Art. XVI.—On the Geology of the Island of Pakihi.
[Read before the Auckland Institute, October 5, 1868.]
The Island of Pakihi lies between the Island of Punui and the main land, at the north-west end of the Firth of the Thames. It is rather more than a mile in length, and a third of a mile broad; the longer axis lying about north-east and south west. Both extremities of the island are high, the north-east end forming a conical shaped hill 433 feet high, known as “Pakihi Hill,” while the centre is a low fertile valley.
The whole of the rocks found on the island belong to the Upper Palœozoic period.*
[Footnote] * Perhaps also partly Triassic.—Ed.
The north-eastern extremity is composed of hard, blue and green sandstones, brecciated in places, and much jointed, the fissures of the joints being filled with peroxide of iron. These sandstones dip about 40° W., and are overlaid by a series of soft blue sandy slates, 1000 feet thick, containing blocks of the harder sandstones which lie under them; they decompose very easily to a soft reddish-yellow clay, in which state they are almost always found at the surface. Near the upper part, a band, about six feet thick, occurs, of a friable green schistose rock, containing small rounded pebbles. (See Section.)
This series is covered by a mass of red, or brown, jasperoid slates, containing large quantities of manganese ore. These jasperoid slates are more than 1300 feet thick. The manganese occurs in numerous bands, about an inch thick, generally running parallel to the cleavage of the slates, but occasionally at right angles with it. The ore appears to be almost entirely Psilomelane, little, if any, Pyrolusite, being mixed with it. On the west coast large quantities could be obtained at a very small expense.
Across the centre of the island runs a fault, which throws up the south-western portion about 500 feet, again exposing the series of soft sandy slates. The dip, however, is increased to about 70° W., which soon brings the jasperoid slates down to the sea level, and the whole of the south-west portion of the island is composed of them.