Acid Pumice Tuffs
The general characters of these deposits are described on pages [10], [218], [220], [222], [223], [231], [233], &c. Such tuffs are restricted to the north-east part of the island east of Lambasa and Tawaki, and are well displayed in the coast cliffs. They are pale yellow or whitish, and are usually non-calcareous. They are composed of the debris of a vacuolar and fibrillar isotropic glass, nearly colourless and in some localities altered. Small crystals of quartz and of glassy felspar with bits of obsidian (up to 3 mm.) and lapilli of rhyolitic glass are inclosed in them. In places inclosed pieces of coral and coral rock indicate submarine deposition.
CHAPTER XXIV
PALAGONITE
From the sea-border to the mountain-top in almost every part of the island, palagonite occurs in a fragmental condition. It is only where tuffs are not found, as in the mountainous mass of Seatura, or where these deposits are formed of acid rocks as in the north-east portion of the island, that palagonite has not been observed. Perhaps, it is not too much to say that the later if not all the stages in the history of Vanua Levu are bound up with the history of this material. In this place I will only deal with certain features in the problem connected with the origin of palagonite which seem to receive further elucidation from my observations in this island. The literature is already extensive, and those interested in the matter will find in Zirkel’s Petrographie and in the Challenger Report on Deep-Sea Deposits by Murray and Renard a good introduction to the subject.
In Vanua Levu we are confronted with the same difficulty that has perplexed geologists in various parts of the world. If we expected to find in this island the source of the enormous quantities of the basic glass that are represented by the palagonite of the tuffs, we should look in vain. Basic or basaltic glass usually occurs in agglomerates in the form of tachylytic pitchstones, as described on page [312], and is also found at times in basic pumiceous tuffs, as described on page [333]; but it is far from frequent. Palagonite-rock, that is to say, a basaltic glass converted in mass into this substance, never came under my notice.
In order to clear the ground for the discussion of my own observations, I will quote from the report on deep-sea deposits above named. Fragments of basic glass undergoing the palagonite change are found everywhere in these deposits and especially in the red-clay areas. The hydro-chemical modifications determining the decomposition of these fragments into palagonite, and at the same time the formation of zeolites, have likewise resulted in the complete transformation of these lapilli into ferruginous argillaceous matter (p. [309]). The authors, however, of this report do not attribute the frequent occurrence of fragments of basic glass on the bottom of the ocean to the buoyant powers of basic pumice. Unfortunately, the problem does not permit of such a simple solution. Basic volcanic glass, writes Prof. Renard, though known only from a few geological formations and from a few eruptions of recent volcanoes at the surface of the continents, appears in abundance and in most typical form among the products of submarine eruptions, as if the deep oceans had been in some way specially favourable to the development of this lithological type (p. [299]).
The palagonite-tuffs of this island are described in detail in [Chapter XXIII.], and a few general remarks are alone needed here. This altered glass enters into the composition, to a greater or less extent and in varying stages of disintegration, of nearly all the submarine basic tuffs and clays. In the volcanic muds, however, and in the tuffs of mixed character, which are the prevailing deposits, it is associated with other components. Here the question of the origin of palagonite within the deposit does not as a rule arise, since there is nothing to indicate that this material was not derived from rocks previously palagonitised, and the point of main interest is connected with the last stages in the degradation of this substance. There are not a few cases, however, where, unless we assume that the lapilli of vesicular basic glass were ejected in the palagonitic condition from a volcanic vent, we must apparently regard the alteration as having occurred in the tuff. But even this will prove to be by no means a necessary consequence if it can be shown, as I have attempted to do below, that the palagonitic condition exists potentially in a particular type of basic glass and that the effect of hydration is not so much to produce but to make evident a condition that was previously latent.
