Foraminiferous Volcanic Mud-rocks
These deposits, which represent the “volcanic muds” forming around the coasts of volcanic islands, are more or less consolidated clay-rocks. They contain in varying numbers the tests of foraminifera with occasionally pteropod-shells. The former are usually minute and of the “Globigerina” type; but in some rocks larger bottom-forms prevail. The original colour of these deposits is bluish-grey, but as generally displayed they are pale-brown and considerably affected by hydration and are known as “soapstones” in the group. The ultimate effect of exposure is the production of a whitish or yellowish soapy rock that has lost all the carbonate of lime and all the organic remains and breaks down easily in the fingers. Such a crumbling material, when examined with a high power, cannot be distinguished from the products of the final disintegration of palagonite as described on page [348].
These deposits are frequently displayed in the lower regions up to elevations of 300 feet above the sea, incrusting the basaltic plains of Lekutu, Sarawanga, Ndreketi, and Lambasa, that occupy such a large area of the north side of the island, and exhibiting there in the prevailing horizontality of their beds the same indication that is presented by the vertical position of the columns of the under-lying basalt, namely, the comparative absence of disturbance during the emergence. At the foot of the mountains these deposits are interstratified with and finally overlaid by coarse palagonite-tuffs, also containing marine remains; and these are in their turn covered over by the agglomerates that often enter so largely into the composition of the mountainous backbone of the island. Such beds form apparently the lowest of a series which begins with the foraminiferous clay and ends with the agglomerate. But in some places, as has been noticed in the cases of the pteropod-ooze rock of Nandua, and of the shelly and foraminiferous limestone of Tembe-ni-ndio, beds composed largely of characteristic palagonite lie beneath.
In the elevated interior of the island these volcanic mud-rocks are usually concealed by the tuffs and agglomerates. Occasionally, however, they are to be seen exposed by landslips high up the flanks of the mountains, as on the slopes of Thambeyu 1,000 feet above the sea. An interesting exposure of them is displayed in the heart of the island in the face of the Mbenutha cliffs where the elevation is about 1,100 feet. Here they are overlaid by a thick bed of agglomerate; and tuff-beds largely made up of vacuolar basic glass debris and showing a few foraminifera are interstratified with them; but they exhibit signs of considerable disturbance (see page [109]).
These deposits contain between 5 and 25 per cent. of carbonate of lime, and as a rule about 90 per cent. of the residue consists of fine clayey material derived from the final degradation of palagonite and of basic rocks. The mineral fragments (plagioclase, augite, rhombic pyroxene, and occasionally hornblende) vary much in amount, their average proportion being 13 or 14 per cent. of the mass. Their size is usually less than ·2 mm. and does not exceed ·4 mm. Casts of foraminifera are nearly always present in the residue and form generally 3 or 4 per cent. of the whole deposit. Sometimes they are black and glauconitic; but more frequently they are white and composed of chalcedonic silica. Such casts represent on a small scale the results of the same silicifying operation to which the flints and silicified corals that occur so frequently on the surface in some localities owe their origin (see [Chapter XXV.]).
With regard to the age of these volcanic mud-rocks of Vanua Levu, it is most likely that as in the case of similar deposits in Viti Levu, which were examined by Mr. H. B. Brady, they are of post-tertiary origin. Samples of the Suva “soapstone” containing 5 or 6 per cent. of lime and displaying shells of foraminifera, pteropods, and other molluscs, were obtained from different heights up to 100 feet above the sea. Since 87 out of the 92 species of foraminifera represented in the deposits are known to be living now in the Pacific, Mr. Brady had no hesitation in assigning the beds to the post-tertiary epoch.[[126]]
Samples of the Volcanic Mud-rocks
A. From districts west of Ndranimako, 100 feet above the sea.
| Carbonate of lime | 20 | per | cent. | |||
| Residue | Fine debris of palagonite and semi-vitreous basic rocks | 62 | " | " | ||
| Minerals | 14 | " | " | |||
| Casts of foraminifera | 4 | " | " | |||
| 100 | ||||||
The organic remains consist mainly of tests of minute foraminifera of the “Globigerina” type, casts of which, both glauconitic and chalcedonic, occur in the residue. About 88 per cent. of the residue consist of fine clayey materials less than ·25 mm. in size. The mineral fragments, which average about ·1 mm. in diameter, are mostly of felspar with a little pyroxene and brown hornblende.
B. From the Mbenutha Cliffs, 1,100 feet above the sea.
| Carbonate of lime | 15 | per | cent. | |||
| Residue | Fine material mainly derived from the degradation of palagonite | 60 | " | " | ||
| Minerals | 23 | " | " | |||
| Casts of foraminifera | 2 | " | " | |||
| 100 | ||||||
This rock is somewhat hard, so that the proportion of fine clayey material, which is however large, cannot be accurately determined. It shows in places dark streaks composed of an abundance of minute and often perfect tabular crystals of zoned plagioclase and prisms of rhombic pyroxene, the size in neither case exceeding half a millimetre, both of them being derived from the acid andesites of the neighbourhood. In the slide it displays minute tests of foraminifera of the “Globigerina” type in a matrix formed mainly of palagonitic debris, fragments of minerals and semi-vitreous basic rocks. The larger fragments of the minerals and of the volcanic rocks do not exceed ·15 mm.; but most of the material is very fine. The tests of the foraminifera are sometimes filled with the matrix, but often they are entirely of calcite and exhibit in polarised light a dark cross.
C. From between Natua and Mbatiri, about 290 feet above the sea.
| Carbonate of lime | 25 | per | cent. | |||
| Residue | Fine material derived from the degradation of palagonite and of semi-vitreous basic rocks | 62 | " | " | ||
| Minerals | 2 | " | " | |||
| Casts of foraminifera | 11 | " | " | |||
| 100 | ||||||
This is a relatively deep-water deposit, the foraminifera being minute and of the “Globigerina” type. About 90 per cent. of the residue consists of fine clayey material, with which the calcite is so intimately mixed that each particle is highly refractive and effervesces freely in an acid. The mineral fragments (pyroxene and felspar) are very scanty, the largest being less than ·25 mm. The white casts of foraminifera, composed of chalcedonic silica, form a conspicuous elements in the residue.
D. From the vicinity of Mbatiri, 100 feet above the sea.
| Carbonate of lime | 4 | per | cent. |
| Fine material derived from the degradation of palagonite | 90 | " | " |
| Minerals | 2 | " | " |
| Casts of foraminifera | 4 | " | " |
| 100 |
About 94 per cent. of the residue consists of fine clayey material. The fragments of minerals are very scanty and are all less than ·2 mm. in size. The casts of foraminifera are white and of chalcedonic silica. From the fineness of the materials and the small size and pelagic character of the foraminifera, this deposit may be regarded as formed in relatively deep water.
E. From the eastern flank of the Wainunu table-land, 200 feet above the sea.—This is a shallow-water deposit and contains, besides small gasteropod shells, large flat tests of foraminifera 5 or 6 mm. in diameter. It possesses 24 per cent. of carbonate of lime, 62 per cent. of palagonitic debris, &c., and 14 per cent. of minerals.