Geology^*

The geological reports, maps, and sections which are issued by the Geological Department of New Zealand indicate our present knowledge of the structure of the Islands and the distribution of the chief groups of rock formations; and the following classification has been adopted in the construction of the geological maps; but, notwithstanding, the large amount of data that has been collected, the extent and rugged nature of the country and the very limited staff have precluded minute surveys being effected, so that any attempt made to express the results obtained in a systematic form must be considered as merely provisional.

I. Posttertiary (Recent).—The deposits belonging to this period have accumulated with great rapidity in New Zealand, owing to the mountainous character of the country giving to the rivers, even when of large size, the character of torrents, which are liable to occasional floods of extreme violence. To some extent, also, the remarkable indications of change which are everywhere manifest must be attributed to alterations of relative level which have affected the surface, some of which have occurred during the present century. Such changes are more easily detected on the sea-coast, where they effect sudden alterations of the shore-line, but there is no doubt that they have been equally potent in inland districts, and have caused, for instance, marked alterations in the courses of some of the rivers.

The Maori race is considered, from the evidence afforded by their traditions, to have been established in New Zealand for little more than five hundred years before the first arrival of Europeans; but during that period, while the Islands were being explored in all parts by this intelligent and adventurous native race, the spread of fires,

^* These numbers refer to the colours on the geological maps.

page 30 causing the destruction of the primæval forests and rank vegetation, was the means of setting free vast accumulations of loose soil and disintegrated rock that were formerly retained on the mountain-slopes. The material thus displaced has accumulated in the river-courses, causing them to raise their beds above the adjacent lands, so that they have broken away from their channels in many places.

The race of gigantic Moa birds (Dinornis) had its maximum development in the New Zealand area, and only became extinct during the recent period, but their extermination must have commenced at an earlier date than the first human occupation, as their bones are found deeply embedded in the gravels and swamps, while the evidences of human occupation are confined to the surface-soil, shelter-caves, and sand-dunes.

II. Pliocene.—This formation belongs to a period when New Zealand was the mountain-range of a greatly-extended land-area, and when, in the North Island, the volcanic forces had their greatest activity, attended with the rapid elevation of local areas of fossiliferous deposits that were at this period forming in adjacent seas. In the South Island no marine deposits of importance belonging to this period are present, but the great area of land above the shore-line intensified the erosive action of the glaciers radiating from the mountain-centres, and gave rise to enormous deposits of gravel, such, for instance, as compose the greater part of the Canterbury Plains, and the Moutere Hills in Nelson.

The economic importance of this formation is very considerable, from its containing the richest deposits of alluvial gold that form the support of the mining population. The beds cover a considerable surface-area, both in the North and also in the South Island.

III. Upper Miocene.—The marine beds of this age consist of a series of sandy, calcareous, and argillaceous strata, the distribution of which, and as a rule also the mineral character, indicate that they were related to a closely adjacent shore-line, as they often pass, almost suddenly, from coarse conglomerates into narrow strips of fine mud and clay, such as are deposited in the centres of deep channels and inlets.

IV. Lower Miocene.—This formation, which is distinguished from the foregoing chiefly by its fossils, is a calcareous and argillaceous formation, widely spread over the east and central part of the North Island and both sides of the South Island, and, when not removed by denudation, can be traced to an altitude of 2,500 feet above the sea. It represents a period of great depression, and the deposits are remarkable for the absence of evidence of volcanic activity in any part of the region, and for the abundance of marine life.

V. Upper Eocene.—This is a very marked formation of calcareous sandstone, composed of shell fragments, with corals and Bryozoa, and is a shallow-water and littoral deposit.

Intense volcanic activity prevailed during this period in both Islands, and the calcareous strata are frequently interbedded with contemporaneous igneous rocks and tufas, and in the North Island are often replaced by wide-spread trachyte floes and volcanic breccias.

The lower part of this formation passes at places into an imperfect nummulitic limestone, or a friable calcareous sandstone, evidently deposited in shallow seas, and forming the lowest member of the proper marine Tertiary series.

VI. Cretaceo-Tertiary.—This constitutes the Cretaceo-tertiary group, being stratigraphically associated and containing many fossils in common throughout, while at the same time, though none are existing species, many from even the lowest beds present a strong Tertiary facies, and in the upper part only a few are decidedly Secondary forms.

The distribution of this formation shows that it was not like the foregoing formations of later date, deposited in relation to a form of the land like that at present obtaining in the New Zealand area, except in the vicinity of some of the oldest and most lofty land-masses in the south, which appeared to have remained above the water-line since the Lower Cretaceous period.

The upper part of this formation is a deep-sea deposit, but the lower subdivisions indicate the close vicinity of land, and are replaced in some areas by true estuarine and fluviatile beds containing coal.

The most valuable coal deposits of New Zealand occur in the Cretaceo-tertiary formation, but always at the base of the marine beds of the formation, in every locality where they occur. The coal-bearing beds always rest upon the basement rock of the district, marking a great unconformity and the closing of a long-persistent land-area at the period to which they belong.

VII. Lower Greensand.—This formation consists of green and grey incoherent sandstones, with hard concretions, and large masses of silicified wood.

It is confined to a few localities of limited extent, is very rich in fossils of the genera Belemnites and Trigonia, with a few Saurian bones and large Chimæroid fishes.

