The Pamphlet Collection of Sir Robert Stout: Volume 66
Igneous Rocks
Igneous Rocks.
XVIII. Basic Volcanic, Plutonic, and Dyke Rocks.
XIX. Acidic Volcanic Rocks.
A. | Volcanic group. Recent and Post-tertiary.
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B. | Trachytic group. Eocene.
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C. | Dolerite group. Upper Cretaceous.
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D. | Propylite group. Lower Cretaceous. | ||||
E. | Diabase group. Triassic. | ||||
F. | Diorite group. Lower Carboniferous. |
The igneous rocks have played an important part in almost every formation in New Zealand, marking great movements of the earth's crust at the different geological periods, while the superficial and later-formed volcanic rocks occupy nearly one-third of the area of the North Island.
They are divided on the map into the above groups, of which the plutonic and dyke rocks include syenite and diorite, with associated breccias, serpentine, and olivine rocks (dunite), the eruption of which took place in the Upper Devonian period.
These rocks are found on a line which extends almost continuously through the South Island; hut diorite rocks reappear in the extreme north of Auckland, and on the Cape Colville Peninsula and Great. Barrier Island. They are generally more or less metalliferous, chrome and copper being the ores of most frequent occurrence.
Basic Volcanic Rocks.—These belong to three different periods, when there were active eruptions, attended by the formation of floes of both compact igneous rocks and tufaceous breccias.
The earliest of these occurred during the Triassic period, and consists chiefly of diabase and serpentinous breccias. The next eruptions took place about the close of the Jurassic period, along the eastern base of the Canterbury Alps, where the rocks occur in dome-shaped mountains as melaphyres associated with felsite (quartz) porphyries which belong to the next group.
In the Cretaceo-tertiary period are massive trappean eruptions of trachy-dolerite and dolerite, while in the same period must be placed the propylite group, consisting of greenstone-trachytes, and fine-and coarse-grained breccia rocks, which form the matrix of the auriferous reefs of the Thames goldfields.
In Eocene times dolerite floes were contemporaneous with the limestones of the period of the Hutchinson's Quarry beds, while lastly in this group have been placed the basaltic lavas of Pliocene age in the northern parts of the colony, and also certain dykes of vesicular lava that cut through and alter the Upper Pliocene gold-drifts in the Maniototo Plain, in the interior of Otago.
Acidic Volcanic Rocks.—The rocks belonging to this group have a similar distribution in time to the foregoing, the earliest being the felsite (quartz) porphyries, while trachyte porphyries and breccias played an important part during Cretaceo-tertiary and older Tertiary periods, scoriaceous lavas and rhyolites being the characteristics of the later outbursts, which have continued down almost to the present time.
The geysers and boiling springs in the North Island give rise to the formation of siliceous sinter, which must be included as the most purely acidic products of volcanic action, and as due to the decomposition of the older rocks by the action upon them of fresh water; but in the case of White Island, and other localities where the decomposition is brought about by the agency of sea-water, the sinter deposits are formed chiefly of sulphate of lime, and not silica.