It will be therefore of interest to determine whether palagonite occurs in this island independently of the tuff-deposits, and under such circumstances that it may be regarded as having been produced within the rock-mass. An example is afforded in the case of a basaltic flow near Soni-soni Island, which is fully described on page [92]. Whilst the lower part of this flow is composed of a hemicrystalline basalt with scanty olivine, the upper portion is made of a basaltic glass which has been broken up or crushed “in situ,” the spaces between the fragments being filled with palagonite. It would seem from the peculiar erosion of the glass fragments that after the crushing a liquid magma occupied the interspaces, and afterwards solidified and underwent the palagonitic change.
Magma-lakelet, ·25 mm. in size, magnified 290 diameters, from a basalt at Navingiri. The groundmass, which is a smoky devitrified glass containing abundant felspar-lathes, is coloured black. The magma-lakelet is pale yellow in the slide and displays concentric lines of congelation. It behaves like palagonite.
In this connection it is noteworthy that in the sections of the lower hemicrystalline portion of the flow there are shown in the groundmass collections of a palagonitic material forming, as I have termed them, “magma lakelets” of microscopic dimensions (·25 mm. in average size). These “lakelets” are irregular in form, and are not uncommon amongst a certain type of basaltic rocks. One of them is figured above; and it may be added that they are best examined when displayed in a groundmass containing much smoky, partly devitrified, glass. They are usually more or less opaque and reddish-brown or yellowish in colour, whilst they have often a marked zoned structure, the concentric bands conforming to the irregular contours of the lakelet. In the least affected stage the zones show fibrous devitrification across their breadth, but as the palagonitic change progresses the material becomes opaque. In the secondary changes, such as those associated with the early alteration of the propylites, these “magma lakelets” are the first affected. They then present alternating layers of calcite and viridite and are often bordered by magnetite.
If these “lakelets” were to be described as collections of residual glass, the description would be insufficient, since they may occur in the midst of a smoky, partially devitrified, glass. During the last stage in the consolidation of the basaltic mass, the magma-residuum that still retains its fluidity collects here and there in the crevices of the groundmass, and forms little pools of usually microscopic dimensions into which the felspar-lathes often protrude from the sides. These little pools or lakelets represent that portion of the yet fluid magma that during the last stage of consolidation is imprisoned in the stiffening mass—like the whey in a cheese—whilst the greater part of it has been squeezed into the cracks of the cooling mass, as occurs in a dyke-like intrusion below described, or has been extruded on its surface, as in the case of the basaltic flow above referred to.
As a suggestive instance of the formation of palagonite “in situ,” I will now refer to a basic tuff-agglomerate on the plateau of Na Savu (see p. [81]) which is penetrated by veins, a few inches thick, apparently composed of a finely brecciated pitchstone-tuff. In the section the material forming the veins is seen to be composed of fragments of basic glass (carrying porphyritic plagioclase and augite) which have been crushed in position, the interspaces being filled up with the finer debris of the glass and of the minerals together with palagonitic material. The glass fragments, which have lost their sharp edges and angles, are often palagonitised at the borders, and we thus get a patch of isotropic brown glass with a yellowish margin formed of a feebly refractive turbid substance. Where this border is not so evident, it is noticed that the edge of the glass is peculiarly eroded. The indication appears to be that the fissures in this agglomerate were filled with a basic magma that after its solidification into a glass was subjected to a crushing process, and that during this process a partial remelting of the glass took place which resulted in the molecular change characteristic of palagonite. Since the unaltered glass-fragments fuse in the ordinary flame, it would seem that the heat developed during the crushing might be sufficient to partially remelt the glass without affecting the rock penetrated by the veins.... It is of importance to note that in the palagonite-tuffs of the Canary Islands the change is often most complete along fissures, which thus present the appearance of being occupied by veins of pitchstone.[[127]]
