(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.
A careful search of the tuff-deposits in this part of the Undu promontory ought to result in the discovery of remains, both of plants and of marine mollusks. Mr. Chalmers informed me that fossilised tree-trunks occur on the coast near Vunikondi; but I was unfortunately not able to discover them.
Brief Summary of the General Characters of the Undu Promontory from Thawaro and Tawaki to Undu Point.—One suggestive negative feature of this region is to be found in the absence, as far as I could ascertain, of any trace of a crater. Here also, as in the area of acid rocks extending westward along the north side of the island to near Lambasa, hot springs are not to be found. The prevailing rocks are in the first place the pumice-tuffs, which not only as a rule form the coast-cliffs, but occur inland as high as the summit of Mount Thuku almost 1,300 feet above the sea. They were probably in great part ejected from sub-aërial vents, though no doubt, as in the vicinity of Undu Point, they were often deposited under the sea. The acid and basic tuffs in the vicinity of Tawaki and Thawaro are, as I imagine, largely derived from marine degradation. Next to the pumice-tuffs, massive quartz-porphyries and oligoclase-trachytes are the characteristic rocks. They are probably in most cases intrusive, and present themselves sometimes as vertical columnar dykes, evidently of considerable dimensions.
The basic rocks, however, are not unrepresented. They occur in one or two places as agglomerates, as in the vicinity of Mount Thuku on the north coast and near Undu Point; whilst they form “flows” overlying the pumiceous tuffs at Vunikondi. Occasional blocks lying on the surface on the north coast are indications of dykes. The basic rocks, nevertheless, take a very secondary part in the composition of the Undu Promontory. They are in most cases to be referred to the augite-andesites with a specific gravity 2·6 to 2·77; but some, as in the case of those forming the agglomerates of Thawaro Peak, are hypersthene-augite andesites with specific gravity of about 2·5. Olivine-basalts are not represented.
The vents, from which the materials forming this promontory were ejected, were arranged in a single straight line for a length of 14 miles. Along this linear fissure, which was probably submarine, vents were at different times formed; and though owing to sub-aerial and marine degradation the present surface has been since shaped and reshaped, their situation may still be recognised in the “necks” of tuff and agglomerate that form the peaks, and in the large dykes or sills of quartz-porphyry and oligoclase-trachyte.
CHAPTER XVII
THE VOLCANIC ROCKS OF VANUA LEVU
The varied character of the volcanic rocks in my collection is brought out in the following Table, where I have grouped about 400 rock-sections, excluding those of the tuffs and finer detrital deposits. The small proportion of plutonic rocks should be noted.
| Olivine-basalts | 23 | per | cent. |
| Augite-andesites | 40 | " | " |
| Hypersthene-augite-andesites | 17 | " | " |
| Acid andesites, including hornblende and quartz-andesites, &c. | 12 | " | " |
| Oligoclase-trachytes, quartz-porphyries or rhyolites | 6 | " | " |
| Hypersthene-gabbros and diorites | 2 | " | " |
| 100 |
In order to avoid the necessity of frequently describing rocks of the same type it has been found requisite to devise a method of classification. In carrying out this somewhat laborious task I have often been surprised at the readiness with which rocks, the relations of which had been previously very difficult to ascertain, fell into their place in the scheme. Although many of my uncertainties have been thus removed, a large number of doubtful points remain. I venture to think, however, that others may be able to employ and also to extend the method of classification here employed.
The general plan followed has been worked out in detail for the olivine-basalts and the pyroxene-andesites of the more basic type; whilst lack of materials has prevented its further elaboration in the case of the acid andesites, oligoclase-trachytes, quartz-porphyries, &c. The treatment of all the classes has been uniform, the scheme being the same whether applied to a basalt or to a dacite.
In describing the general method I will take the Augite class. As will be seen in the Synopsis that follows, this class is first divided according to the absence or presence of a groundmass into two sub-classes, the first referring to the plutonic rocks, which, however, are not represented in my collection, the second comprising the augite-andesites which make up 40 per cent. of the total. These andesites are again divided into four orders, according as the groundmass presents parallel or non-parallel felspar-lathes, or short and stout felspars (orthophyric), or displays a felsitic character. The last two orders are practically unrepresented here, though many examples of them are found amongst the more acid andesites. Each order is then split up into three sub-orders depending on the nature of the pyroxene of the groundmass, whether granular, prismatic, or ophitic.
Each sub-order is broken up into two sections, one displaying plagioclase-phenocrysts, the other without them, or possessing very few of them. The first section is divided into two genera, according to the character of the plagioclase-phenocrysts, whether glassy or opaque, the second genus often comprising rocks allied to the porphyrites. The second or aphanitic section is subdivided into two genera according to the character of the pyroxene-phenocrysts; in the one case they are macroporphyritic; in the other they are either small or absent. The genera are split into four species according to the length of the felspar-lathes, a method which readily separates out the doleritic rocks. In cases where the materials are abundant, the genera have been first divided into porphyritic and non-porphyritic sub-genera, based on the macroporphyritic or the micro-porphyritic character of the plagioclase-phenocrysts, when present. The species can be also split into sub-species, according to the degree of basicity of the rocks, as indicated by the specific gravity.
