xxiii SUBMARINE BASIC PUMICE TUFFS 333 



organic remains, and palagonite when present is usually scanty, 

 whilst viridite and similar materials represent the decomposition of 



the pyroxene A hard breccia-tuff found on the flanks of Mariko 



and in one or two other localities contains vesicular fragments, 

 where the steam-holes are filled with opal or chalcedony, and the 

 cracks traversing the matrix are also filled with this mineral. 



The alteration of these tuffs has evidently arisen from a variety 

 of causes. In some cases the change appears to be purely inter- 

 stitial. In other times it has arisen from contact-metamorphism, 

 or from hydro-thermal agencies, as in the case of the altered tuffs 

 near the hot springs at Nukumbolo (p. 161). 



Submarine Basic Pumice Tuffs 



These deposits, which, however, are not of frequent occurrence, 

 are interstratified with volcanic mud-rocks in certain localities, as 

 at the Mbenutha Cliffs (p. no), and in the vicinity of the hill of 

 Korolevu (p. 47). They indicate periods of volcanic activity 

 during the deposition of the foraminiferous muds, with which they 

 are associated, when the fine materials ejected from sub-aerial vents 

 fell into the seas around. 



Such tuffs are more or less compact and usually fine in texture. 

 When the glass fragments are but slightly altered, the tuff-rock is 

 dark grey ; but when the palagonitic change is well advanced, it 

 becomes pale and yellowish. They are made up chiefly of small 

 fragments of a bottle-green basic glass, which are as a rule vacuolar 

 and sometimes fibrillar ; but it never happens that the pumiceous 

 character is as pronounced as in acid pumice ; and in some cases 

 the vacuoles or steam-pores are to be observed only in the minority 

 of the fragments displayed in a slide. The size of the glass 

 fragments is as a rule small, in some tuffs averaging only t mm. 

 and in others '5 mm. ; but occasionally they may be i or 2 mm. 

 in diameter. 



Fragments of minerals (plagioclase and pyroxene) correspond- 

 ing in size to the glass fragments are as a rule well represented, 

 forming a fourth or a third of the mass. A little fine detritus of 

 a semi-vitreous basic rock also occurs. Tested in an acid several of 

 the tuffs either do not effervesce or give an indication of a small 

 percentage of carbonate of lime ; whilst others effervesce freely. 

 They usually display a few minute tests of foraminifera of the 

 " Globigerina " type, the cavities of which are filled with fine 

 palagonitic debris. 