In this connection allusion may be made to a dyke-like mass of a rubbly semi-vitreous basaltic rock exposed at Vatu-lele Bay, described on page [184]. It is penetrated in all directions by veins, 1 to 3 inches thick, of a tachylytic glass which begins to fuse in the ordinary flame. The glass is traversed by cracks which sometimes contain palagonite. The basalt, penetrated by the veins, has a smoky groundmass displaying imperfect felspar-lathes in a feebly refractive glassy base and containing a few small “magma-lakelets” that cannot be distinguished from palagonite.[[128]]
Near the mouth of the Narengali valley (see page [149]) I found what appears to be a palagonite-tuff overlain by agglomerates formed of tachylytic pitchstone and of semi-vitreous amygdaloidal basalts. The tuff consists of fragments of a brown basic glass, the larger 1 to 2 millimetres in size, carrying porphyritic plagioclase, and fractured in position, the interspaces being filled with palagonite. The glass fragments possess the eroded margins indicated in the accompanying figure. It may be remarked that this type of tuff differs from that of the prevailing palagonite-tuffs in being rarely vacuolar, in the absence of marine organic remains, and in its homogeneous composition. It is described on page [334] under the head of “crush-tuffs.” Whether it is derived from the destruction of a mass of basic glass that had previously undergone crushing and partial palagonisation I cannot say; but its characters point in the direction of this conclusion.[[129]]
In the foregoing pages it has been attempted to show that palagonitisation has taken place in the veins of basaltic glass traversing in one case a basic tuff agglomerate and in another case an intrusive basaltic mass, and that it has also occurred in the upper vitreous portion of a basaltic flow and in the materials now composing a so-called “crush-tuff.” In order to explain this group of facts I venture to propose this theory.
In certain types of basaltic lava,[[130]] when cooling and consolidation take place under peculiar conditions, such as we would expect to find in submarine eruptions, there is a residuum of the magma with relatively low fusibility that remains fluid after general solidification of the mass is well advanced. As the rock continues to consolidate, portions of this magma residuum become imprisoned in the mass, like whey in a cheese, giving rise to the “magma lakelets” above described; whilst other portions, during the contraction and fissuring accompanying the cooling process, are squeezed out into the cracks thus formed, or are intruded on the surface of the consolidating mass, as in the case of a submarine lava-flow. This solidified magma-residuum differs from the ordinary basic glass not only in its lower degree of fusibility but in its mineral composition and in its molecular condition. It probably in the first place does not differ much in appearance from the typical glass, but it is an unstable substance and is capable under certain hydro-chemical conditions of developing the characters of palagonite.
Showing fragments of glass with eroded borders and of plagioclase with more even edges in a matrix of palagonite traversed by cracks. The length of the largest fragment is half a millimetre. The glass has been evidently fractured in position and this is true of one of the felspar fragments. It is also apparent that whatever its cause the erosion of the margins of the glass has been produced since the fracture.
In those cases where the occurrence of palagonite is associated with evidence of crushing, the process appears to be in a sense reversed, since partial palagonitisation of an ordinary basic glass takes place as a result of the elevation of temperature due to the crushing. The heat thus developed is sufficient to partly fuse the glass; but since it is not great, it only affects the most fusible constituents, and the remelted material corresponds therefore to the magma-residuum of the consolidating mass, which is referred to in the previous paragraph. It has the same unstable characters and the same tendency to assume the palagonitic condition.
This theory centres around the relatively low fusibility of the magma-residuum that gives rise to palagonite. This degree of fusibility has yet to be ascertained, since according to the views here advanced it may even be much lower than that of tachylyte. It is, however, noteworthy that the melting-point of tachylyte is far below that of the more crystalline basaltic rocks, since it can be readily determined, as I have done in the instance of a dyke-like mass penetrated by tachylyte-veins before referred to, that the veins are composed of a much more fusible material than the rock-mass. From a very crude experiment I would infer that the melting-point of ordinary tachylyte is not much above that of lead (335° C). The fusion-point of an ordinary hemicrystalline basalt, according to the well-known experiments on the lavas of Vesuvius and Etna, would probably be over 1,000 degrees C.