This method is fully worked out in the later pages and need not be further described here. With abundant material from different regions it appears to me that a ready mode is here afforded of assigning to a rock its place in the scheme. In this way it would be possible to follow the systematist in his method of comparing plants and animals from different localities. To facilitate this end, I have suggested in the synopsis the employment of abbreviations, so that the description of the critical characters of a rock can be condensed into a formula capable of easy interpretation.
As an example of the use of these abbreviations I will take the instance of a common form of augite-andesite which is represented by the formula:—“Plag, aug, matr, flu, gran, non-phen, parv, ·1-·2 mm.” This is the formula for an aphanitic augite-andesite, and it signifies that it is a rock of the plagioclase-augite class possessing a groundmass showing parallel felspar-lathes (between ·1 and ·2 mm, in average length) and granular pyroxene, and displaying no phenocrysts of plagioclase or only a very few of small size, whilst pyroxene phenocrysts if present are micro-porphyritic.
As another example the following formula for a type of porphyrite found in this island may be given:—Plag, hypersth-aug, matr, orth, prism, phen, opac. This is an andesite in which rhombic and monoclinic pyroxene are associated both in the phenocrysts and in the groundmass where the pyroxene is prismatic and not granular. The plagioclase phenocrysts are opaque and the felspars of the groundmass are of the orthophyric type.
There are one or two points that require further reference. In the first place the early employment in the scheme of the characters of the felspars of the groundmass for distinguishing the orders scarcely seems needed in the cases where they take the lathe-form; but the importance of its early use is shown in the acid andesites where it is certainly of prime importance in an early stage of the classification to distinguish the rocks by the character of the felspars of the groundmass, whether lathe-like, orthophyric, or felsitic. It may also be objected that the two orders obtained by dividing the lathe group into “parallel” and “non-parallel” divisions are not equivalents of the two other orders, the orthophyric and the felsitic. The distinction, however, between the flow or non-flow arrangement, though in practice not always readily established, is a far-reaching one. On à priori grounds the first division might be expected to have no plutonic equivalent; whilst in the second division, it is easy to trace the gradations through the doleritic stage, where the felspar-lathes are very large, to the granitoid condition. Then, again, the ophitic habit is as a general rule confined to rocks with a doleritic or semi-doleritic groundmass, where the felspar-lathes are coarse and form a mesh-work. Two quite distinct lines of development unite in the felspar-lathes and begin to diverge with the difference in their arrangement in the groundmass.
The nature of the difference between the flow and non-flow arrangement of the felspar-lathes is well brought out in some dykes of basalt and augite-andesite that I examined in this island and also in the Valle del Bove on the Etna slopes. In the outer vitreous portion the felspar-lathes, which are fairly well represented, are all about the same length and are more or less parallel with the sides of the dyke. In the central more crystalline portion two sets of lathes can be distinguished, one (A) corresponding in the average length and in the flow-arrangement with the lathes of the borders, the other (B) being about half the length and forming a plexus between the larger parallel lathes. Those of the A set, which are those that usually catch the eye in a section, are contemporaneous in their origin with those in the margins of the dyke; whilst those of the B set have been subsequently formed. In the preliminary “stiffening” of the first stage of consolidation, the whole mass of the dyke would be affected. To this stage the lathes of the A set belong; whilst to the later stage of consolidation which would proceed much more slowly in the interior than at the margins of the dyke, the lathes of the B set are to be referred. This distinction so plainly illustrated in a dyke must be postulated for all intrusive masses; but I have not yet found it possible to make much use of it. Much ground will have first to be cleared before it can be safely employed, since it is apparent, for instance, that there are often all gradations in a slide between a lathe and a phenocryst, and that the term “phenocryst” is applied to crystals having very different histories.
Diagram illustrating the two sets of felspar-lathes in a dyke: A, long and parallel, dispersed through the mass. B, short and non-parallel and found only in the centre.
With regard to the ophitic habit of some of the basaltic rocks the following conclusions may be drawn:
(a) Typical ophitic “plates” are not very common in the slides. More frequently the habit of the pyroxene is semi-ophitic.
(b) This character is as a general rule associated with the plexus or non-fluidal arrangement of the felspar lathes.
(c) The felspar-lathes are nearly always large, frequently averaging more than ·2 mm. in length. This coarse doleritic groundmass is almost diagnostic of an ophitic rock.
It is not always possible to allow for the influence of locality in drawing up such a classification as this, since it is well known that in each volcanic region the rocks have a particular facies recognisable in hand-specimens as well as in the slide, though it is not easy to express such a distinction in a definition. Perhaps this is represented in “adaptation” as we find it in the organic world, and the question arises as to the value of such characters for critical purposes. Regional variation plays such an important part that it cannot be ignored in rock-classification.