Mayor Island lies to the north of Tauranga, twenty-three miles distant from the entrance to the harbour. Discovered by Captain Cook, it was for some time a secure stronghold for small parties of natives. It was first described by Goldsmith in 1884 (4, pp. 417–427); von Wolff in 1904 described some of the rocks that had been collected by Thilenius (10); Bell gave a short account of the island in The Wilds of Maoriland in 1914 (3). More recently Thomson (9) wrote a brief description of its field geology; while the rock specimens that he collected were described with great care by Bartrum (1). Sladden (7) described the surface features of the island as well as its fauna and flora. These three papers were published in 1925.
The island, which is three miles long from north to south and two miles wide, is the upper part of a volcano composed entirely of alkaline rhyolites, that are most properly classed as comendites. Bartrum (1) points out that some of them are almost pantellerites. The lower part of the volcano for a height of 500 feet above its surroundings is covered by the ocean waters. Its highest point, Opuahau on the west, rises to 1210 feet above sea-level, while Tutaretare on the south is 1162 feet high. In the portion now above sea-level the slopes rise gradually, from the crest of the sea-cliffs, which average about 100 feet in height in the south, but in places in the northern portion of the island as much as 500 feet. There is always a steep drop internally to the floor of the crater. Except in the southern part of the island the sea has eroded away almost all of the exterior slopes of the volcano, and a mere ridge of varying width now separates the sea from the crater floor. On the eastern side at Taratimi this ridge is in one place no more than 100 feet high, and here the crater-wall is formed of coarse tuff. On the south side the exterior slopes are nearly a mile long, and the craterwall 1000 feet high. The crater is about a mile and a-half in diameter, and within it there is a small cone 800 feet high which is densely covered with trees and lower vegetation. It has a low slope, and on the west its base reaches halfway up the inner slope of the main crater, but in both north and south directions the cone retreats slightly from the crater-wall, and in the east there is an interval of a quarter of a mile between it and the outer crater-ridge. Part of this space is now occupied by two small lakes. All the surface of the interior cone—the only part that is now visible—is formed of obsidian. The southern slopes of Mayor Island to a height of 600 feet at least are coated with a thick covering of pumice which completely conceals the real rocks of the island. The pumice is slightly greyish in colour and contains a considerable amount of
hypersthene. It is evident that the pumice has floated over from the mainland, which is mainly formed of hypersthene-andesite and of rhyolites containing hypersthene.
The island is somewhat dissected by stream valleys, but it is only during the climax of the heaviest rainfall that any water flows in them, and then for a brief period only. In none of the four stream valleys that were examined could any solid formation of the rocks of the island be seen. On the sea-coast solid rocks are continuously exposed, and, except at Taratimi where tuffs only are found, the formations have an upper and lower selvage of obsidian. There are boulders of hard rock scattered promiscuously but somewhat sparsely on the pumice, and these have the characteristic mineral composition of the rocks of the island. Almost everywhere the descent to the crater-floor is extremely abrupt. In the south the surface descends in one unbroken precipice for a height of more than 1000 feet, but on the west it abuts against the slope of the interior cone, and on the east it is very low. The fact that no pumice has been found within the crater, though on the exterior slopes it has been found at far greater heights than that of the present lip of the crater, is an indication that at the time when the pumice fields floated over from the mainland the marine erosion of the crater-wall had not proceeded nearly so far as at present.
Fig. 1.—Imaginary bird's-eye view of Tuhua or Mayor Island, Bay of Plenty, New Zealand. 3 × 2 miles, 1210 ft. high. Rock: comendite and obsidian.
Many of the features of the land are picturesque and interesting. In places striking arches have been formed in the coast by wave-action. At the north end there is a dyke which has been worn back into some prominent stacks. Usually only a single formation is exposed on the sea-cliffs, but on the north and west where the cliffs are perhaps 50 feet high as many as three or four may be seen. Unfortunately on the occasions on which the island has been visited no landing was possible on these coasts.