The Savu-savu Peninsula
I include in this district the promontory west of Naindi Bay and Sava-reka-reka Bay. Although its surface is much cut up, it has, when viewed from a distance, a fairly even profile and attains a maximum height of rather over 800 feet. From the region east of it, it is separated by the Naindi Gap. Here one can cross the peninsula between the two bays above named without rising more than 50 feet above the sea. The elevated interior is divided into two parts, which are divided by a col, about 250 feet in elevation, which is ascended in crossing from Naithekoro on the south coast to Na Kama on the north coast. Much of the surface is clothed with the usual “talasinga” vegetation. Close to the north shore, with which it is connected by the reef-flat, rises the small island of Na-Wi, and off the extremity of the peninsula, which is known as Harman’s Point, is the islet of Naviavia, formed of raised reef-limestone as described on p. [8]. The celebrated boiling springs known as Na Kama are situated on the north coast opposite Na-Wi. It may be remarked in passing that besides finding an exit in the springs, the hot water oozes through the beach and below the tide-marks for several hundred yards along the shore. These springs are described in detail on p. [25].
This is one of the few districts of the island in which elevated reef-masses occur at the sea-border. These old reefs, which attain a maximum elevation of 250 feet above the sea, are principally restricted to the neighbourhood of Naindi Bay. (They are referred to in detail in [Chapter II.]) But they indicate only a part of the submergence which this region has experienced. There is an exposure of a very interesting rock in a stream-course that is crossed on the road from Yaroi to Naindi, less than a mile from the first-named place, and about 30 feet above the sea. Here we find a dark, impure “Globigerina” limestone, or, as it might be also designated, an altered calcareous palagonitic clay-tuff.[[84]] The larger fragments in it average only ·2 mm., and it affords evidence of a period of submergence during which the hill-tops of the Savu-savu Peninsula were below the sea-level.
We get the same indication, but in a more pronounced degree, in the stratified sedimentary clay-tuffs which are exposed on the shore-flat of the south side of the neighbouring Sava-reka-reka Bay. These beds, which within a distance of fifty paces are inclined 10-15° to the south-west and the same amount to the north-west, have apparently a quâquâversal dip. In places they exhibit a spheroidal and concentric structure, and are penetrated by cracks containing some calcite, but mostly filled with a white zeolitic mineral.[[85]] One of these rocks is a bright green, hard and compact deposit, containing but little lime, and evidently an altered palagonitic clay-tuff. It contains a few minute tests of the “Globigerina” type; and on account of the small size of its fragments of minerals, which range from .01 to .04 mm., it may be regarded as a relatively deep-water sediment.[[86]] It is interstratified with a coarser, somewhat altered palagonite-tuff, which shows but little lime and only a suspicion of tests of foraminifera. The size of the larger included fragments does not exceed half a millimetre.... The low hill, near Yaroi, on which the magistrate’s house is built, is composed of fine and coarse tuffs, probably submarine. It is doubtful whether any but sedimentary tuffs occur in this peninsula.
In the hills of the western part of the peninsula, that is, west of Na Kama and Naithekoro, a particular type of basaltic andesite prevails, characterised by rhombic pyroxene as well as augite phenocrysts, and referred for the most part to genus 13 of the hypersthene-augite andesites. Their specific gravity ranges from 2·76 to 2·83, and the interstitial glass may be fair or scanty in amount. The average length of the felspar-lathes is unusually small, ·04-·06 mm. In these respects the basaltic andesites of the Savu-savu Peninsula differ from the basaltic andesites found in most other parts of the island, where, as exemplified by those of the Wainunu, Solevu, and Seatura regions, the felspar-lathes average between ·1 and ·2 mm. in length, and there is practically no rhombic pyroxene. A somewhat scoriaceous semi-vitreous form of pyroxene andesite is exposed on the south slopes above Nukumbalavu, where it is covered by basic agglomerates. The pyroxene in the groundmass is here prismatic, and not granular, and for the most part rhombic; and the rock is referred to the prismatic sub-order of the hypersthene-augite andesites described on p. [287].
