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Volume 19, 1886
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Volcanic Rocks.

I noticed four principal centres of eruption, but no doubt there are others.

1. Oamaru Volcano.—In passing along the shore from the breakwater at Oamaru towards Cape Wanbrow, we first find rocks dipping 25° N. The upper beds (Pl. XXVI., Section I., a), under the Flagstaff, are basaltic agglomerate and ash, the former with bands and pieces of fine-grained limestone. It is this limestone that in 1874 I mistook for included fragments of Ototara stone, altered into a kind of lithographic limestone. By Mr. McKay they are shown as regular beds, interstratified with the agglomerate. A careful inspection, however, has convinced me that they are all veins running between blocks of lava in the agglomerate. They are segregation veins, formed from the calcareous cement in the agglomerate and ash beds, and are of later age than the main body of the rock. The volcanic rocks in contact with these veins are not in the least altered, and the veins are usually compact and solid throughout, often with a banded structure parallel to the margin. In one instance I noticed that there was a compact layer on each side, while the central portion, varying from 6 to 12 inches in thickness, was filled in with broken shells and corals; the two inner surfaces of the limestone were quite smooth, and the organic fragments appear to have been washed in from above. Associated with these beds are tachylyte breccias, consisting of angular fragments of glossy tachylyte, rarely exceeding an inch in thickness, cemented together by crystalline calcite. Round their margins the fragments are often altered into a rich yellow-brown palagonite. The basalt of the agglomerates is compact, bluish-black in colour, finely crystalline, and with olivine more or less abundant; S.G. = 2.80. Under the microscope it is seen to consist of a microcrystalline ground-mass of felspar laths, magnetite, and pinkish brown augite grains, containing here and there crystals of slightly dichroic olivine, much decomposed round the margins

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into a dark-brown mineral. The tachylyte is formed of a pale smoky-brown glass, in which are numerous felspar laths. Here also the olivine has undergone much decomposition, a description of which I reserve for another occasion. Its specific gravity is 2.72.

Below the agglomerates comes a series of thin-bedded greenish-brown sandstones (b) containing fossils, and inter-stratified with ash beds. Below these is a coarse scoriaceous sandstone, which is underlain by grey current-bedded sandstones. Then, at the next point, comes a remarkable agglomerate (a1) formed of large basalt bombs, the interstices between which are filled up with compact fossiliferous limestone. These bombs vary from one to six, or more, feet in diameter, and some of them on the lower surface curve round those below, showing that they were soft when they fell into their places. Each bomb is encased by a coating of tachylyte about 1 inch thick, which is decomposed in places into reddish-yellow palagonite. The basalt of these bombs is rather coarser in texture than that of the agglomerate first mentioned, and I could detect no olivine with the naked eye; but under the microscope both the basalts and the tachylytes are much alike. Beyond the agglomerate, in descending order, comes (3) a series of thin-bedded sandstones and clays, dipping 20° N. Next below are (2) coarser scoriaceous sandstones, dipping 30° N., and then (1) agglomerate, (a2 in section), which gradually changes round to an easterly dip, so as to look nearly horizontal in the cliff. Then comes a fault with a hade to the north. On the south side of this fault the beds dip 25° S.E. At the top of the cliff are the thin-bedded sandstones and clays (3), underlain by the coarser scoriaceous sandstones (2), so that the downthrow of the fault is to the south, or, in other words, it is a reversed fault; the throw, however, is small. The sandstones and clays (3) extend to the next point, which is quite low; and in the following bay all the rocks are obscured by the silt deposit, which here comes down to the sea. The next point is Cape Wanbrow, formed of grey scoriaceous sandstones, dipping 10° S.E. I have given a somewhat detailed account of this section, for I am under the impression that it is at the fault that Mr. McKay supposes an unconformity to exist between his upper eocene and cretaceo-tertiary formations; but, if so, he is undoubtedly wrong. At the same time, I saw no other place where any break occurred at all.

Past Cape Wanbrow the dip changes gradually to 15° S.S.E.; then to S., then to 15° S.S.W.; and ultimately to 35° S.W. The grey scoriaceous sandstones of Cape Wanbrow occupy most of this section (Pl. XXVI., Section II.), but are overlain by a bed of pale grey tuffaceous limestone, 6 or 7 feet in thickness, containing minute fragments of coral. This is followed by

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sandstones with fossils, and then the Otatara limestone, 45 feet thick. The details of this part of the section I will defer until treating of the Hutchinson's Quarry beds.

It will be seen that the beds all round the east side of the Oamaru Peninsula form a single periclinal curve, as shown by me in 1875.* Mr. McKay's section is very different, and I am at a loss to account for it, as he gives no details.

2. Deborah Volcano.—Between the Deborah railway-station and Totara are the relics of another volcano, which has been almost entirely destroyed by denudation. The rocks are basic, but I neglected to collect specimens. So far as I could see, they always underlie the Ototara limestone, which surrounds the volcano on all sides but the south-east. Mr. McKay, however, mentions a lava flow overlying the limestone somewhere in the neighbourhood. He gives no precise locality, and I failed to find it; but as I arrived late in the day I could not make a sufficiently careful examination. In the Waireka Valley, opposite Deborah, a tachylyte tuff, probably erupted from this volcano, underlies the Ototara limestone, but I will give its position when describing the sedimentary rocks of the Ototara series. This tuff is compact, grey in colour, and with a lens shows minute black shining spots, and occasionally small pieces of vesicular tachylyte. It effervesces freely with acid, S.G.=2.47. Under the microscope it is seen to be made up of minute angular fragments of vesicular tachylyte in a calcareous cement. The tachylyte is of a pale yellow-brown colour, without any felspars, but contains a few scattered microliths. The vesicles are ovoid, not much elongated. It is much like a tachylyte tuff, presently to be described, from Lookout Bluff.

3. Enfield Volcano.—The railway at Enfield runs through an old volcano which extends as far as Elderslie (Section III.). It is formed principally by lava flows, which are compact and finely crystalline. Some are dark grey in colour, with small white pearly flecks, and cavities filled with limonite; these rocks weather reddish-grey. Others are darker, and without white flecks. S.G.= 2.64. I could see no olivine in any of them. Under the microscope these rocks are seen to have a microcrystalline ground-mass of felspar laths, brownish augite grains and ilmenite, more or less decomposed into leuxocene. There are no porphyritic crystals. In the absence of chemical analysis, I feel inclined to call these rocks augite andesites. At the road cutting close to the Waireka Presbyterian Church, there is a palagonite tuff composed of fragments of tachylyte and fragments of black magma-basalt with olivine. S.G.=2.35. The tachylyte is altered in places into a yellow-brown or brownish-green

[Footnote] * “Geology of Otago,” p. 55, fig. 7.

[Footnote] † “Rep. Geol. Expl.,” 1876–77, p. 50, section No. 3.

[Footnote] ‡ “Rep. Geol. Expl.,” 1876–77, p. 58.

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palagonite. I could obtain no direct evidence of the age of these rocks, and it is quite possible that the andesitic lava flows may belong to a later period than the palagonite tuffs. This is a point that requires more investigation than the time at my disposal would allow.

4. Kakanui Volcano.—The Kakanui River runs into the sea between two low hills formed of scoriaceous sandstone overlain by the Ototara limestone, here generally more compact than usual. (Section IV.) The sandstones of the northern hill form a periclinal curve, which extends across the river so as to include the rocks seen in the river-bed between the bridge and the sea. The south head is a separate and smaller periclinal curve, showing two foci of eruption; but I did not ascertain which of the two is the younger. I saw no lava streams.