Art. XLVII.—On the Geology of Te Moehau.
[Read before the Auckland Institute, 10th October, 1898.]
The Moehau is the highest point in the northern portion of the Cape Colville Peninsula. From the cape the range rises somewhat abruptly, culminating in two castellated peaks 2,900 ft. above sea-level. To the south, however, a gentle fall terminates in a low saddle, about 800 ft. high, between Cabbage Bay and Waikawau. Though thus apparently an isolated mountain, it is certainly a part of the main axial elevation of the Hauraki Peninsula, an elevation that has its northern termination in the Great Barrier Island.
From the superstitious dread of the higher parts of the mountain entertained by the natives, and from there being no inducement for settlers and others to ascend it, the summit has been but rarely visited. The Maoris, who were formerly very numerous in this district, averred that on the higher slopes of the mountain there exists a race of men, small in stature and ruddy in appearance. These men they called “Turehu,” and on foggy days—which, indeed, are neither few nor far between on Moehau—their voices and those of women and children may be heard piercing the misty silence. The origin of this legend, unique in Maori folk-lore so far as I know, at any rate in its present setting, is unknown, and the most feasible explanation that has so far been advanced to account for it is that the legend arises out of the enmity that ever exists between coastal and inland tribes. My own theory, however, traces the legend to a much more insignificant source. Remembering that the Coromandel district is practically the sole habitat of the rare little New Zealand frog (Liopelma, hochstetteri), and Moehau its only certain place of abode, I think that we have here the basis of the legend. It is extremely improbable that this animal was more common during the occupation of the peninsula by the Maoris than it now is, and the rarity of the animal, its similarity to a lizard—always an object of dread to the Maori—its singular mode of progression, its colour, and finally its many attitudes, so grotesquely human, must have appealed strongly to the Maori mind, already prone, as we know from their carvings, to a belief in distorted humanity.
The first ascent of Te Moehau was made by Mr. James Adams, B.A., of the Thames High School, in January, 1890,
and since then the summit has been visited some half-dozen times only, and then mainly by surveyors using the “trig.” during the late “boom.”
Mr. Adams ascended from Waiaro, on the western shore; but the easier route, and the one that is generally used, is that from Port Charles. From Port Charles the track crosses to Sandy Bay, and thence up the Okahutahi Creek, which has its source in Moehau. The andesitic lavas and breccias, seen on the coast, continue for about a mile up the creek, when slaty shales and mudstones appear in the bed of the stream, though high up on the spurs on either side the overlying igneous rock, which has resisted the denuding efforts of nature, still appears. Taking one of the slate spurs to the left, after a somewhat arduous climb of nearly 2,000ft. the main ridge is reached, along which a track, worn into mud by the wild cattle that abound on Moehau, runs north for about three miles to the foot of the peak. The whole of this ridge, averaging about 2,100 ft. above sea-level, is composed of decomposing yellow slaty shales and mudstones, showing occasionally sheddings of quartz. At a height of about 2,400 ft. the vegetation becomes stunted and covered with lichen, clear indications of the average climatic conditions. Here also was found the first evidence of the igneous nature of the summit, in the shape of boulders of much-decomposed andesite lying in the water-holes of a small creek. The ascent now becomes very sharp, and with the rapid rise the trees gradually diminish in height until they are only breast-high. Rimu (Dacrydium cupressinum) are seen 4 ft. high with trunks 1 ft. in diameter. Higher still the trees disappear, and the last 100 ft. of ascent is accomplished over a grassy sward. The summit is steeply precipitous with cliffs 50 ft.—100 ft. in height on three sides.
The view from the top is, to say the least, magnificent. Under-foot, apparently, lie the green fiats of Port Charles, Waiaro, and Cabbage Bay, constituting a pleasing relief to the prevailing sombre hues of the densely clad bush ranges. On the one hand lie the glancing waters of the Hauraki Gulf, studded with island gems; over and beyond are the Waitakerei Ranges, fading away to a blue haze. On the other hand, and in front, the horizon is unbroken, save for the rugged outlines of the Great Barrier. To the south the eye travels over valley after valley, range after range, apparently interminable—mute monuments of nature's sculpture.