Two interesting experiments, the one artificial, the other natural, may be here cited in connection with this view. Bunsen[[131]] more than half a century ago, as a result of some experiments in which he produced palagonite, arrived at the conclusion that the tuffs formed of this material are submarine deposits derived from the breaking up of previously formed palagonite-masses. Having obtained this substance by placing powdered basalt in an excess of melted potash-hydrate (Kalihydrat) and then adding water to the silicate of potash thus formed, he concluded that palagonite results from the reaction between glowing augitic-lavas and rocks rich in lime and other alkalies. Although Zirkel quotes in this connection the example where this material has been produced in the Cape de Verde Islands by basic lava flowing over limestone, he rejects Bunsen’s explanation as inapplicable to extensive palagonite districts, such as occur in Iceland, though allowing that it would account for the local production of this substance.
I venture to think, however, that in these two experiments the general principles involved in the production of palagonite are partly illustrated. We may accept the results of an experiment without acquiescing in its interpretation. As I take it, it is in the partial wet fusion of the powdered basalt that the secret of this successful production of palagonite lies. In both these experiments some of the conditions of a submarine flow have been reproduced.
Whilst Rosenbusch established the true character of palagonite as the product of a peculiar alteration of a basic glass, Renard pointed out the conditions under which it was most typically and in greatest abundance formed. But Bunsen was happy in his suggestion that palagonite-tuffs are submarine deposits derived from the breaking up of previously formed palagonite masses. The question thus resolves itself into one concerning the conditions of submarine eruptions and the behaviour during consolidation of a submarine basaltic flow. In the nature of things the field of investigation is mainly restricted to the examination of ancient submarine basaltic flows that have been raised above the sea.
A remarkable series of beds exposed in a stream-course below the Nandua tea-estate may be here described in connection with the question of the origin of palagonite formations. As observed on page [86], this locality lies on the flanks of a basaltic plateau, which are incrusted with recent submarine deposits. A pteropod-ooze, containing also the tests of large and small foraminifera and the shells of small bivalves, is displayed on the sides of the stream-course for the first 150 feet of the descent. Below this, as shown in the diagram, is a declivity with a drop of 60 or 70 feet where there is a waterfall. Horizontal beds of the pteropod-ooze rock are exposed in the upper-third of this declivity; but below, they pass into a chocolate-coloured marl-like deposit also horizontally bedded, and sometimes having a banded appearance from the alternation of layers of different degrees of fineness. This rock contains 5 or 6 per cent. of carbonate of lime and incloses a few scattered tests of minute foraminifera of the “Globigerina” and “Nodosaria” types. In the slide the rock appears to be of massive palagonite inclosing a few felspar-lathes ·1 to ·3 mm. in length, and exhibiting a zeolite and calcite in the crevices and cracks. But it was not until I had discovered the tests of the foraminifera and had observed some fragments of larger crystals of plagioclase and a little detritus of a semi-vitreous basaltic rock that its clastic character was disclosed. The palagonite change has here to a great extent disguised the character of the deposit.
This palagonite-marl formation is 20 or 30 feet in thickness. It passed downward into a reddish-brown rubbly unstratified rock which falls to pieces in one’s hands, breaking up into little cube-like masses an inch or two across. These masses display in their interior a radiate prismatic structure; but after drying they crumble into small fragments exhibiting the same minute prismatic structure, the miniature prisms being about a millimetre in diameter. Fine cracks, filled with calcite and a zeolite, traverse this rock in all directions, and no doubt this peculiar structure arises from shrinkage.
My idea that I was dealing with a clay-rock affected by the proximity of an igneous intrusion was dispelled when the powdered material presented itself as pure palagonite with scarcely any mineral fragments. Unlike the marl above, it does not effervesce with an acid; and appears as a mass of compacted minute fragments of basic glass converted into palagonite, which is seemingly non-vacuolar, and containing about 15 per cent. of water.
In connection with the diagram it should be remarked that I did not find the palagonite-rock actually passing down into the basalt which, however, is exposed in the river-bed below. The whole district is characterised by columnar basalt, and the series of deposits here described have been formed on the flank of the great basaltic table-land of Wainunu. It is noteworthy that in the uppermost deposits of the pteropod-ooze palagonite forms a noticeable proportion (10-20 per cent.) of the residue; and perhaps most of the fine clayey material is thus derived. As noted on page [321], minute pellets of pure palagonite are not infrequent in the residue. Probably about 90 per cent. of the underlying marl consists of palagonite. In the lowest palagonite-rock the proportion would be quite 98 per cent.
