Geological Structure of the Kaimanawa Range

Transactions and Proceedings of the Royal Society of New Zealand, Volume 3, 1870, Page 16

3. The next communication by Dr. Hector, on the interior of the North Island, (See Reports of Geological Survey, 1870,) gave the leading features of the geology of the Kaimanawa and Ruahine ranges, which had been recently examined by him. The modern tertiary rocks that form the eastern portion of the Hawke's Bay province, were described as rising in the interior to an altitude of 2,700 feet, but that it was probable that the Kaimanawa range and certain parts of the Ruahine mountains had always remained as islands above the tertiary sea. The tertiary rocks comprise three groups—1. Limestone containing a large percentage of existing shells; 2. Clay marls, containing few shells; and 3. Sandstones and conglomerates with irregular seams of coal, some of which might yet prove valuable as fuel. The upper group is of much later date than the others, but all are distinctly tertiary. The axis of slate rocks, which divided the tertiary series at the time of the development of the conglomerates, is within twenty miles of the present East Coast line, but is broken through by several modern rivers which rise in the Taupo plains; so that easy passes exist from Napier to the interior, a circumstance which has an important bearing on the opening up of the country. The Kaimanawa range is formed of the same slate and sandstone rocks as the Ruahine, but it lies at a considerable distance to the west of the proper axis of the island. The space left between them is occupied by the same tertiary rocks as on the east side, and which slope gradually to the sea coast at Wanganui. As the tertiary rocks are quite free from any trace of volcanic matter, the eruption of the central volcanoes must have commenced after their deposit was completed. In referring to the auriferous specimens which had been found on Mr. Lyon's run at Kereru, Dr. Hector stated that chemical analysis had

proved that, notwithstanding its granitic appearance, the rock to which the gold quartz was attached was only an altered form of the sandstone, as it contained traces of graphite, and 91 per cent. of silica. This is strongly in favour of the view that it is derived from the Ruahine range, as the sandstones in them have been previously mistaken for granite. After alluding to the recent increase in the activity of the volcanic forces in the Tongariro district, Dr. Hector described the route to the West Coast from the interior, and drew attention to maps and reports by Mr. Geo. Swainson and Mr. Field. He also exhibited a new geological map of the central district. The Hon. Mr. Fox considered that there was no doubt of the practicability of a route to the Taupo district from the Wanganui coast. He believed that the track through the bush country was almost completed, and he was glad to find that no insurmountable obstruction would be encountered beyond that point. In reply to Mr. Mantell, Dr. Hector stated that he did not think that any rich auriferous quartz had been obtained in the Kaimanawa, but that his opinion remained unchanged as to the probability that gold would yet be met with in the district he had described.