XXIII SUBMARINE PALAGONITE TUFFS 331 



substance occur irregular patches formed of a colourless semi- 

 isotropic mineral which is either zeolitic or a form of opal. 

 Plagioclase and augite compose the mineral fragments, the former 

 prevailing. Although these tuffs are derived from a vent that was 

 probably the last in eruption in this island, they display con- 

 siderable alteration which is mainly connected with the secondary 

 changes affecting the palagonite since the deposition of these 

 materials. 



C. As an example of the banded tuffs composed of coarse and fine 

 materials I will take a compact grey rock forming one of the 

 horizontal beds in the natural section exposed near the hill of 

 Korolevu (p. 49). It is also an example of those tuffs which 

 whilst not effervescing with an acid display a few casts of fora- 

 minifera in the slide. The alternating bands which are about a 

 centimetre in thickness pass gradually into each other. The bands 

 of finer materials are made up of sub-angular fragments, t to 

 •2 mm. in size, of the dark opaque groundmass of a semi-vitreous 

 basic rock and of a grey hemi-crystalline groundmass of an augite- 

 andesite, together with palagonite more or less decomposed, and 

 fragments of plagioclase and augite, whilst the interspaces are 

 filled with the finer debris. The layers of coarse materials have 

 much the same composition, the fragments varying usually between 

 •5 and I mm. in diameter, with occasional larger pieces of pala- 

 gonite 2 to 5 mm. in size representing original lapilli of a vacuolar 

 basic glass. The tests of the foraminifera, which occur scantily in 

 the layers of fine material, are all minute. They are filled with 

 palagonitic material. 



D. The tuffs prevailing on the higher flanks of the mountainous 

 backbone of the island are well represented by those exposed at 

 an elevation of 1,200 feet above the sea on the south slope of the 

 Korotini Range behind Mbale-mbale. It is a somewhat coarse- 

 grained rather hard grey rock effervescing feebly with an acid. It 

 is composed of sub-angular or partly rounded fragments, i to 4 mm. 

 in size, of various basic rocks, and of rather smaller fragments of 

 plagioclase and augite, the interstices being filled up with fine 

 debris of the same materials, in which a few minute tests of fora- 

 minifera of the " Globigerina " type may be observed. The basic 

 rocks of which the fragments are formed comprise the following : 

 (a) a grey aphanitic augite-andesite, with but little interstitial glass, 

 presenting a parallel arrangement of the minute felspar-lathes 

 which have an average length of '05 mm. ; (l>) a grey augite-ande- 



