grits, coloured yellow-brown by hydrous ferric oxide, which we had no time to examine sufficiently. Rhyolite grit with quartz occurs also on the Island of Mokoia to an altitude of about 100ft. above the lake.

page break
Plate II.

Plate II.

Mt Tarawera from Near Galatea, 4TH July, 1886.

page 7

South of the Hemo Gorge, by which the Taupo Road leaves the Rotorua basin, there is another old lake-basin, now entirely filled up, from the middle of which rises Haparangi, like Mokoia in Rotorua. This basin is ten or twelve, miles long by about four or five in breadth; but we had no time to examine it.

Waikoura basin, or Earthquake Flat, where the roads from Rotorua to Paeroa and to Galatea diverge, has the appearance of being another old lake-basin; but, unlike the others, it is entirely filled up by volcanic ejectamenta. On the north-east side, near the broader end, there is a remarkable crateriform hollow, about (50ft. long and 30ft. broad, and 25ft. deep. It is of irregular form, a projection extending into it. On the north-west side. An old watercourse leaves it on the western side; but this is not more than 6ft. or 8ft. deep. On the southerly side a very-good section, exposed by the late earthquakes, shows that the lower half is formed by rhyolite-tuff and agglomerate covered by beds of red and yellow fumaroles-clay. These have suffered considerable denudation, and are overlain unconformably by the following:—
4.Rhyolite-sand;
3.Pumice-dust without quartz;
2.Rhyolite-grit;
1.Pumice-sand with quartz;
the whole being covered by a deposit of yellow sand. The northern and eastern sides are obscure, but appear to consist of rhyolite agglomerate nearly to the surface.

Old siliceous sinter deposits are found in many places on the south side of Rotorua; and the hill called Pukeroa, behind Ohinemutu, as well as Tamate Heria on the lake, are composed entirely of it. The sinter, both here and in the Rotorua Township, is overlain by the lacustrine pumice-sands already mentioned. We also saw some siliceous sinter by the side of the Taupo Road between Haparangi and Horohoro.

* "Volcanic Eruption at Tarawera," 1886, H.-26, p. 2.