CHAPTER XII

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)

THE KORO-MBASANGA RANGE

AS is illustrated in the accompanying profile-sketch, the relatively level-topped range of Koro-tini gives place at its eastern end to a broken line of mountains, of which the round-topped Koro-tambu, 2,753 feet in height, and the pinnacled Koro-mbasanga,[77] 2,537 feet, are the highest peaks. Further east lies the broad Vuinandi Gap which separates the Koro-mbasanga and Mount Thurston, or Thambeyu, ranges. The twin-peaks of Mount Mbatini, the highest mountain of the island, appear in the background in the sketch, and to the left rises Thambeyu, the second highest summit.

We enter here another complex region of mountains; and if the character of the rocks are sometimes different we shall yet have to bear in mind in our interpretation of its geological features the lesson derived from the examination of the Koro-tini Range. Before and behind all our facts of observation lie the two great periods of marine-erosion and the later ages of sub-aerial denudation.

When approached from the north, the western part of the range has a rude crescentic form, and looks like the remnant of a gigantic crater-cavity about two miles across. At the back rise, as shown in the second of the profile-sketches, the precipitous slopes of Koro-mbasanga proper; whilst the two spurs descending from it, one on the west, the Sokena spur, towards Koro-utari, the other on the east, at the back of Nasawana, give the crescentic figure open to the north. The last-named village lies nestled in this great hollow, the floor of which, though not in its lowest part below the level of the Lambasa plains, is not over 200 feet above the sea. However, the facts adduced in the following description of this region do not give much support to this view of its surface-configuration.

For the convenience of description, I will first describe the peak of Koro-mbasanga, and then the Sokena ridge and, lastly, the Lovo valley that cuts through the range to the eastward.

(1) KORO-MBASANGA.—The ascent of Koro-mbasanga is best made from Nasawana, a village at its base, elevated rather over 200 feet above the sea, and distant about a mile and a half north-east from the peak. On the way to the foot of the mountain we traverse an undulating region of basaltic andesite,[78] which is merely the extension to the base of the mountains of the basaltic Lambasa plains. After commencing the ascent of the steep slopes, we find exposed in a stream-course, 700 to 800 feet above the sea, a sedimentary basic tuff, presenting layers of coarse and fine materials, and partly palagonitic in composition. It is a little calcareous, and apparently incloses tests of foraminifera. These submarine deposits have evidently been stripped off the basaltic low-lands beneath. It thus becomes evident that the structural features of the Lambasa plains (basaltic rocks overlain by submarine deposits) are preserved to the base of the range.

These submarine deposits, as exposed in the stream-course, lie beneath agglomerates which repose horizontally upon them; and from this level up to the bare rocky peak of the mountain, agglomerates and agglomerate-tuffs are alone displayed either as large detached masses or in cliff-faces.

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.

I did not make the ascent of Koro-tambu, the other principal peak of the Koro-mbasanga Range. This round-topped mountain is well seen from the summit of Mbatini from which it bears N. 30° W. by compass. It is probably the peak marked 2,753 feet in the Admiralty chart, and is connected with Mbatini by a saddle not under 1,500 feet in elevation.

Some of the most important features in the above account of this district may here be emphasised. We have seen that in the peak of Koro-mbasanga and in the Lovo valley agglomerates and agglomerate-tuffs, several hundred feet thick, overlie sedimentary submarine tuffs. In the last-named locality the deeper massive basic rocks are also exposed; and we may infer in both instances that the agglomerate-formation is a submarine deposit. On the other hand, in the Sokena Ridge, which is a spur of the main range, we have apparently the accumulation of materials on a sea-bottom, directly ejected from a vent without the intervention of the agency of marine-erosion. In regard to this and other districts in this part of the island it should be remembered that east and west occur undoubted evidences of extensive submergence. It has already been shown that submarine tuffs containing tests of foraminifera and other organic remains occur at heights of 2,000 feet and over on the summit of the Koro-tini Range, and it will be subsequently shown that similar deposits are to be found on the neighbouring slopes of Thambeyu as high as 2,100 feet.

