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The Pamphlet Collection of Sir Robert Stout: Volume 65

Eruptive Rocks

page 215

Eruptive Rocks.

The oldest of our eruptive rocks are found in the Manapoúri System at the West-Coast Sounds, in the form of dykes of white granite, minette, eurite, &c. They do not penetrate any higher, and no eruptive rocks have yet been noticed belonging to the Tákaka System.

Maitaí System.—The pink granite found at Preservation Inlet, as well as the granites along the ge-anticlinal axis through Westland and Nelson, have penetrated some of the rocks of the Maítai System, but are found as rolled fragments in the rocks of the Kaihíku Series at the base of the Hokanúi System *. Their eruption therefore must have taken place some time during the deposition of the Maítai System, and they are probably contemporaneous with the dykes of syenite, diorite, olivine rocks, and serpentines as well as the greenstone ashes found in various localities in the Maítai System.

Hoikanúi System.—There is evidence of eruptive rocks belonging to this date near the Hurinúi Plains, where the river Mandamus cuts through a volcanic region in which ash-beds and lava-streams are interbedded with slates containing remains of plants .

Waípara System.—In the South Island extensive eruptions of white or light-coloured quartz-rhyolites and dolerites, the latter now often altered into melaphyres, took place along the western margin of the Canterbury plains at the Malvern Hills, Alford Forest, Mt. Somers, and Gawler Downs, during the deposition of the older rocks belonging to this system . Quartz-rhyolites of the same character form the base of the western portion of Banks's Peninsula, but the rest of this volcanic system is of later date. On the west coast of the South Island basic volcanic rocks occur at Paringa and other places south of Bruce Bay, which may belong to the Waípara System or to the next §. In the North Island volcanic rocks, said to be of this age, occur on the east coast of Wellington, at Red Island, south of Cape Kidnappers , and perhaps near Castle Point .

Oamarú System.—In the South Island basaltic rocks are interbedded with sedimentary rocks of this system at Oamarú Capo **; at Culverden, and at Pahau †† on the north side of the Hurinúi Plains. In the Trelissick basin on the Waimakariri, beds of volcanic tuff overlie and underlie a limestone considered to be the equivalent of the Ototara stone ‡‡. At Limestone Bluff and at the Two Brothers, on the south branch of the River Ashburton, a "Palagonite tuff" §§

* Cox, Rep. Geol. Surv. 1877-78, p. 47.

Haast, Rep. Geol. Surv. 1870-71, p. 46, and Hutton, Rep. Geol. Surv. 1873-74, p. 34.

Haast, Rep. Geol. Surv. 1871-72, p. 12, and l. c. 1873-74, p. 7; Hutton, Rep. Geol. Surv. 1873-74, p. 40; Daintree, Trans. N. Z. Institute, vii. p 458.

§ Cox, Rep. Geol. Surv. 1874-76, p. 8; Haast, Geol. Canterbury, p. 302.

M°Kay, Rep. Geol. Surv. 1874-76, p. 45.

M°Kay, l. c. 1874-76, p. 59.

** Geology of Otago, p. 55.

†† Rep. Geol. Surv. 1873-74, p. 46.

‡‡ M°Kay, Rep. Geol. Surv. 1873-80, p. 60.

§§ Haast, Geology of Canterbury, p. 313.

page 216 occurs, which, while agreeing well in ultimate analysis with specimens obtained in other countries, does not contain any palagonite visible to the naked eye.

The andesites and trachytes forming the centre of the Dunedin volcanic system are interbedded in the peninsula with sedimentary rocks, probably of this age; but the later eruptions of basaltic rocks which surround the andesites may belong to the Pareóra System *.

According to Dr. von Haast, the results of whose extensive researches on the structure of Banks's Peninsula I can in great part confirm, there have been here, in addition to the quartz-rhyolites of the Waípara System already mentioned, three periods of activity. To the first of these belong the caldera of Lyttelton, Little River, and Akaróa, in which the lava-flows are chiefly augite-andesites and occasionally trachytes. The dykes are chiefly trachytes, but occasionally augite-andesites, and at least one is rhyolite §. To the second period belong Mt. Herbert and Mt. Sinclair, which are formed of andesites, but without any visible dykes. To the third period belongs only Quail Island in Lyttelton Harbour, also composed of andesites with dykes of trachyte (?). The first and second of these periods of eruption are quite evident, and both appear to have been entirely subaerial in character. There does not, however, seem to be any means of distinguishing the third from the second period, and it is comparatively insignificant. The quartz-rhyolites had suffered severely from denudation, and thick beds of sandstone had been formed by their disintegration, before the more basic eruptions took place; consequently we may consider these latter as younger than the Waípara System. On the other hand the whole have suffered too much from denudation to allow us to put any of them later than the Pareóra System; and as both periods of eruption were subaerial, we have the interval between the Waípara and Oamarú Systems or that between the Oamarú and Pareóra Systems to choose between. I think it probable that the calderas of Lyttelton and Akaróa belong to the Oamarú System, but Mt. Herbert may belong to the Pareóra System.

In the North Island the trachytes (?) of Hick's Bay, near the East Cape, are distinctly overlain by beds of the Táwhiti Series , and they may therefore belong to the Oamarú System.

Pareóra System.—In the South Island basalts and basaltic tuffs are interbedded with rocks of this system at Mt. Cookson, north of the Hurinúi plains. The basalts of Moëraki Peninsula are clearly seen to overlie a Pareóra clay (the Onekakára clay of Mantell ), and as they have undergone great denudation, we cannot put them into the Wanganúi System. The volcanic rocks of Timarú may also

* Geology of Otago, p. 55.

