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.