Ast. XLIV.—On a Carbonaceous Mineral from Whangarei Harbour.
[Read before the Auckland Institute, September 12, 1870.]
At one-tree Point, on the western side of Whangarei Harbour, a carbonaceous mineral, resembling coal in appearance, occurs under such curious circumstances that I think a short notice of it will prove interesting.
One-tree Point consists of low cliffs, some twenty feet high, of soft horizontal quaternary sandstones, with casts of marine shells (Pecten, etc.) The upper stratum is yellowish sand, the middle one more ferruginous, while the lower one is blackish or brown, from the quantity of carbonaceous matter scattered through it. On the face of the cliffs, several irregular oblique fissures, from one to six inches in breadth, are seen traversing all the beds. These fissures are generally filled up with sand, but some of them, when they enter the dark coloured lower stratum, are filled with carbonaceous matter, which is much mixed with impurities near the sides of the vein, but in the centre is nearly pure. Sometimes the centre of the vein is empty, with the pure carbonaceous mineral lining both sides. These fissures are similar to ordinary veins, and do not bear any resemblance to holes made by roots of trees, etc.
The mineral from the centre of the vein has very much the appearance of cannel coal. Its colour is black, with a shining resinous lustre. Streak and powder black. Very brittle, but does not dirty the fingers. Hardness about 2. In the flame of a spirit lamp it burns to a white ash without altering its shape, and without giving off any odour or smoke, but it will not burn if taken out of the flame. It appears to be nearly pure carbon, without any admixture of bitumen.
These phenomena appear to me to be inexplicable, except by supposing that the carbon has passed in a state of solution from the sandstone into the fissures, and that it has then been deposited on the sides of the veins. If the process had been one of sublimation, the mineral would also be found at a higher level than the upper surface of the dark coloured sandstone, which is not the case. And if both it and the carbonaceous matter that colours the sandstone had been sublimed from below, and impregnated both the vein and the rock, it is evident that the vapour would have ascended further up the fissure than through the rock. What, however, the solvent could have been I cannot even conjecture. Specimens of this mineral were forwarded to the Colonial Museum by me, in 1866. See Colonial Museum and Laboratory Reports, 1866–67, page 17.
[The following is the composition of the carbonaceous mineral, which appears to be a non-caking lignite approaching jet, burning with difficulty, giving but little flame and a white ash:—
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