MOUNT MBATINI

According to the Admiralty chart this is the highest mountain in Vanua Levu, its elevation being 3,437 feet. It has twin peaks which lie either N.W. and S.E. or W.N.W. and E.S.E. with each other. The northerly or westerly peak is pointed and tooth-like. Hence probably arises its name of Mbatini (mbati-tooth). The southerly or easterly peak is known as Soro-levu. It has a broadly conical outline with a truncated summit. The mountain is named Koro-mbasanga in the Admiralty chart, a name that really belongs to a peak lying about 3 miles nearly due north (N. 5° W.). The natives are very clear in this matter; but it must be remarked in this connection that Koro-mbasanga, which signifies “a forked eminence,” would be a very suitable appellation for the double-peaked summit of Mbatini.[80] By the natives of the surrounding district the whole mountain is known as Mbatini; but by the natives of the eastern shores of Natewa Bay, it is usually known as Soro-levu, since the western peak is often more or less hidden from view or is less conspicuous. The profile of this mountain and of the neighbouring region is shown in the accompanying profile-sketches and also in one of those illustrating the Koro-mbasanga range on page 167.

As viewed from the top of Mariko to the southward, Mbatini presents itself as a long mountain-ridge, trending W.N.W. and E.S.E., which is connected on the north with Koro-tambu, the highest peak of the Koro-mbasanga Range, by a saddle probably not over 1,500 feet above the sea, and on the south with the mountain-ridge of Mariko by a _col_ which appears not to be under 1,000 feet in elevation.

My ascent of this mountain was made from the north by the way of the Lovo valley. In ascending the Lovo valley one reaches, at an elevation of about 1,000 feet, the foot of the north slope of Mbatini. The slope is somewhat steep up to 2,000 feet, the rocks exposed on the surface being closely similar in the groundmass to those displayed in the upper part of the Lovo valley. They are compact-looking blackish augite-andesites (sp. gr. 2·7), the very small felspar-lathes of the groundmass, which are in flow arrangement, averaging only ·05 mm. in length. Like the rocks below, they are a little altered; and here the interstitial glass is also scanty. But they differ in the absence of rhombic pyroxene and are therefore referred to the augite-andesites (genus 13).

At 2,000 feet, where one crosses the foot-track from Nukumbolo to Korolau, the ascent of the true Mbatini ridge begins, the summit lying nearly two miles to the south-east. Whilst following along this lofty mountain-ridge we were for the greater part of the time in the rain-clouds, so that very little was seen of our surroundings. The crest is densely wooded so that our progress was very slow. The rocks are but sparingly exposed. At the commencement of the ridge (2,100 feet) is displayed an altered hypersthene-augite andesite, rudely columnar blocks of which, up to 2 feet in diameter, were lying about. It belongs to genus 1 of this sub-class (see page 286) which also includes the rocks exposed farther along the ridge. In these rocks the felspar-lathes are small (·05-·07 mm. long) and are not in flow arrangement. The interstitial glass varies in amount, and the specific gravity is about 2·7.

The ascent is very gradual for the first one and a half miles, when an elevation of 2,600 feet is attained. From here one ascends the steep-sided peak of Mbatini, which rises some 700 or 800 feet from the ridge. As one nears the highest point the crest becomes very narrow, between 15 and 20 feet across; and on either side there is apparently a drop of several hundred feet. The actual peak, which is bare and rocky, is yet narrower; and when it is enveloped in dense mist as it was in my instance, it is not a very secure situation for a geologist. It is highly magnetic, as is the case with most of the other bare peaks of the island. The rocks exposed in the upper 500 feet, that is, in the peak proper, are highly altered semi-vitreous, but extensively weathered, hypersthene-augite-andesites which are referred to genus 1 of that sub-class. Much of the glassy groundmass is replaced by viridite, silica, calcite, &c. Less altered specimens display in a brown opaque glass small felspar-lathes averaging less than ·1 mm. in length. They exhibit phenocrysts of rhombic pyroxene and augite, the first prevailing.