Diagram showing the succession of deposits below the Nandua tea-estate. The total thickness is about 250 feet. The figures refer to the proportion of palagonite.
Whilst it is apparent that we have represented in this series the covering of a submarine basaltic flow with submarine deposits, it is also evident that the mode of junction between the flow and the overlying deposits is of an unexpected nature. Before drawing any inferences, it is necessary to point out that when we begin on à priori grounds to frame our notions as to the course of events on the surface of a submarine basaltic flow, we are entering a little known region of inquiry. I would, however, suggest in the light of the theory before advanced, the following explanation of the appearances presented by this series.
During the consolidation of the flow much of the magma-residuum that still retained its fluidity was extruded on the surface, where after solidification it became palagonitised. According to my view this would be the typical behaviour of submarine basaltic flows; but, owing to the unstable and perishable nature of the palagonitic crust of the flow, it would be rarely preserved in upheaved volcanic regions. There would probably be, as in the case of the Nandua series, no sharp line to be drawn between the palagonite-crust and the deposits subsequently covering it, deposits indeed that would derive no inconsiderable proportion of their materials from the disintegration of the crust itself. During and after the emergence of such a district of submarine eruptions the unstable palagonitic crust would be further subjected to the hydration resulting from weathering and similar agencies; and as a result of its final degradation there would often alone remain a bed of reddish argillaceous material.
In concluding these remarks on palagonite the following summary of the principal points here dwelt upon may be added:
(a) The basic glass, that undergoes the palagonitic change, is the vitreous form of the magma-residuum that in a particular type of basalt and under certain conditions remains fluid after the mass of the rock has solidified. During the last stage of the consolidation it is in part imprisoned in the “magma-lakelets” of the groundmass; whilst the rest of it is squeezed into cracks and fissures, or extruded on the surface of the flow.
(b) This glass differs from ordinary basic glass in its molecular condition, its mineral composition, its low degree of fusibility, and in its unstable character.
(c) The formation of palagonite in connection with the crushing of a basic glass is to be explained by the hypothesis that the heat developed during the crushing is sufficient to partially re-fuse the glass, the material thus produced corresponding to the magma-residuum of low degree of fusibility, which is above referred to.
(d) In submarine eruptions are to be found the conditions favouring the production of palagonite on a large scale. In the case of such basaltic flows it is probable that their upper portions are formed entirely of palagonite arising from the alteration of a vitreous magma-residuum extruded on the surface in the manner above described. Such a crust, as a result of shrinkage and other processes, would probably present itself to the geologist as a somewhat friable material, passing gradually into the overlying submarine deposits.
Note on the type of basalt found associated with palagonite.
The type is characterised, it would appear, rather by its structural features than by its mineral composition. It is the basalt of ophitic or semi-ophitic habit that would seem to be usually associated with palagonite; and since this habit is as a rule to be found where the groundmass displays large felspar-lathes in plexus arrangement, coarse augites, and at least a fair amount of smoky glass, it follows that a hemi-crystalline, ophitic or semi-ophitic, doleritic basalt is the type to be associated with palagonite.
This is the type of rock that forms the lower part of the basaltic flow near Kiombo, the upper part of which is largely palagonitic. To this structural type also belong most of the basalts in my collection where palagonite exists in the form of “magma-lakelets” in the groundmass. These “lakelets” are almost diagnostic of this type of basalt. Here also belongs the famous globular basalt of Acicastello on the coast of Sicily.[[132]] In such rocks the felspar-lathes form a mesh-work and vary usually in average length between ·1 and ·3 mm. The augites of the groundmass, typically semi-ophitic, range up to ·1 mm. in size. They are always large, that is, over ·03 mm., and this coarseness is another important indication.