In the lower part of the mountain the blocks, usually sub-angular, are about a foot across; but they become smaller as one ascends towards the summit, where they are 3 or 4 inches in diameter. At the top there are extensive exposures in cliff-faces of the agglomerate-tuffs; and here the finer materials of the matrix include a few rounded pebbles not exceeding half an inch in size. This is a fact of importance in connection with the submarine origin of these formations.
As regards their composition, the blocks of the agglomerates have not the uniform character we would expect to find in the case of materials directly ejected from a volcanic vent. The most frequent type of rock represented is a grey hypersthene-augite-andesite, having a specific gravity of 2·72-2·78. It displays small phenocrysts of plagioclase and of rhombic and monoclinic pyroxene, but in other respects it exhibits much variety, not only in the arrangement and average length of the felspar-lathes (·08 to ·18 mm.) but in the form of the pyroxene of the groundmass (either granular or prismatic) and in the amount of residual glass, sometimes abundant, sometimes scanty. Two distinct genera (1 and 5) of the sub-class are therefore represented.
Other rocks found in these agglomerates contain no rhombic pyroxene, and are referred to genera 13 and 16 of the augite-andesites according to the presence or absence of plagioclase phenocrysts. In the last case we have a dark aphanitic rock (sp. gr. 2·74), sometimes scoriaceous, where the average length of the felspar-lathes may be as little as ·04 mm. One of the blocks was composed of a highly scoriaceous semi-vitreous rock, the cavities being filled with a zeolite. Another was composed of a black porphyritic augite-andesite, showing large crystals of plagioclase.... The matrix of the agglomerate-tuff is formed of sub-angular and rounded fragments, up to a centimetre in size, of the same andesites, the interstitial material being formed of fine detritus and palagonitic debris.
Though the agglomerates of the peak of Koro-mbasanga are composed of a variety of rocks, all the rocks are to be referred to the pyroxene-andesites with specific gravity below 2·8 but above 2·7. They are therefore less basic than the olivine-basalts and basaltic andesites of the Koro-tini range, where the density is usually 2·8 and over. Their variation, however, is more consistent with the characters of an agglomerate formed by marine erosion. The same may be said of the sorting of the blocks according to their size and of the occasional occurrence in the matrix of small rounded pebbles. That these deposits of agglomerates were formed under the sea is indicated also by their overlying submarine sedimentary tuffs near the base of the mountain.
(2) The Sokena Ridge.—To the west of Koro-mbasanga, and forming a spur of the same range, is the flat-topped hill of Sokena, which rises about 1,100 feet above the country at its base and about 1,600 feet above the sea. From a distance it has the appearance of being formed in its higher portion of nearly horizontal strata dipping gently northward. In its upper part it terminates in a line of cliffs about 200 feet in height, and there is a similar line of cliffs lower down the slopes. These cliffs are composed of bedded fine and coarse non-calcareous tuffs, dipping about 10° N.N.W., in which are imbedded without any arrangement blocks, ranging in size from 2 or 3 inches to 3 or 4 feet, of a remarkable blackish pitchstone-like rock displaying opaque plagioclase phenocrysts. It is referred to genus 18 (see page [289]) of the hypersthene-augite andesites, both rhombic and monoclinic pyroxene being represented in the phenocrysts and in the groundmass where they take the form of minute prisms (·03 mm.). There is a considerable amount of pale brown glass. A rock very similar occurs in the Thambeyu agglomerates (see page [178]). The tuffs are formed largely of palagonitic materials, the angular fragments in the coarser beds being ½ to 2 centimetres in size, the palagonite being often vacuolar but much affected by hydration.
These tuffs and agglomerates of the Sokena cliffs apparently contain no organic remains. They appear to have accumulated under water as the result of the eruptions of a neighbouring vent without the intermediate agency of marine erosion.
(3) The Ascent of the Lovo Valley.—About two miles to the east of the peak of Koro-mbasanga the picturesque Lovo valley cuts deeply in a southerly direction into the mountainous backbone of the island. The site of the old town of Lovo lies within the valley about two miles from its mouth. “Lovo” is the Fijian word for a cannibal-oven; and I gathered from my natives that in the old times this vale was noted for its cannibal orgies. It is occupied by the Nasawana tributary of the Lambasa River, and often becomes so narrow that it may be described as a gorge. I followed the valley from its mouth, where it is elevated about 300 feet above the sea, for some miles in a southerly direction up to a height of 1,000 feet, where the northern slope of the great mountain-mass of Mbatini commences.
On either side of the Lovo valley rise precipitous mountain-slopes, displaying in their cliff-faces and in the large detached rock-masses basic agglomerates. The same formation is also usually displayed in the sides of the river. The blocks composing the agglomerates are formed of the usual type of hemi-crystalline or semi-vitreous blackish basaltic andesite so characteristic of these deposits. It is generally compact, but is at times amygdaloidal. Some distance below the old site of Lovo, and at an elevation of about 500 feet above the sea, there is an interesting exposure in the river-side, where the agglomerates overlie bedded coarse calcareous basic tuffs containing large flat tests of foraminifera with pieces of molluscan shells, and dipping about 15° S.W. These tuffs can be traced up the valley towards Lovo.
Displayed in mass in the bed of the river in the same locality, and beneath the submarine tuff just referred to, is a porphyritic basaltic andesite (sp. gr. 2·79) containing but scanty interstitial glass, the felspar-lathes being ·15 mm. in average length. It is referred to genus 1 of the augite-andesites. The same rock is exposed at intervals in the river-bed as far as Lovo, which is about 850 feet above the sea. At one place it exhibits a rudely columnar structure, the columns being horizontal and 2 to 2½ feet in diameter, the trend of the dyke-like mass being W. by S. and E. by N. Near Lovo a small dyke, 6 feet thick and trending N.N.W. and S.S.E., pierces the agglomerate. It is composed of a somewhat aphanitic augite-andesite closely resembling the rocks exposed in the river-course for a mile or so above Lovo up to an elevation of 1,000 feet. In this upper part of the valley whilst agglomerates are exposed in the cliffs and precipitous mountain-slopes on either side, pyroxene-andesites, somewhat aphanitic in texture and with a specific gravity of 2·68 to 2·7, are displayed in mass in the river bed. These last-named rocks, which are closely similar to those found on the lower slopes of Mount Mbatini (see page [173]) are, as I should have also remarked in the case of the basaltic andesite above mentioned, a little altered, as is indicated by the existence of calcite and viridite in the groundmass.[[79]]
From this instructive ascent of the Lovo valley we may learn that whilst the mountain mass is formed, to a considerable depth, of agglomerates with underlying submarine tuffs, the deeper seated rocks exposed in the river-beds are massive intrusive rocks. The overlying agglomerates have preserved the submarine tuffs from destruction, and there is no difficulty in assuming that they also were accumulated under the sea, but in shallow water, as evidenced by the character of the tuffs. I found no signs of alteration in these tuffs, and except in the case of the small dyke above noticed there is no sign of the dykes penetrating the agglomerates. We have here a section into the heart of the mountain-range; and assuming that the large intrusive masses of basic andesites had penetrated these deposits, there would certainly have been some evidence of this in the extensive exposures of agglomerates far up the mountain-sides. As it is, however, we find such rocks only in the deeply excavated river-bed. If we imagine a submarine volcanic mountain, or one but slightly raised above the surface of the sea, to be subjected during a long period of emergence to marine erosion, the “basal wreck” of the mountain would ultimately be covered over by submarine tuffs and agglomerates. This is the condition that seems to be presented here.