The basaltic andesites of the peninsula are often extensively decomposed through the weathering process, a spheroidal structure being then displayed. It rarely happens that the basaltic rocks of this locality assume a propylitic character. Yet, if this change is due to hydrothermal metamorphism, we ought to find altered rocks of this kind in the vicinity of the boiling-springs. Such rocks did not come under my notice at the surface; but this only indicates that if this alteration has taken place here, it has been effected at some depth; and, indeed, it would seem probable that the alteration known as “propylitic” is a change produced generally in deep-seated rocks.
A semi-ophitic basaltic andesite that is exposed in the small stream-course at the back of the springs, and not 100 yards distant, displays no propylitic change, and is only affected by hydration. The basaltic andesites found on the hill-slopes further inland from the springs exhibit no change of such a nature. However, rocks of this description occur at and near the coast about a mile to the westward. One of them, which is light green in colour, might be taken for a limestone, since it effervesces with an acid. When examined in the slide it is shown to be the prevailing basaltic andesite greatly altered. The porphyritic rhombic pyroxene is replaced by viriditic material; the plagioclase phenocrysts are replaced by calcite, secondary silica, and other alteration products; and the structure of the groundmass is disguised by chalcedony, calcite, viridite, &c. Another rock from this locality displays great alteration. The structure of the groundmass is obscured by secondary silica, and is traversed by fine cracks passing through the felspar phenocrysts and filled with blood-red films of hematite.
On the hill-slopes behind Harman’s Point, at an elevation of 300 to 400 feet, blocks of a reddish, volcanic rock, greatly altered by the deposition of silica, were displayed on the surface. The ground was here strewn in places with beautiful pyramidal prisms of clear quartz, ranging up to an inch in length. They contain numerous inclusions, their faces being sometimes deeply etched or eroded. These crystals appear to have been formed rather rapidly in some highly siliceous thermal underground waters.
I did not ascend the hills of the portion of the peninsula lying east of Na Kama and Naithekoro. But whilst crossing the saddle between these two places, I perceived that the prevailing basaltic andesites extended up the slopes to the east. The neighbourhood of Naindi Bay offers several features of interest. The bay, which is circular in shape, is closed in on the east and west by projecting points, where we find elevated reef-limestone, 40 or 50 feet above the sea, displaying massive corals and large “Tridacna” shells in their natural position, and overlaying a cement-stone composed of blocks of volcanic rocks in a calcareous matrix. On the beach on the west side of the bay there is exposed a reddish-grey altered pyroxene-andesite, which, as regards the size of the felspars of the groundmass and other characters, appears to be an altered form of the prevailing basaltic andesites of the peninsula. In the midst of the low passage that isolates the peninsula, which I have termed the Naindi Gap, there is displayed a highly altered basic andesite which contains a white, zeolitic mineral in its numerous cracks.
The small island of Na-Wi consists of two low hills, the highest 130 feet in height, connected by a mangrove swamp and a sandy beach. There is no trace of a crateral cavity. The prevailing rock is a porphyritic, compact, basic andesite, differing from the other rocks of the neighbourhood in the greater amount of glass it contains. Though it is not easy to find a good, unweathered specimen of the rock, it would appear that Na-Wi represents an old volcanic neck.
We may infer from the above description of this peninsula that it has a history similar to that of most other parts of the island. There is evidence in the upraised reefs and in the “Globigerina” clays and limestones of considerable submergence at one period; and it is highly probable that the prevailing basaltic andesites are the products of submarine eruptions. In my account of the hot springs given on page [26], reference is made to the absence of any trace of a crateral cavity in that locality. The same is true, as far as my observation goes, of the whole peninsula. Altered rocks do not occur in the vicinity of the springs, but they are to be found at distances a mile and more away. It does not seem possible to restore in imagination the original form of this part of the island. The present contours are the results of more than one reshaping of the surface through the agencies of marine erosion and sub-aerial denudation.