I did not climb Soro-levu, the other of the twin-peaks. Its ascent should be made either from Nukumbolo or from one of the villages on the neighbouring shore of Natewa Bay. My acquaintance with Mbatini, although very incomplete, enables me however to point out a few of its general features. As remarked before, there is a general uniformity in the type of its rocks. The olivine-basalts and basaltic andesites, prevailing in the Koro-tini Range, are not here represented, nor are the dacites or acid andesites to be found. The characteristic rocks are more or less altered hypersthene-augite-andesites having a specific gravity in the least altered and least vitreous condition of about 2·7; whilst the average length of the felspar-lathes is always less than ·1 mm. The same type prevails from the upper part of the Lovo valley to the summit of Mbatini; but it is only in the actual peak that these rocks show much glass in the groundmass, though extensively affected by alteration. Neither tuffs nor agglomerates came under my notice; but they might be expected to occur on the other slopes. I am inclined to regard this mountain-ridge as a huge dyke-like mass or sill, representing the remains of a volcanic vent that has been subjected at different periods to marine-erosion and in later ages to sub-aerial denudation.

THE VUINANDI GAP

I have given this name to the break between the Thambeyu (Mount Thurston) and Koro-mbasanga ranges, where the level of the mountainous backbone of the island descends to about 1,200 feet above the sea. This is the route taken by the track from Vuinandi on the shores of Natewa Bay across the island to Lambasa.

At Vuinandi the mountains recede from the coast leaving a broad level plain extending about two miles inland to the village of Tarawau without rising over 60 feet above the sea. Basaltic rocks are exposed in the spurs that descend from the mountains to the coast on each side of the plain. After traversing the low-lying region that lies between Vuinandi and the main range, one finds on ascending the eastern slopes, _en route_ to Lambasa, basaltic andesites of the usual type prevailing up to 1,000 feet. The upper portion of the dividing range, 1,000 to 1,200 feet, is composed of a more compact basaltic andesite which is often rubbly and in this condition is penetrated by fine cracks, 1/8 of an inch broad, filled with chalcedony. This rock, which has a specific gravity of 2·85, has a very fresh-looking appearance in the slide, and the segregation of silica does not therefore appear to arise from an alterative change. The felspar-lathes, which are in flow-arrangement, average ·11 mm. in length, and there is a little residual glass.

The mountains rise on either side of the Vuinandi Gap to about 2,000 feet. Descending on the west side of the range one follows a stream-course down to a level of 400 feet above the sea, agglomerates and coarse basic tuffs being exposed on the way. The rocks forming the agglomerates are for the most part to be referred to genus 1 of the hypersthene-augite andesites. They are sometimes compact and sometimes amygdaloidal, the amygdules being formed of chalcedony and other minerals, whilst the glass of the groundmass is often altered.

The track then lay across a spur, 800 feet in height, principally composed of a greyish porphyrite, exhibiting large opaque crystals of plagioclase, 4 to 7 mm. long, in an almost holo-crystalline groundmass formed of stout lamellar felspars with large augite granules. It is described on page 268 under the porphyritic sub-genus of genus 2 of the augite-andesites, and is an unusual type of rock for this island. After this I descended into the picturesque gorge of the Satulaki River, which is only elevated about 200 feet above the sea, agglomerates prevailing. In the vicinity of Satulaki a rather compact basaltic andesite (sp. gr. 2·82) is commonly exposed in position. It is referred to genus 13 of the augite andesites and belongs to the species with felspar-lathes less than ·1 mm. in average length. It occurs both north and south of this place and in the hill-spurs on either side. This is the bed-rock of the Lambasa plains which here begin and extend to the north coast, being usually covered with submarine tuffs and clays.

THE THAMBEYU OR MOUNT THURSTON RANGE

Mount Thurston is the name given in the Admiralty charts to the highest peak (3,124 feet) of this range. There does not appear to be any general native name. The highest peak visible from the Lambasa side is known as “Thambeyu.” The lofty mountain-mass, as it is viewed from Vuinandi, is known as Ulu-i-ndiri-ndiri.[81] The whole mountain-range has yet to be properly explored. It is a much more complicated system of mountain-ridges than is indicated in the chart, my acquaintance with it being restricted to the Thambeyu ridge, the elevation of which is 2,600 feet above the sea. It trends N.N.W. and S.S.E.; but its relation to the highest peak of the range could not be ascertained, as we were in the rain-clouds during the two days we were on the mountain.

