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.
Altered Volcanic Mud-rocks
This group includes compact hard foraminiferous usually dark-brown rocks, which exhibit evidence of alteration in their induration, in the presence of pyrites, and in the chalcedonic quartz filling fine cracks in the rock-mass. Occasionally special types of alteration occur, one of which will be referred to in the description of some of the rocks given below. The proportion of carbonate of lime is generally small; but sometimes it amounts to 10 per cent. or more.
They admit of being examined in thin sections; and their true nature is at times so much disguised that I have taken them at first for aphanitic basic andesites. In the slide they display a few scattered tests of foraminifera of pelagic habit in a matrix composed of the fine debris of palagonite and of basic rocks, together with fragments of plagioclase and pyroxene. Most of the material is very fine, and the size of the largest mineral fragments does not exceed ·2 mm.
Such rocks, however, are not very frequent. They may be displayed on the flanks of mountain-ranges buried beneath basic tuffs and agglomerates, as in the case of Mount Mariko (page [187]) and of the mountain-slope behind Mbale-mbale (page [158]); or they may be found at much lower levels as at Savarekareka Bay (page [190]), where, however, they assume sometimes a peculiar character. Though in the last-named locality the alteration is possibly connected with thermal metamorphism, it is probable that in most instances it is a normal interstitial change occurring in beds of some antiquity which are covered over by a considerable thickness of later deposits. In places, where these rocks have been subjected to much hydration in the weathering process, they become red in colour, as is found on the flanks of Mount Mariko.