(b) South coast between Tawaki and the foot of Mount Thuku.—The tall cliffs that rise to a height of from 200 to 300 feet behind Tawaki are composed of white tuffs and agglomerate-tuffs derived from the acid rocks of the district. Eastward from Tawaki to the base of Mount Thuku the coast scenery is particularly fine. A little inland a line of hills, named “Na Kula,” rises precipitously to a height of 700 or 800 feet, and in the vertical sides are displayed tuffs and agglomerates probably of the character of those above noticed. Light-coloured tuffs are sometimes exposed at the coast in which are inclosed fragments varying in size of a pitchstone[[104]] (sp. gr. 2·36) approaching in structure a trachytic glass. At one place the tuffs were evidently sedimentary and bedded, the dip being about 15° N.W.
The massive rock most frequently exposed at the coast and on the hill-slopes between Tawaki and Mount Thuku is a quartz porphyry[[105]] displaying abundant porphyritic crystals of quartz and felspar in a groundmass originally semi-vitreous but now obscurely felsitic in character. The shore-flat for more than half a mile west of Mount Thuku is strewn with great numbers of detached columnar blocks, 12 to 15 inches across, of a slightly vesicular oligoclase-trachyte of the type described on page [308].
(c) North coast between Thawaro Bay and the foot of Mount Thuku.—Coarse and fine tuffs prevail at the coast and on the neighbouring hill-slopes; and in the bare rocky faces of the hills inland they are also to be observed. The streams have worn deep gorges into their mass. Towards Thuku they are acid and pumiceous, and are evidently the products of eruption. Towards Thawaro, they are more basic and darker, and are in part at least to be attributed to marine degradation.
(d) Mount Thuku.—My ascent of this hill, which is 1,288 feet in height, was made from the north coast. I found it to be composed from the foot to the summit of white pumiceous tuffs without any evident arrangement. It has a narrow top and shows no sign of a crateral cavity. The hills east and west, as viewed from its summit, are ridge-shaped and display nothing in their configuration at all suggestive of craters. The pumice-tuffs of Mount Thuku are non-calcareous, and exhibit greyish pumiceous lapilli in an abundant white matrix formed of fine pumice-debris. Under the microscope it shows the characteristic vacuolar and fibrillar structure; but the material has not the fresh appearance of ordinary pumice and the minute cavities are often filled with alteration products. The two rocky points on the north coast opposite the hill are formed in one case of a somewhat altered oligoclase-trachyte and in the other of a quartz-porphyry. Both no doubt represent intrusive masses, the almost horizontal columns, 12 to 15 inches in diameter, of the former indicating a nearly vertical dyke.
(3) The Undu Promontory East of Mount Thuku
East of Mount Thuku the hilly backbone of the promontory is of much less elevation. About three miles to the eastward the highest hill is 630 feet, and thence to within a mile or two of Undu Point the hills retain a height of 400 to 500 feet.
(a) The north coast between Mount Thuku and the coast village of Nuku-ndamu.—On this stretch of coast, about five miles in length, the shore-cliffs are composed of white and pale-yellow, coarse and fine stratified pumice-tuffs, the beds being either horizontal or with a gentle dip northward. They are as a rule non-calcareous, and contain some quartz grains and small bits, 1 to 3 millimetres in size, of bottle green compact obsidian, much as one finds in Lipari pumice-tuffs. In general character, both naked-eye and microscopic, they correspond to the Mount Thuku pumice-tuffs above described. Large blocks of basic rocks are occasionally to be observed on this coast, sometimes probably indicating dykes, but in one place near Mount Thuku forming an agglomerate. The rock is a dark-grey augite-andesite with a specific gravity of 2·77. It is compact and has a hemi-crystalline groundmass.[[106]]
(b) The south coast between Mount Thuku and Moala, a distance of about five miles.—Pumice-tuffs and agglomerates are displayed at the coast, the former often bedded and in one place having a dip of 35 or 40 degrees to the north. A quartz-porphyry, somewhat banded and a little altered, and displaying rounded quartz crystals 3 or 4 millimetres in size, is the prevailing massive rock exposed on the hill-slopes and occasionally at the coast. It is well exhibited about a mile east of Mount Thuku. Blocks of a grey oligoclase-trachyte also occur. These rocks are described on pages [308], [309].
(c) The terminal portion of the promontory from Nuku-ndamu and Moala to Undu Point.—The same pumiceous tuffs, usually non-calcareous, form the shore-cliffs on the south coast from Moala to Mr. Bulling’s station at Ndothiu, which lies about 2 miles from the point. On the corresponding part of the north coast between Nuku-ndamu and Ndothiu these tuffs are often calcareous; and near the first-named place they contain sub-angular bits of coral of the size of a walnut. On the beach at Vunikondi in this locality they are overlain by nearly horizontal beds of basic lava, the upper surface of which when exposed displays the smooth, “ropy” crust of a lava flow. The rock is a dark slightly vesicular augite-andesite, hemi-crystalline in structure, and containing a fair amount of residual glass.[[107]] Since the underlying tuffs were evidently deposited on a sea-bottom, it follows that this is a submarine flow. I intended to revisit this locality, but was prevented. A detailed examination of it would be worth undertaking.
From Ndothiu to Undu Point, about 2 miles distant, the interior of the promontory has an undulating surface, the elevation being usually 200 or 300 feet and rising to 400 feet. On the coasts are exposed bedded pumiceous tuffs, steeply inclined and usually calcareous. As displayed in the hill-slopes of the interior they are horizontally stratified and as a rule non-calcareous. These deposits sometimes exhibit a spheroidal arrangement indicative of the proximity of a dyke. In one or two places at the coasts occur basic agglomerates, formed of the same augite-andesite lava-rock of which the Vunikondi beds are composed, but scoriaceous and containing more glass in the groundmass. In the hand-specimen beside me, the steam cavities are of all sizes, from that of a pin-prick to a third of an inch (8 mm.) and are generally elongated.