Geology of Canterbury and Westland, p. 324, and Trans. N. Z. Institute, xi. p. 495.

For the knowledge that these rocks are andesites I am indebted to Prof. G. H. F. Ulrich, who has examined them microscopically.

§ Ulrich.

Cox, Rep. Geol. Surv. 1876-77, p. 112.

Geology of Otago, p. 61.

page 217 perhaps be placed here. No trace of a scoria-cone nor of a tuff-crater exists anywhere in the South Island; all the volcanic rocks, even Banks's Peninsula, which is 3000 feet high, appear to have suffered from marine denudation.

In the North Island volcanic ash beds and andesitic breccias are found associated with the Waitemáta Series near Auckland; and the trachytes (?) of Whangarei, the Great Barrier Island, and Coromandel are no doubt of the same age. Fossil wood of Podocarpium dacrydioides, Ung., was obtained by Dr. Hochstetter from the trachyte tuffs of both the Great Barrier and Coromandel; and from the much decomposed basaltic rocks behind Drury he obtained wood of Nicolia zelandica, Ung., which was also found in the Pareóra gravels of Moutere Hills near Nelson. On the Great Barrier Island the trachytic cone of Ahumáta, 1500 feet high, still retains a well-marked tuff crater *, as also does Arid Island .

The andesites , and gold-bearing propylites (?) of the Thames may he of the same age, or they may possibly date back to the Oamarú System; but we have no evidence that any andesites in New Zealand are older than the Oamarú System §. A piece of carbonized wood impregnated with iron pyrites, but showing plainly annual rings of growth, was obtained from the gold-bearing propylites in the "Maid of England" claim .

Wanganúi System.—There is no trace of volcanic action having taken place in the South Island during this period or later; but in the North Island, on the western side of the main range, volcanic eruptions on a large scale occurred from the commencement of the Pareóra and are even now not quite over. At Mt. Egmont in Taranaki the first eruptions, perhaps of Pareóra date, were trachytes containing (according to Zirkel) both sanidine and oligoclase, and are much like some of the rocks of Banks's Peninsula. These were succeeded by dolerites and basalts. The bases of Ruapêhu and of Tongaríro also appear to consist of trachytes, but here the later eruptions have been dark-coloured rhyolites and pumice. These siliceous eruptions appear to have commenced during the formation of the upper part of the Wanganúi System (Keréru Series), for no pumice occurs in the lower beds, Rhyolites are extensively developed round Lake Taupo, and in the Hot-spring district. In the valley of the Thames it appears that they are of later date than the dolerites (? augite-andesitcs) of Cape Colville peninsula Around Auckland and at the Bay of Islands basalts were the only lavas erupted.

* Hep. Geol. Surv. 1868-69, p. 6.

Trans. N. Z. Inst. i. p. 164, new ed. p. 108.

I am indebted to Prof. Ulrich for the information that, the rocks called by me dolerites. in Rep. Geol. Surv. 1808-69, p. 20, are typical augite-andesites According to Dr. Hector they contain olivine (Geol. Rep. 1868-69, p. 42, no. ix.); I saw none myself.

§ See "On the Geological Structure of the Thames Goldfields," in Trans. N Z Inst. vi. p. 272; and Cox, Rep. Geol. Surv. 1882, p 4.

Rep. Geol. Surv. 1868-69, p. 44.

Cox, Rep. Geol. Surv. 1883, p. 20.

page 218

In the neighbourhood of Auckland many scoria-cones with well- preserved craters occur at very slight elevations above the sea. These are younger than the clays belonging to the Wanganúi System round Mánukau Harbour, and have nover undergone marine denudation; but some of the tuff-craters, such as Lake Pupuki on the north shore, are of submarine origin.

Obsidian occurs in many places and is particularly plentiful on Mayor Island in the Bay of Plenty. Perlite is found in the Hot-springs district; and a finely laminated rhyolite, called lithoidite by Richthofen, is found at Totara on the east shore of Lake Taupo *. Tachylyte occurs but rarely. We have no glassy basaltic lava-streams, as in the Sandwich Islands and Friendly Islands. Leucite has been found in a basalt of uncertain age near Castle Point on the cast coast of Wellington . Anorthite occurs in the volcanic rocks of Campbell Island , but the age of these also is unknown. The only active volcano in New Zealand is Tongaríro, and its eruptions are feeble. White Island, in the Bay of Plenty, appears to be in the solfatara stage. It has never been known in eruption, and there is no appearance of recent lava-streams §.

Two interesting sections of volcanoes occur in the sea-cliffs of the North Island. I described the one on the west coast, between Port Waíkato and Raglan, some years ago . The other, which is situated at the west head of Támaki River, near Auckland, I have the late Mr. Heaphy's authority for saying is the same as the one figured by him , and copied into the works of Scrope and Judd**. This is the only crater near Auckland that is cut completely through by a sea cliff; and I quite agree with Dr. Hochstetter that it is a tuff-crater only, without any lava-stream ††.

* Hochstetter, 'Reise der Novara.' Geol. i. p. 113.

Colonial-Museum Laboratory Reports, x. p. 48.

Filhol, Comptes Rendus, Fe'b. 1882.

§ Edwin, Trans. N. Z. Inst. i. p. 57 (new ed. p. 463); Hector, Trans. N. Z. Inst. iii. p. 278.

Quart Journ. Geol. Soc. xxv. p. 13.

Quart. Journ. Geol. Soc. xvi. p. 242.

** Volcanoes, p. 165, f. 66.

†† 'Reise der Novara,'Geology, i. p. 176, no. 28, and Map.