Considerable variation of opinion exists between previous writers on the subject of the geology of Te Moehau. Mr. McKay, the Government Geologist, from information supplied to him, considers that the peak is formed by one large dyke of andesite, and that the mass of the mountain consists of
slaty shales. So far as I could ascertain, however, the igneous rocks on the summit do not appear to be intrusive, and in this view I am confirmed by the fact that the section of the rock under the microscope conclusively proves that it is not, and never has been, holocrystalline. In other respects my observations tend to confirm Mr. McKay's views, more especially as to the height to which the slates reach. Mr. Park, on the other hand, shows* a section across Te Moehau in which he represents the mass of the mountain to consist of solid andesites, the andesites reaching nearly to sea-level. My examination, so far as it went, of the spurs and ridges on the slopes of the mountain does not, however, tend to support this view.†
From stratigraphical and petrological considerations, therefore, I am disposed to consider these volcanic rocks as forming part of an old andesitic flow which formerly extended far to the east, and which had its origin in the dykes that are now found on the eastern slopes. One of these dykes is 50 ft. in width, approximately vertical, and strikes north and south, parallel -with the main axis of elevation of the peninsula. The microscopical examination, as will be seen from the detailed description of the sections, does -not discourage this view. The feldspars are identical, and the augite of the andesite is represented in the dyke rock by its plutonic congener, hornblende. In both, the feldspars are highly corroded, and, in fact, everything points to a common origin, the difference in texture, in size, and nature of the ferro-magnesian silicate being clearly due to different conditions of cooling.
Appended are detailed descriptions of the rocks mentioned above, together with notes on a hornblende andesite which, overlies the slate spur from Moehau on the low saddle between Cabbage Bay and Waikawau.
Summit of Moehau.
Augite Andesite.—A compact, dark, greenish-grey, non-porphyritic rock. Specific gravity, 2.63. Base abundant and much decomposed. Microlites and laths of feldspar up to 0.05 mm. in length. Feldspars much kaolinized, showing polysynthetic twinning. From their extinction angle (about 30°) they must be grouped with labradorite. Phenocrysts much corroded. Pyroxenes monoclinic, faintly zoned, ranging tip to 0.8 mm. in length. Purplish and very slightly pleo-
[Footnote] *“Geology and Veins of the Hauraki Peninsula”: James Park, F.G.S.
[Footnote] † Mr. Park has since informed me that I am in error in supposing that his section as shown is across Te Moehau, it being in reality a section six miles to the north of the mountain. This at once explains the apparent discrepancy.
chroic. Chlorite and magnetite abundant. Chalcedony, showing a black spherulitic cross under crossed nicols, appears along planes of fracture.
Dyke Bock from Eastern Slope of Mountain.
Hornblende Porphyrite.—A coarse - grained porphyritic grey rock with large black crystals of hornblende. Under the microscope the base is seen to be completely holocrystal-line. It is abundant, and is formed of feldspar grains, laths, and plates up to 0.02 mm. in length. Feldspars are porphyritic, and range from 3 mm. long by 2 mm, broad to 7 mm. long. They show marked polysynthetic twinning both on the albite and pericline types, the latter crossing the former at right angles. Phenocrysts of feldspar idiomorphic and zoned. From their extinction angle of about 30° they must be placed in the labradorite group. Hornblende is highly porphyritic, as may be seen from hand specimens, ranging up to 12 mm. (½ in.) long and 4 mm. (⅙ in.) broad. They are strongly pleochroic, and show alteration to chlorite. Ophitic plates are not uncommon. Magnetite abundant. The amphiboles and feldspars, from their corroded outlines, evidently crystallized long before the base, or at great depths. Specific gravity, 2.71. Considering, then, the three main features of the rock—viz., its holocrystalline base, its basic plagioclase feldspars, and its porphyritic ferro-magnesian silicate—the rock must be classed as a “hornblende porphyrite.” I use the term “porphyrite” very reluctantly, as the use of this term is one of the moot points in the nomenclature of the igneous rocks. Continental petrologists include under the name “porphyrite” the “older” andesites, while some British authors apply the name to andesites altered by atmospheric action, and others again use its in the sense in which I have used it above.
Saddle between Waikawau and Cabbage Bay.
Hornblende Andesite.—A compact greenish-black rock showing no porphyritic minerals. Section: Ground-mass abundant, microlitic, chiefly feldspar plates ranging up to 0.2 mm. in length. Porphyritic minerals are hornblende and feldspar. The amphiboles reach 2 mm. in length, and show remarkable multiple twinning, a feature very rare indeed in hornblende. In one crystal alone there are as many as thirty lamellæ present. Phenocrysts slightly decomposed, showing resorption border and alteration to magnetite along cleavage-planes. Feldspars reach 5 mm. in length, showing polysynthetic twinning both on the albite and pericline types. Zonal bands strongly marked. Phenocrysts much corroded. One section of the rock contains an included fragment of greywacke. This Waikawau rock differs from that from the
summit of Moehau only in the ferro-magnesian silicate, the base, feldspars, &c, being precisely similar.
In conclusion, I may be permitted to state that the above does not by any means constitute an exhaustive paper on my subject. From its inaccessibility, and its densely wooded spurs and ravines, years must necessarily elapse before finality can be reached in that respect.