324 A NATURALIST IN THE PACIFIC CHAP. 



and felspar) are very scanty, the largest being less than -25 mm. 

 The white casts of foraminifera, composed of chalcedonic silica, 

 form a conspicuous elements in the residue. 



D. From the vicinity of Mbatiri, loofeet above the sea. 



Carbonate of lime 4 per cent. 



Fine material derived from the degradation of palagonite 90 



Minerals 2 ,, 



Casts of foraminifera 4 



100 



About 94 per cent, of the residue consists of fine clayey 

 material. The fragments of minerals are very scanty and are all 

 less than '2 mm. in size. The casts of foraminifera are white and 

 of chalcedonic silica. From the fineness of the materials and the 

 small size and pelagic character of the foraminifera, this deposit 

 may be regarded as formed in relatively deep water. 



E. From the eastern flank of the Wainunu table-land, 200 feet 

 above the sea. This is a shallow-water deposit and contains, 

 besides small gasteropod shells, large flat tests of foraminifera 

 5 or 6 mm. in diameter. It possesses 24 per cent, of carbonate 

 of lime, 62 per cent of palagonitic debris, &c., and 14 per cent, of 

 minerals. 



ALTERED VOLCANIC MUD-ROCKS 



This group includes compact hard foraminiferous usually dark- 

 brown rocks, which exhibit evidence of alteration in their indura- 

 tion, in the presence of pyrites, and in the chalcedonic quartz 

 filling fine cracks in the rock-mass. Occasionally special types of 

 alteration occur, one of which will be referred to in the description 

 of some of the rocks given below. The proportion of carbonate 

 of lime is generally small ; but sometimes it amounts to 10 per 

 cent, or more. 



They admit of being examined in thin sections ; and their true 

 nature is at times so much disguised that I have taken them at 

 first for aphanitic basic andesites. In the slide they display a few 

 scattered tests of foraminifera of pelagic habit in a matrix 

 composed of the fine debris of palagonite and of basic rocks, 

 together with fragments of plagioclase and pyroxene. Most of 

 the material is very fine, and the size of the largest mineral 

 fragments does not exceed '2 mm. 



Such rocks, however, are not very frequent. They may be 



