Submarine Basic Tuffs of Mixed Composition
These tuffs, which are composed not only of palagonitic materials but also of the fine detritus of usually semi-vitreous basic rocks, rank first in frequency amongst the volcanic sedimentary deposits of the island. In their character they pass on the one hand into the foraminiferous volcanic mud-rocks or clay rocks and on the other into the tuff-breccias and tuff-agglomerates. We have here a series beginning with the agglomerate and ending with the clay that represents in a general sense the successive stages of the degradation of the same materials.
These tuffs occur at all elevations from the sea-border, where they may form the shore-cliffs, to the upper slopes and summits of the mountain-ranges, where they are found at elevations between 2,000 and 2,500 feet above the sea. In the interior of the island they are generally to be observed underlying the basic agglomerates. Wherever an extensive exposure of the agglomerates exists in the mountainous districts, these tuffs are as a rule to be found at the base of the cliffs. The precipitous bluff of agglomerate, that so often gives a character to the mountainous interior, and the line of cliff of the same formation that runs along the slopes, represent the work of landslips, as is shown by the huge masses of agglomerate lying on the ground below. These “slips” are not uncommon, and are due to the undermining influence of the springs that percolate through the tuffs and clays underlying the agglomerates.
When the tuffs are well displayed they as a rule show stratification. The bedding may be indicated either by distinct parting-lines or by alternating bands of varying degrees of coarseness. That these deposits, when occurring in mass in the upland regions, are often horizontal or but slightly inclined, is evidenced by the Nganga-turuturu Cliffs, 1,200 feet above the sea, which are described in [Chapter VIII.], in the line of cliffs behind Sealevu ([Chapter XI.]), and high up the slopes of Mount Thambeyu ([Chapter XII.]) as high as 1,500 feet. This is also shown in the circumstance that the line of junction with the overlying agglomerate, except in rare cases, as in that of the Mbenutha Cliffs, is generally horizontal. It is, however, not uncommon to find the beds exposed on the mountain-flanks dipping away from the axis of the range at a small angle, as on the slopes behind Mbale-mbale and in the Sokena Cliffs. In the lower regions, where these deposits are associated with the volcanic mud-rocks on the basaltic plains, they are but slightly inclined. On the other hand, as in the Kumbulau district, the sea-cliffs for some miles may be composed of tuffs more or less steeply tilted.
These tuffs are generally more or less compacted and have a greyish colour; but as usually exposed in a weathered condition they are often pale brown or yellowish and are more friable. They may be uniform in structure, or they may display thin seams of a marl-like clay, or they may contain numerous lapilli of vesicular basic glass extensively palagonitised. Not uncommonly they contain larger fragments of basic rocks, and when these are at all frequent the terms “agglomerate-tuff” or “tuff-agglomerate” have been employed according to the preponderance of either material.
Many of these tuffs show no effervescence with an acid; and this is especially the case with specimens at all weathered. On the other hand there are just as many that contain a little carbonate of lime, not usually more than 3 or 4 per cent., but sometimes amounting to 12 or 13 per cent. It often happens in the case of a series of tuffs, apparently non-calcareous, that an occasional thin band of a fine clay-like rock contains a good percentage of lime. It is pointed out below, however, that the absence of effervescence does not necessarily imply the absence of foraminiferous tests.
Tests of foraminifera, often macroscopic bottom forms, together with shells of small gasteropods, are displayed at times; but they are as a rule in such cases not frequent. I found foraminiferous tuffs at considerable heights in some localities, as for instance between 2,000 and 2,500 feet on the slopes of Mount Thambeyu (page [178]), at an elevation of 1,850 feet on the south slope of the Korotini Range above Vatu-kawa (page [158]), and between 2,000 and 2,400 feet on the summit of the range between Waisali and Sealevu (page [154]). In the last-named locality, where the tuffs are coarse and often of the nature of agglomerate-tuffs, they are highly fossiliferous; but such a character is exceptional.
The submarine origin of the tuffs can often be demonstrated in the absence of evidence of organic remains, as by their interstratification with foraminiferous clay rocks, such as we find at an elevation of 1,000 to 1,100 feet on the top of the “divide” between the Ndreketi and Lambasa basins. A single seam of marl-like rock displaying only a solitary test of a foraminifer in the slide may throw light on the origin of the coarser tuffs with which it is associated. The use of the microscope is essential in the case of some of the harder tuffs, where there has been a little alteration. Here casts of foraminifera may be observed, although no carbonate of lime is indicated by an acid. In some localities where no organic remains are evident in the tuff, fine waterworn gravel is to be noticed.
These deposits are composed as a rule of sub-angular fragments of semi-vitreous basic or basaltic rocks and of palagonite, together with fragments of plagioclase and pyroxene, the interspaces being filled with fine debris of the same materials. The relative proportion, however, of the three principal constituents varies considerably, the palagonite, for instance, being sometimes scanty and sometimes abundant. The size of the larger fragments in a tuff of the most common kind is about a millimetre; but deposits rather finer and rather coarser are also frequent. In the very coarse tuffs and in the breccia-tuffs, where the larger materials are mostly of palagonite, the larger fragments may be a centimetre in size and even more, the interspaces being filled with fine debris of the same character cemented together often by carbonate of lime.
The fragments of semi-vitreous basic rocks forming a regular constituent of these tuffs are usually dark and opaque and display a few plagioclase lathes. They correspond with the type of the semi-vitreous basalt or basaltic andesite, of which the blocks of the overlying agglomerates are as a rule composed and are doubtless derived from the same source. Fragments of unaltered basic glass are rarely to be observed in these tuffs. It is as a rule all converted into palagonite. This material presents itself in various stages of secondary alteration, from the compact greenish or yellowish waxy mass to a white friable pulverulent substance, which represents the last stage of degradation. These changes will be found described on page [348]. It can, however, be stated here that they are mainly concerned with hydration. In the case of the lapilli of finely vesicular basic glass, that is, of basic pumice, which are inclosed in some of the tuffs, all stages of the secondary alteration of palagonite are often exhibited, and the last stage of the change is merely indicated by a white powdery patch containing a few minute siliceous amygdules. The puzzling little white patches so common in basic tuffs merely represent lapilli of basic pumice that have been palagonitised, and then bleached and disintegrated by hydration.
The minerals are more or less abundant and may constitute a third of the whole mass. They include plagioclase, augite, rhombic pyroxene, and magnetite, olivine being rare and scanty. Entire crystals of any size are infrequent. However, crystals of augite, 5 or 6 mm. in length, are found in the tuffs at Naivaka and of the coast cliffs near the Salt Lake Passage.... It may be observed that zeolitic minerals which are frequently developed in the tuffs consisting almost entirely of palagonite are not typical of the tuffs of mixed composition.
There is nothing suggestive of recent eruptions in any of these formations. They were formed ages since on the sea-floor at varying depths around volcanic vents. Sometimes a cone was able to rear itself above the level of the sea; but in most cases it rapidly succumbed to breaker-action. Three agencies, concerned with sub-aerial eruptions, submarine eruptions, and marine denudation, have co-operated in the production of these deposits, but their parts in the process have varied greatly. The last is indicated when the tuff is formed of a variety of basic rocks with but little palagonite. The tuffs containing much palagonite representing an original vacuolar basic glass are regarded as mainly the products of submarine-eruptions. In those cases where lapilli of altered basic pumice occur in the deposit, sub-aerial eruptions are directly indicated. When an extensive exposure of these tuffs occurs, as in the case of the Nganga-turuturu Cliffs and in that of the section displayed near the hill of Korolevu (page [48]), all three agencies are often illustrated.