Pteropod-ooze Rocks
These rocks are bluish-grey when not exposed; but through the hydration accompanying exposure they become much lighter in colour. They are crowded with pteropod shells, and contain also small gasteropod and lamellibranchiate shells together with tests of foraminifera both microscopic and macroscopic. They yield between 30 and 40 per cent. of carbonate of lime, the residue being made up of disintegrated palagonitic debris and fine clayey material derived from the same source, together with a fair amount of mineral fragments (10 per cent.) which include plagioclase, pyroxene, and brown hornblende, and measure in the case of the larger fragments between ·1 and ·4 mm. in diameter. Such rocks are somewhat friable and correspond with the pteropod-ooze rocks of the Solomon Islands; but they are not very frequent, being best represented on the flanks of the basaltic table-land between the Wainunu and Yanawai rivers, as in the vicinity of the Nandua tea-plantation where they extend up to 500 feet above the sea. In this particular locality (see page [345]) they overlie horizontally-stratified tuffs and clays, composed of the debris of a basic glass usually vacuolar but now for the most part converted into palagonite, and showing a few small tests of foraminifera.
These deposits are always either surface or incrusting formations. The circumstance of their passing down into characteristic palagonite-formations is repeated in the case of the Tembe-ni-ndio limestones, as observed on page [131]; and there is no doubt that much of their non-calcareous material is derived from the disintegration of palagonite.
Sample of pteropod-ooze rock from below the Nandua tea-plantation.
| Carbonate of lime | 38 | per | cent. | |||
| Residue | Palagonitic debris and clayey material | 51 | " | " | ||
| Minerals | 8 | " | " | |||
| Casts of foraminifera | 3 | " | " | |||
| 100 | ||||||
Fine clayey material makes up the greater part (72 per cent.) of the residue. It presents the microscopic characters of material derived from the degradation of palagonite. Amongst the mineral fragments, of which the larger are ·2 to ·3 mm. in size, occur brown hornblende, pyroxene, felspar, magnetite, &c. The casts of foraminifera are usually glauconitic, but a few are of crystalline silica. A number of curious little pellets of palagonite, oblong or oval in form, occur in the residue. Their size is ·3 to ·6 mm., and they apparently represent the minute amygdules of palagonite that occupy the vacuoles in an altered basic glass.