VIII. Jurassic.—These beds, which are the youngest of the Lower Secondary formation in New Zealand, consist in the upper part of estuarine beds, marine fossils being absent or rare.

Following these are marlstones, represented in southern districts by coarse-grained sandstones, which pass near the base of the forma- page 32 tion into conglomerates with bands of indurated shale, enclosing plant-remains and irregular coal-seams, which have been included in the next group as its upper member.

They are all of marine origin, and contain Middle and Lower Oolite fossils.

IX. Lias.—This formation consists in its upper part of conglomerates and sandy grits, with plant-remains too indistinct for identification; and in the lower of marly sandstones in banded layers of different colours, at the base having a concretionary structure, which has led to their being termed "the cannon-ball sandstone;" similar sandstones also occur in the Otapiri formation.

X. Trias.—It has been found necessary to include in this formation a thickness of strata which is quite unusual in other parts of the world; but the close connection which exists throughout, founded on both palæontological and stratigraphical grounds, and the clearly-defined Permian character of the next underlying formation, renders this classification absolutely necessary.

XI. Permian.—The mineral character of this formation is grey and green sandstone with breccia and heavy conglomerate beds. Marine fossils have only been found at 1,000ft. below the great conglomerate that divides its two sections.

XII. Lower Carboniferous and Upper Devonian.—This formation is of considerable importance from the large share it takes in the structure of the great mountain-ranges, and from the occasionally great development in it of contemporaneous igneous rocks, with which are associated metalliferous deposits. In the upper part this formation consists of fine-grained argillaceous slates (Maitai slates of Hochstetter), becoming calcareous and passing into true limestones at their base. These limestones, which close the Maitai series, contain Lower Carboniferous fossils.

Succeeding these is an enormous thickness of greenstone breccias, aphanite slates, and diorite sandstones, with great contemporaneous floes and dykes of diorite, serpentine, syenite, and felsite belonging to the Upper Devonian period.

XIII. Lower Devonian.—These, as determined by their fossil contents, have only been distinguished in one locality, viz., Reefton, although from their mineral character they are evidently present in many other parts of the South Island.

XIV. Upper Silurian.—Many areas of metamorphic schists should probably be included in this formation, but it has only been distinguished by its fossil contents in the north-west district of Nelson, where both Upper and Lower Silurian rocks are present, page 33 The Upper Silurian rocks consist of grey cherts, sandstones, and calcareous slates, with occasional beds of blue limestone.

In the Baton River they contain a great variety of fossils in the calcareous strata, and not infrequently in the sandstones and cherts, of which thirteen species have been determined, besides which a great variety of corals and corallines occur; crinoids also are very abundant.

XV. Lower Silurian.—These rocks form the mass of Mount Arthur and the range to the north-east as far as Separation Point, and they consist chiefly of a dark bituminous slate, associated with a blue or grey submetamorphic limestone, which is in places developed to a very large extent. White crystalline limestones are also associated with these beds throughout the whole length of the district from Mount Owen to Motueka.

The whole series is disturbed by eruptive hornblendic and syenitic rocks, which are probably of Devonian age.

Fossils have been found in two localities only, and these consist entirely of encrinite remains, one species of coral not yet determined, and Graptolites which occur in the slates.

The central axis of these beds consists of true micaschists, to the east and west of which the limestone and bituminous slates overlie.

XVI. Foliated Schists.—The metamorphic rocks under this division have as yet been only subdivided according to their mineral character; but they probably consist chiefly of altered Silurian rocks, and even those of formations as young as the Maitai or Lower Carboniferous beds. The less metamorphosed areas of Lower Palæozoic rocks in the South of New Zealand have yielded no fossils. They were formerly classed as the Kaihiku series, but this name has latterly been transferred to the Permian formation of which the Kaihiku Range is more largely composed.

XVII. Crystalline Schists and Granite.—The south-western portion of the District of Otago is composed of crystalline rocks, forming lofty and rugged mountains, of which the chief characteristic is their cubical form, due to their being intersected in all directions by profound but narrow valleys, with abrupt precipitous sides to three-fourths of the extreme height of the adjacent mountains. The valleys are occupied on the west by arms of the sea, and on the east by those of inland lakes that resemble the Norwegian fiords, and present most wonderful mountain scenery.

The base rock of this formation is foliated and contorted gneiss corresponding to Humboldt's gneiss-granite of South America, and associated with it are granite, syenite, and diorite, which belong to the next group.

Wrapping round these crystalline strata, and sometimes rising to an altitude of 5,000ft. on its surface, is a series of hornblende schists, soft micaceous and amphibolic gneiss, clay-slate, and quartzites, associated with felstone dykes, serpentine, and granular limestone. I believe these latter to be metamorphic rocks of not very ancient date, probably of Devonian age.

Areas within the crystalline schists where true granite occurs, either metamorphosed or in the form of perfect dykes, have been distinguished under this group.

Granites of a light-grey colour and very fine grain are found in the Nelson and Westland Districts, forming isolated hills along the boundary of the Foliated Schists on the east and Lower Devonian beds on the west. In the south-western extremity of New Zealand, at Preservation Inlet, coarsely crystalline granites, of white and flesh-colour, appear to break through and overlie the younger members of the crystalline schists.