I made the ascent from the village of Numbu-ni-a-vula about three miles to the westward, which is only 200 feet above the sea. In the intervening low district a basaltic andesite is exposed in the stream-courses. The structure of the ridge, as indicated by the ascent of its western slope, is shown in the accompanying diagram. The core or central axis is formed of massive basic rocks which protrude at the summit and in one or two of the crests of the spurs. The flanks are composed of submarine tuffs and clays overlaid by agglomerates of considerable thickness. The tuffs reach to within 50 feet of the top, whilst the agglomerates extend to within 400 feet of the summit. The results obtained from this ascent are specially interesting, since it afforded me the opportunity of studying in a satisfactory manner the junction of the agglomerates with the tuffs.

There are two caves on the mountain-side which can be used for night-shelter by those exploring the range. The lowest, 1,500 feet above the sea, is the Taloko Cave (na-ngara-taloko). The highest is the Ndromo Cave,[82] 2,100 feet, known to the natives as “na-ngara-vatu-ni-ndromo.” Like most of the caves all over the island they occur at the junction of the agglomerates and tuffs, and are to be attributed to the more rapid weathering of the underlying tuffs.... In describing the results of my examination of this mountain-ridge, I will deal in succession with the tuffs, the agglomerates, the junction between these two deposits, and the axis or core of basic rocks.

(1) _The submarine tuffs and tuff-clays._—As exposed in the stream-courses near and at the foot of the mountain and as high as the Taloko Cave, these deposits are bedded horizontally. At higher levels, owing to insufficient exposure the bedding is not so clear. Up to 700 or 800 feet coarse palagonite-tuffs prevail; but they do not effervesce with an acid, and apparently contain but scanty organic remains. At 950 feet coarse and fine sedimentary tuffs alternate, the last being greenish foraminiferous tuff-clay rocks, somewhat compacted and containing 10 per cent. of carbonate of lime. The tests of the foraminifera, which are abundant and of the Globigerina type, are filled with calcite. Several fragments, of a semi-vitreous basic rock, not however exceeding ·2 mm. in size, are inclosed in the deposit; but the mass of it is made up of yet finer materials of the same rock, palagonitic detritus, plagioclase fragments, fine calcitic debris, tests of foraminifera, &c. These fine tuff-clays were evidently formed in relatively deep-water.

At the Taloko Cave (1,500 feet), where there are exposed rather coarse tuffs containing bands about a centimetre thick of a fine clay-tuff, the last-named effervesce freely with an acid, whilst the first contain only a little carbonate of lime. No sections have been made of these deposits; but when powdered and examined under the microscope they appear to have the same general composition as the deposit described above from an elevation of 950 feet. They are probably foraminiferous though scantily. The tuffs found at the Ndromo Cave (2,100 feet) contain 4 per cent. of carbonate of lime and small tests of foraminifera are visible with a lens. The mineral fragments include plagioclase and rhombic pyroxene, and there are inclosed rounded gravel-fragments, 5 mm. in size, of a semi-vitreous rock. Palagonitic debris make up the mass of these tuffs. A coarse deposit from 2,500 feet is non-calcareous, but has the same general composition.

(2) _The agglomerates._—These deposits are best represented in the upper part of the mountain, between 1,500 and 2,200 feet above the sea. Here they often present vertical precipices having a drop varying between 100 and 400 feet, with the submarine tuffs exposed at their base. Such cliffs, however, display no structure. Their vertical faces are to be attributed to joints and to the extensive “slips” that frequently occur on these slopes, when large masses of agglomerate, undermined by the percolation of springs through the tuffs beneath them, roll far down the mountain-sides. The blocks of the agglomerates are fairly uniform in size, being usually 4 or 5 inches across. They are composed of a semi-vitreous hypersthene-augite andesite, containing both augite and rhombic pyroxene, but of an unusual type. It is a blackish rock carrying opaque phenocrysts of plagioclase, and is characterised by the prismatic form of the pyroxene (monoclinic) of the groundmass. A very similar rock from the Sokena agglomerates has been before described. It is referred to genus 18 of the class, and the prismatic sub-order to which that genus belongs is described on page 289.

