Note on the Geology of the Outlying Islands of New Zealand

Transactions and Proceedings of the Royal Society of New Zealand, Volume 28, 1895, Page 736

Paper.—” Note on the Geology of the Outlying Islands of New Zealand,” by Sir James Hector. A large collection of specimens and photographs, with, maps, &c., were exhibited to illustrate the paper. Abstract. The lecturer gave the results of observations made during a visit to the islands, as a guest of His Excellency Lord Glasgow, from the 29th January to the 25th February, 1895. The distance steamed in the “Hinemoa” was 2,600 miles, and, with the exception of the visit to the Macquaries and the landing at the Bounty Islands, the programme was fairly well accomplished, notwithstanding that unusually boisterous weather was encountered for the season of the year. The lecturer's researches on the botanical features, bird life, and general topography of the islands haying been already fully described,* Chapman, F., “Transactions of the New Zealand Institute,” vol. xxiii., p. 491; and Kirk, T., F.L.S., in the Report of the Australasian Association for the Advancement of Science, vol. iii., p. 213. only those parts relating to the geology of the islands need be recorded. 1. The Snares.—These islands form two groups about five miles apart. They, on all sides, present precipitous cliffs to the sea, rising from 75 to 90 fathoms depth of water, without shingle or sandy beaches, there being only a few indentations of the coast-line where landing from a boat is possible. Only at one of these, the Boat Harbour, was the nature of the rock-formation actually observed. It is a very singular form of red granite (?), the only rock in New Zealand to which it bears resemblance being the red granite at the entrance to Chalky Inlet; but it differs in being less compactly crystalline, the quartz being granular, and the mica, though in large crystals, being in small proportions. The rock-masses decompose freely, having large naked domes on the surface, between which are profound hollows filled up to the level with peaty matter and bird-guano to a great depth, as proved by the excavations made for a lighthouse-site some years ago. The cliffs rise 300ft. to 500ft. vertical from the sea, and are especially bold on the west coast, where they display a stratified and columnar structure, which suggests a doubt as to the granite nature of the island as a whole. 2. Auckland Island is clearly connected with the Snares by a ridge or plateau, the soundings ranging from 86 to 196 fathoms only. So

far as seen, Auckland Island is altogether volcanic, and closely resembles the rock-structure of Banks Peninsula. The northern part recalls the aspect of the country north of Waikouaiti, with conical hills formed by denuded basaltic sheets; but further south along the east coast there is a succession of harbours or inlets like those in Banks Peninsula, the steep shores of which are formed of successive sheets of basaltic lava, often columnar, and varying from 10ft. to 80ft. in thickness. These are separated by brightly-coloured layers of volcanic tufa or ashes containing interspersed blocks of all sizes. The lava-flows can be traced by the eye to the highest peaks overlooking the west coast of the island, all dipping eastward at from 5° to 10°, but at the north and south ends of the island the slope is towards these directions. Along the west coast there are no indentations, only precipitous cliffs, and one off-lying rock, “Disappointment Island,” which is probably a “neck” or “dyke” through which some of the igneous rocks have been extruded. Carnley Harbour divides Adam's Island from the mainland, and appears to have been formed by a dislocating east-and-west fault. The south side of Adam's Island is the most exposed part of the Aucklands, and presents cliffs composed, as exposed in section, of horizontal layers of basalt and tufa to a height of 1,900ft., some of the cliffs being 1,400ft. sheer down. The lava-sheets vary in thickness from 10ft. to 80ft., and there is evidence of not less than seventy disinct outpourings still preserved above the sea-level. At sounding three miles off shore from the South Cape gave 95 fathoms (575ft.). The average dip of the lava-floes to the eastward is 7°; the width of the island in this section is ten miles, and the height of the western cliffs 1,000ft.; so that, by adopting the usually-accepted curve for volcanic deposits, we have the following result: Auckland Island is the remnant of a great volcanic cone that was 12,000ft. in height and fifty miles in diameter in early Tertiary times, the chief centre having been about eight miles west of Disappointment Island. Four-fifths of the original mass has been removed by the denuding force of the westerly waves. 3. Campbell Island is a volcanic mass, but has the peculiar feature of having slightly above the sea-level the original rock-formation on which it is founded. On the north and south it presents fantastic peaks and precipices, carved out of rocks of the same character as those which form Otago Peninsula. Towards the southern end it is traversed by Perseverance Harbour east and west, which is by a moderately high saddle connected with West Harbour. On the north and south of this rift are frowning cliffs and peaks of basaltic lava-sheets; but at the sea-level there is an exposure, both on the east and west coasts of the island, of the Upper Cretaceous rocks, with chalk flints and fossil wood, such as is found in New Zealand in the Upper Amuri series. These have been brought to the surface-level together with the volcanic outburst, as they are injected and interstratified with dykes and tufa-rocks of the same kind. The wonderful mystery of the occurrence of fossilised dicotyledonous wood in the far south latitude of Campbell Island is therefore thrown entirely out of the distribution of plant-life in Tertiary times, and must be referred to the Cretaceous epoch. At an altitude of 800ft. clear evidence of the former existence of a true corrie and moraine of the first order was obtained, but it is purely local, and is the only evidence of former glacier action observed. 4. Antipodes Island is the result of four distinct centres pouring out scoria and basaltic lavas with enormous deposits of volcanic breccia, which proves the great local violence of the eruptions. The erosion of the coast-line has been very slight considering the friable nature of the rock, so that the eruptions must have occurred in a very late geological time, coincident probably with those of the Auckland Peninsula. 5. Chatham Islands have a very complex structure, the northern

part being composed of the oldest metamorphic rocks that occur in New Zealand, and the southern part and Pitt's Island of Lower Tertiary and Cretaceous rocks. The soundings between the Chathams and Banks Peninsula did not exceed 300 fathoms. The results of the voyage indicate so far that New Zealand is the remnant of a mountain-chain that formed the crest of a great continental area that stretched far to the south and east, and which is now submerged to a depth not much more than 2,000ft. The President felt sure that all were delighted with Sir James Hector's most interesting lecture. It was rather late to take the discussion on it, and it was proposed and carried that the discussion be postponed to that evening fortnight.