(c) The coast between Waimotu and Natasa Bay.—A low belt of land often forms the sea-border. Between these two localities there is a broad estuary, the village of Vanuavou being situated on the right side and Malati on the left side. Tuffs usually calcareous and probably submarine are here displayed together with basaltic andesites.
(d) The coast between Natasa Bay and the vicinity of Tawaki.—This extensive stretch of sea-border, nearly 15 miles in length, is characterised by hilly spurs and long intervening low-lying tracts. The prevailing rocks exposed are tuff-clays, somewhat hard and altered, and coarser basic tuffs sometimes calcareous and overlying the former. These sedimentary deposits, which are evidently submarine, are often bedded and have a fairly constant dip over most of this region of 10 to 15 degrees to N.W. or W.N.W. They are frequently pierced by dykes, 6 to 10 feet thick, of compact basic rocks. A specimen of one of these dyke-rocks from between Natasa and Vatu-karoa is a doleritic basalt with scanty olivine (sp. gr. 2·81)[[92]]; but the rock of the dykes east of Vatu-karoa is less basic (sp. gr. 2·72), and may be described as an augite-andesite[[93]] with a doleritic structure in part disguised by alteration. Between Natasa and Sangani there is a remarkable exposure of an intrusive opaque white rhyolitic rock associated with altered tuffs. This rock, which has undergone some degree of alteration, is described on page [311]. It is the only indication of the vicinity of a region of acid rocks that I came upon on this tract of coast. It probably, if traced inland, would be found connected with the district of acid rocks. In following along the coast, however, towards Undu Point, the region of acid rocks is not reached until within 2 or 3 miles west of Tawaki. Reference should be made here to two hot springs that, as described on page [33], rise up on the coast at Natuvu, a little east of Lakemba.
The Inland Mountainous Region between the Eastern
Foot of the Mount Thurston Range and the Vicinity of Tawaki
This inland region corresponds for the most part to the tract of coast before described between Vuinandi and the vicinity of Tawaki. We have here a very much broken area traversed in a northerly and southerly direction by mountain-ridges and penetrated in places on the north side by prolongations of the Wainikoro and Kalikoso plains. The two loftiest summits, the Ndoendamu and Savu-riti peaks of the chart, rise respectively to heights of 2,481 and 2,238 feet. The former is the Hale Peak of the Wilkes’ Expedition; but as regards the native names there is some confusion and I have not been able to clear it up. Several of the lesser peaks rise to over a 1,000 feet, and four or five of them to rather over 1,500 feet.
With the exception of the fixing of the positions of the more conspicuous peaks, the interior of this part of the island is unsurveyed. I was able to lay down my positions approximately; but before a systematic examination can be made of the geology of this region a survey is necessary. Several traverses and ascents were made by me; but much more in the way of exploration remains to be done. I venture, however, to think that from the data below given a fairly accurate notion of some of the leading geological features of this region may be formed. The districts visited will be described in their order from east to west.
(a) The mountainous district east of the mountains of Vungalei and Nailotha, extending to the vicinity of Tawaki.—Mr. Thomson[[94]] in his map of the sea-border of this district shows a continuous coast range from near Tawaki to Natasa lying about two miles inland. This is the appearance when the region is seen in profile from a distance; but when viewed from an elevation in its interior its surface is seen to be for the most part traversed by a series of mountainous and hilly ridges trending roughly north and south, the greatest height not exceeding 1,500 or 1,600 feet.
The geological character of the sea-border of this district has already been described on page [208]. It is there shown that fine and coarse sedimentary tuffs, sometimes calcareous, and often penetrated by basic dykes, here prevail. Their general story is one of great denudation, and we have the same testimony impressed on us when examining one of the inland mountain-ridges.
I followed the crest of one of these ridges from the hamlet of Nawi, on the headwaters of the Vui-na-savu River, in a south-easterly direction for about 4 miles to the peak of Uthulanga, which overlooks Lakemba on the shore of Natewa Bay. During the walk my level rose fairly gradually from that of Nawi, less than 100 feet above the sea, until near Lakemba, where a height of 1,400 feet was attained. Here the ridge abruptly terminates in the round-topped peak of Uthulanga, which rises steeply from its crest for another 150 feet or rather more, and has a precipitous face on the southern side descending far down the mountain-slope. This peak, which is about 1,550 feet above the sea, when seen from the opposite shores of Natewa Bay is very conspicuous and looks like a thumb or a nose. The first part of the Fijian name of this peak signifies a nose.
The hamlet of Nawi, from whence the ridge begins, is built on a low mound-like “rise,” which is composed of a dark-grey hypersthene-gabbro. Since plutonic rocks are of very rare occurrence in this island, the Nawi gabbro has a particular interest. It belongs to the group of plutonic rocks described on page [250]; and is to be referred to the more basic kinds, its specific gravity being 2·84. In appearance it is like a diallage-gabbro, and in the slide displays monoclinic and rhombic pyroxene filling the interspaces between the large plagioclase crystals and undergoing respectively the diallage and bronzite stages of schillerisation. This rock forms the type of the group and need not be referred to in more detail here. It should be added that in the bed of the neighbouring river occur blocks of a basaltic andesite (sp. gr. 2·86) referred to genus 1 of the hypersthene-augite andesites. It is a less crystalline form of the gabbro just mentioned.
The prevailing rock exposed for the first 2 miles on the crest of the ridge (by starting from Nawi) was a greenish porphyrite displaying large opaque crystals of oligoclase, but showing much alteration of the propylitic kind, its specific gravity being 2·5, but the structure of the groundmass is much disguised. During the next mile, basic agglomerates and a massive hypersthene-augite andesite were exposed. The last-named is semi-vitreous, and on account of the prismatic pyroxene of the groundmass is placed in genus 5 of its sub-class. Its specific gravity is 2·7. In the last and fourth mile of the ridge was exposed a dark grey doleritic basalt (sp. gr. 2·85), which rings like clinkstone under the hammer, and weathers in a honeycombed fashion. The felspar-lathes average ·3 mm. in length, the residual glass being scanty. The pyroxene phenocrysts include some of rhombic pyroxene; and the rock is therefore referred to genus 4 of the hypersthene-augite andesites. The actual peak of Uthulanga, as it rises abruptly about 150 feet from the end of the ridge, is of agglomerate, the blocks being composed of a compact grey andesite.