(3) _The junction of the agglomerates and submarine tuffs._—This is well displayed at the Taloko Cave. Here the agglomerates lie conformably on the sedimentary tuffs; but the line of junction is sharply defined and the only evidence of transition is afforded by the great diminution in the size of the blocks of the agglomerates, which are 1 to 2 inches across. Immediately beneath the agglomerate is a layer 2½ centimetres thick of a rather coarse sedimentary palagonite-tuff having the composition of the deposits above described, but not effervescing with an acid, and showing no foraminiferous tests. The size of its “grain” is about a millimetre. This passes downward rather abruptly into a chocolate-coloured marl-like rock, a centimetre thick, which is formed of the same materials but in a clayey condition. Beneath this is the calcareous foraminiferous palagonite-tuff referred to in the first paragraph.

It is apparent that for some time before the agglomerates began to accumulate on the sea-bottom there had been a fairly uniform deposit of submarine tuffs, evidently in rather deep water. Then followed a period during which the finest mud was deposited which is represented by the thin layer of chocolate-coloured clay. This was succeeded by the deposition of coarser sedimentary tuffs forming a layer about an inch in thickness. Then commenced the accumulation of the agglomerates, of which the materials were at first small and afterwards larger in size.

(4) _The core or axis of volcanic rocks._—This is represented on the summit by masses, 2 to 5 feet across, of two kinds of hypersthene-augite andesite, which are referred to genus 1 of that sub-class. One is a compact grey rock (sp. gr. 2·72) carrying phenocrysts both of rhombic and monoclinic pyroxene, the former prevailing, and displaying a small amount of interstitial glass. It is magnetic and exhibits marked polarity, as noticed in Chapter XXVI. The other is a scoriaceous rock containing numerous round steam-pores, ranging up to 5 millimetres in diameter and generally filled with clear quartz-crystals and lined by chalcedony. It contains semi-opaque glass in abundance, and is apparently a semi-vitreous form of the rock just described. Both rocks are to some extent altered.... On the crest of a spur, 500 feet below the summit, is exposed in position an augite-andesite, assigned to genus 13, sub-genus 1, species B, of that sub-class. It is non-scoriaceous and exhibits a considerable amount of greenish alteration products. (Sp. gr. 2·79.)

THE AVUKA RANGE

This high range, which lies immediately to the east of Lambasa, attains its greatest elevation in Mount Avuka, which is 1,976 feet above the sea. It represents the extension northward to the coast of the inland Thambeyu mountains that culminate in Mount Thurston. In its upper portion Mount Avuka presents bare precipitous faces apparently of agglomerates and some hundreds of feet in height. My acquaintance with this range is scanty. In a traverse from Lambasa to Ngele-mumu I crossed it a mile or more south of Mount Avuka, where it is only 700 feet in elevation. I also rounded the end of the range where it reaches the coast between Lambasa and the valley of Mbuthai-sau. This last locality, which is described on page 218, derives especial interest from the circumstance that here the regions of basic and acid rocks meet. The basic rocks that occupy nearly all the sea-border from Naivaka to Lambasa here become mingled with, and finally give place to, the acid rocks which prevail in all the region eastward as far as Undu Point.

In crossing the range on the way from Lambasa to Ngelemumu, I noticed as high as 450 feet basic non-calcareous tuffs displaying a concretionary arrangement suggestive of the proximity of an intrusive igneous rock. Further up the western slope occur basic agglomerates, whilst at and near the top (700 feet) there lie on the surface large boulders of a dark grey hypersthene-gabbro having a specific gravity of 2·7 and belonging to the type of plutonic rocks described on page 249. It is very probable that this gabbro forms the axis of the range; and we have here no doubt one of the oldest of the mountain-ridges in the island.