32 o A NATURALIST IN THE PACIFIC chap. 



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 t 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 pala- 

 gonite, 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 disinte- 

 gration of palagonite. 



Sample of pteropod-ooze rock from below the Nandua tea- 

 plantation. 



Carbonate of lime 38 per cent. 



/• Palagonitic debris and clayey material . . 51 ,, ,> 



Residue -! Minerals 8 ,, „ 



I 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 foramin- 

 ifera are usually glauconitic, but a few are of crystalline silica. A 



