334 A NATURALIST IN THE PACIFIC chap. 



The palagonitic alteration is to be noticed in all cases ; but it 

 varies considerably in its extent. In the dark grey tuffs it affects 

 the margins only of a few of the glass-fragments. In the paler 

 tuffs it has extended more into their substance, although the 

 alteration is never more than partial. The pale greyish material 

 filling up the interspaces is composed of disintegrated palagonite. 

 The steam-pores or vacuoles are sometimes empty ; and at other 

 times, especially where the palagonitic change has begun, they are 

 filled with a granular alteration product. The glass fuses readily ; 

 but is not affected by acids. It is clear and isotropic, showing 

 however a few scattered microliths. These tuffs correspond with 

 the hyalomelan-tuff from the island of Munia in this group as 

 described by Wichmann ; but in that instance no mention is made 

 of inclosed tests of foraminifera. 



"Crush-Tuffs" formed of Basic Glass 



This is a remarkable group of compacted tuff-like rocks which 

 as hand-specimens would be generally regarded as pitchstone-tuffs. 

 Their detrital origin is, however, often very doubtful. They are 

 composed of fragments of basic glass, carrying plagioclase pheno- 

 crysts, with the interspaces occupied by palagonite and by the 

 finer debris of the glass and felspar. The larger glass fragments, 

 which vary in different rocks from i or 2 to 4 or 5 mm. in size,. 

 have been crushed in situ, the broken portions often remaining 

 more or less in position. These fragments are invested by pala- 

 gonite and have eroded borders, as shown in the figure on page 342. 

 The glass is bottle-green, non-vacuolar, fuses readily, and only at 

 times displays incipient crystallisation. The explanation of the 

 origin of these rocks is attempted in Chapter XXIV. They 

 contain neither carbonate of lime nor organic remains. The most 

 typical example is present in a bed underlying a pitchstone- 

 agglomerate near Narengali (see page 149). It is not uncommon 

 to find evidence of crushing in the glassy matrix of a pitchstone- 

 agglomerate or of rubbly pitchstone, as in the Va Lili Ridge (i42)> 

 the Korotini Bluff (157), and Mount Soloa Levu (313) ; and here 

 also palagonite has been produced around the crushed fragments. 



Coarse Zeolitic Palagonite-tuffs 



These deposits represent coarse kinds of the submarine tuffs of 

 basic glass, in which the palagonitic change is far advanced, and 



