They are usually hard in texture and greyish or pale yellow in colour. They contain between 25 and 45 per cent. of carbonate of lime; whilst the residue consists of fragments of minerals (10 to 15 per cent.), including plagioclase, monoclinic and rhombic pyroxene, and occasionally brown hornblende, with siliceous casts of foraminifera (4 to 20 per cent.), mostly formed of chalcedonic silica but sometimes black and glauconitic; the remainder (30 to 40 per cent.) being composed of rounded and sub-angular portions of palagonite and semi-vitreous basic rocks, of which the larger vary from ½ to 1 millimetre in diameter. In some cases the carbonate of lime of the inclosed organic remains has been mainly replaced by more or less crystalline silica. In others a recrystallisation of the calcitic material is in progress, as described on page [131]; and the matrix presents in places a mosaic of calcite.
These rocks have therefore been subject to some degree of alteration, the causes probably lying within the mass. They are, however, far from frequent. They are best represented in the upper valley of the Sarawanga River in the vicinity of Tembe-ni-ndio, where they reach to a height of about 250 feet above the sea. The greatest elevation at which I found them was in the mountainous interior of the Waikawa Promontory, where they occur at a height of 1,100 feet above the sea.
As samples, the results of the examination of two rocks from the Tembe-ni-ndio district are here appended:—
A.
| Carbonate of lime | 46 | per | cent. | |||
| Residue | Fragments of palagonite and of semi-vitreous basic rocks | 30 | " | " | ||
| Minerals | 12 | " | " | |||
| Secondary silica replacing the carbonate of lime in the organic remains | 12 | " | " | |||
| 100 | ||||||
It displays to the naked eye fragments of shells including pteropods, and numerous tests of foraminifera 2 or 3 millimetres in diameter. In the section it displays in addition coral debris and a considerable quantity of rounded and sub-angular pieces of palagonite and semi-vitreous basic rocks, usually less than a millimetre across, with smaller fragments of minerals (pyroxene and plagioclase), and much calcitic material in the matrix.
B.
| Carbonate of lime | 25 | per | cent. |
| Debris of palagonite and of basic volcanic rocks | 45 | " | " |
| Minerals | 10 | " | " |
| Siliceous casts of foraminifera | 20 | " | " |
| 100 |
The organic remains mainly consist of tests of foraminifera, many of which occur in the residue as colourless siliceous casts. The fragments, whether of minerals or of volcanic rocks, are usually less than half a millimetre across. The foraminifera are mostly small and of the “Globigerina” type; but there is a cast of a tube of some boring-mollusc, and fragments of shells also occur.