336 A NATURALIST IN THE PACIFIC CH. xxin 



materials composing them are exceedingly fine, the largest frag- 

 ments not usually exceeding "Z mm. As a rule they contain a 

 little carbonate of lime and sometimes as much as 10 per cent., 

 whilst a few tests of minute foraminifera are to be noticed in the 

 slide. These deposits are horizontally bedded, and underlie a 

 pteropod-ooze rock at Nandua and a shelly impure limestone at 

 Tembe-ni-ndio. They are not very frequent, and sometimes 

 approach in characters the volcanic-mud rocks, which, however, are 

 much more mixed in composition. I regard them in the main as 

 sedimentary deposits derived from the disintegration of the pala- 

 gonitised vitreous surface of a submarine basaltic flow. They pass 

 downward at Nandua, as described on page 345, into a rock of 

 pure palagonite ; and they are only to be found in localities where 

 basaltic plains or plateaux are covered over with submarine deposits. 



ACID PUMICE TUFFS 



The general characters of these deposits are described on 

 pages 10, 218, 220, 222, 223, 231, 233, &c. Such tuffs are restricted 

 to the north-east part of the island east of Lambasa and Tawaki, 

 and are well displayed in the coast cliffs. They are pale yellow or 

 whitish, and are usually non-calcareous. They are composed of 

 the debris of a vacuolar and fibrillar isotropic glass, nearly colour- 

 less and in some localities altered. Small crystals of quartz and 

 of glassy felspar with bits of obsidian (up to 3 mm.) and lapilli of 

 rhyolitic glass are inclosed in them. In places inclosed pieces of 

 coral and coral rock indicate submarine deposition. 



