Report on Samples of Stone from the Tokatea Tunnel, was communicated by His Honour the Superintendent
1. The following Report on Samples of Stone from the Tokatea Tunnel, by Dr. Hector, was communicated by His Honour the Superintendent:—
“11th October, 1873.—On the 11th July, I learn from Mr. Aitken's letter and plan, that the length of the drive, which runs north-east and south-west, was on the west, or Coromandel side, 750 feet; and on the east, or Kennedy Bay side, 250 feet, making a total of 1,000 feet, or about half the total distance which has to be driven. The drive is divided into sections of 100 feet in
length, and from each, what I presume to be an average specimen of the rock passed through, has been sent, together with samples of the reefs cut.
“1st Section on West Side.—A dark, very compact base, containing tabular crystals of felspar, and grains of a red mineral not determined. The structure is only visible in the heart of the fragments, which are weathered on the outside, of a mottled green, and dirty from earth.
“2nd Section.—Soft rock of a light buff colour, mottled as if formed by the decomposition of a felspathic paste, containing an imperfectly crystallized mineral.
“3rd Section.—Fine-grained breccia containing rolled pebbles of the tufanite rock in the 4th and 5th sections, and iron pyrites in small quantity; evidently a rock that has been decomposed.
“4th Section.—Compact white paste, speckled with blue spots, that have no defined shape, and containing much pyrites.
“5th Section.—Sharp granular rock of light grey colour, containing large quantities of pyrites in brilliant crystals and grains. This is the characteristic tufanite of the Thames Gold Fields.
“In this rock the first quartz vein was met with, lying very flat, the dip being 22° to south-south-east. Its thickness on the plan is stated to be 1 foot 6 inches, and its yield 19 dwts. per ton.
“The sample sent of the quartz from this vein showed distinct traces of gold, but the quantity was too small to be determined.
“6th Section.—Dark coloured, compact, heavy rock, containing much carbonate of lime and pyrites.
“This section is cut through at 560 feet by a quartz vein 6 inches thick, running north and south, with an easterly dip of 25°. The quartz is stated to yield 6 ozs. 10 dwts., but the sample received only gave traces of gold. After a short interval there is a second vein, 1 foot thick, with nearly the same dip, resting on a dyke said to be diorite, but no sample seems to have been sent. The vein-stone is calcspar, with druses lined with crystals of arragonite, and only containing small threads of quartz.
“7th Section.—Light coloured, calcareous, and pyritous rock. This is cut by a quartz vein 6 feet thick, and nearly flat, or with a slight underlay of one in twenty to east. The quartz is crystalline, and the sample sent gave at the rate of 25 grains per ton of gold.
“In the 8th section, at 720 feet, a small vein of similar size, and underlying, was cut, the sample of which only gave traces of gold. It is noticed that on cutting this vein a heavy flow of water was met.
“The rock where the tunnel had reached to from the west side, in June, was compact, granular, and of a dark grey colour, charged with pyrites, and only feebly calcareous.
“On the Kennedy Bay side—1st Section is a coarse granular, or sub-crystalline rock, light in colour, and containing pyrites, but weathering freely to a dark brown. On the west side of this band of rock are two quartz veins, 5 and 3 inches respectively, trending north-east, and underlying to the west at 20°.
“The samples of these sent gave at the rate of 28 grains per ton of gold.
“2nd Section and 3rd Section.—Very compact felstone, or indurated claystone full of pyrites in large masses and crystals.
“A glance at the suite of specimens submitted shows that, while there is considerable variety in the rocks cut through in the tunnel, after passing the 3rd section of 300 feet from the west end they all belong to the same group. The presence of carbonate of lime in large quantities, both as a constituent of the hardest and densest parts of the rock, and as secondary deposits in veins, is very interesting, and shows that, even if these rocks had at first a volcanic origin, as has been supposed, they have since undergone much alteration. The marked change at 300 feet, and the presence in the rock at section 3 of rolled fragments of the more interior rock, is sufficient proof of the existence of two formations belonging to different periods.
“All the rocks will be exhaustively analyzed and microscopically examined, so that the fullest benefit to science may be obtained from this most interesting work.”
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An animated discussion ensued.
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Mr. T. B. Gillies was chosen to vote in the election of the Board of Governors for the ensuing year, in accordance with Clause 7 of the New Zealand Institute Act.