Art. V.—On the Occurrence of Three Bands of Marble at South Peak, near Hampden, Otago.
[Read before the Otago Institute, 10th December, 1918; received by Editor, 27th December, 1918; issued separately, 14th May, 1919.]
In the early part of last November, while investigating the extension of the Shag Point beds to the landward side of Hampden, I was informed by Mr. A. Craig, of that place, that an attempt had been made some twenty years ago to burn limestone in a kiln somewhere near the upper end of Baghdad Road. As the result of two days' search, with the assistance of Mr. Craig, I found a small outcrop of a grey crystalline limestone on the north side of Skinner's Creek, at an altitude of 450ft. above the sea, at a point about 300 yards from Baghdad Road. The outcrop had been opened out by blasting, and I concluded that this was the place from which the material for the experimental burning had been excavated. I continued the search towards South Peak, and succeeded in discovering three well-defined bands of limestone, two on the south side of Skinner's Creek and one on the north. These bands are interbedded in the altered argillite that forms the core of the coastal range lying behind Hampden. They strike almost north and south (true), and dip east at angles that vary from 50° to 65°.
Band A is about 5 ft. thick; band B, 12 ft.; and band C about 2 ft. Bands B and C are separated from one another by 32 ft. of argillite. Band A, geologically the lowest, is perhaps 200 ft. below band B.
When traced along the outcrop the limestone bands are found to occur as short lenses. Band A is a lens about 450 yards long, and bands B and C peter out in a distance of 70 yards.
Like the blue crystalline limestone at Dunback, the Hampden limestone bands occur in the semi-metamorphic rocks of the Kakanui series of Hector, the age of which, is still unknown. The relationship of the Hampden and Dunback limestones can be determined only by a detailed survey.
The Hampden limestone is a fine-grained grey marble of good quality. The larger blocks will form good building-material, and the small pieces may be utilized for grinding into material for agricultural purposes.
An average sample of marble from the lens marked A on the accompanying sketch was analysed at the Dominion Laboratory, Wellington, with the following results:—
|Insoluble in acid||0.86|
|Alumina and iron oxide||0.55|
|Calcium carbonate (CaCO3) by difference||97.86|
I am indebted to the Director of the Geological Survey for obtaining the above analysis for me.