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 | ||||||