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Geological and other Reports

Igneous Rocks

Igneous Rocks.

True igneous rocks occupy but a small portion of the surface of the Province, but the more one investigates, the more one finds the eruptive rocks exposed in the great fractures, and sees in the jasperoid rocks the evidence of their action.*

The igneous rocks of the Province may be classed in two grand groups.

1st.The volcanic rocks of the central chain of Ruapehu and Tongariro.
2nd.The eruptive rocks of Rimutaka, Tararua, and of the East Coast.

Looking at the geological sections of the Province, these igneous rocks at once strike one as forming a grand series of ribs and buttresses, which lock up and support the sedimentary rocks and the North Island.

In the centre, the grand volcanic chain of Ruapehu and Tongariro rising from an elevated plateau to a great altitude, forms the great striking feature of the landscape.

Turning now to the eruptive rucks, I believe we must look to Tapuanuka, the inland Kaikoura, as their chief centre of action, whence slightly divergent ribs have been sent.

1st.Up the East Coast of this Province penetrating and tilting the sedimentary rocks in that direction.
2nd.To the Muka-muka rocks and thence to the N.N.E., changing the overlying sedimentary into jasperoid rocks.
3rd.Towards Sinclair Head and thence to N.N.E., with a somewhat similar result.

The intervention of the Strait does not militate against this view. The Strait would seem to be a zone of “fault” and submergence.

Those who consider that the deep sounding in Palliser Bay, where the depth changes suddenly from 74 fathoms to 142 fathoms (no bottom), shows the site of a submarine crater, may probably look upon that point as the focus of present action, but, I would suggest that a submarine crater ought to show shallow instead of deep soundings, that the soundings would also indicate the outward slope of a crater, which they do not do, and that there is really no evidence to show, that any earthquake phenomena originated from that place as a centre.

It is true that after the earthquake of 1855, dead fish and a bituminous substance, were washed up on the coast near Wellington, shewing that volcanic action in the Strait had been attended with the emission of gases, or lava, or noxious products of some kind, but the evidence that this action occurred, in particular, at the deep sounding in Palliser Bay, amounts to nothing.

To consider fully the igneous phenomena, one ought, however, to be acquainted with, and to include the whole of the igneous rocks of the North Island, As I have, however, no right to travel beyond my own “Province” in this enquiry, I will merely indicate that the principal seat of the volcanic rocks of New Zealand lies in the Province of Auckland, that Taranaki has its one volcanic cone, and that Hawkes Bay possesses no igneous rocks “in situ,” that I am aware of.

Although a thorough investigation of the volcanic rocks, with a view of escablishing at what geological period their action commenced, would occupy far more time than has been at my disposal, I will state, as a starting point for future enquiry, that the eruptions of the great central chain appear to me to have begun after the era of the “blue clay,” possibly intermediate between the older and newer sedimentary tertiuries.

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I have not found igneous boulders in the “blue clay,” while they are common in the overlying rocks In connection with this question the sedimentary tertiaries of the West Coast would require a very careful and detailed examinination, which would occupy a good deal of time. Slickensides occur frequently in our old rocks.

* A red rock of Rimutaka, which I consented to put down as a lava, is now decided by the Rev. W. B. Clarke, to be a jasperoid rock. The supposed granitic character of a rock of the Upper Hutt, must, I believe, also be abandoned.

Note.—It may appear that I am mixing up together old and new igneous action. I do not positively know when the first intrusion of eruptive rocks occurred; but, whenever that action may have commenced, it has probably continued until now.

It has plenty of deposited pumice, however, brought down from Tongario.