322 A NATURALIST IN THE PACIFIC chap. 



flanks of the mountains, as on the slopes of Thambeyu i,ooo feet 

 above the sea. An interesting exposure of them is displayed in 

 the heart of the island in the face of the Mbenutha cliffs where the 

 elevation is about 1,100 feet. Here they are overlaid by a thick 

 bed of agglomerate ; and tuff-beds largely made up of vacuolar 

 basic glass debris and showing a few foraminifera are interstratified 

 with them ; but they exhibit signs of considerable disturbance (see 



page 109). 



These deposits contain between 5 and 25 per cent, of carbonate 

 of lime, and as a rule about 90 per cent, of the residue consists of 

 fine clayey material derived from the final degradation of palagonite 

 and of basic rocks. The mineral fragments (plagioclase, augite, 

 rhombic pyroxene, and occasionally hornblende) vary much in 

 amount, their average proportion being 13 or 14 per cent, of the 

 mass. Their size is usually less than -2 mm. and does not exceed 

 •4 mm. Casts of foraminifera are nearly always present in the 

 residue and form generally 3 or 4 per cent, of the whole deposit. 

 Sometimes they are black and glauconitic ; but more frequently 

 they are white and composed of chalcedonic silica. Such casts 

 represent on a small scale the results of the same silicifying opera- 

 tion to which the flints and silicified corals that occur so frequently 

 on the surface in some localities owe their origin (see Chapter 

 XXV.). 



With regard to the age of these volcanic mud-rocks of Vanua 

 Levu, it is most likely that as in the case of similar deposits 

 in Viti Levu, which were examined by Mr. H. B. Brady, they are 

 of post-tertiary origin. Samples of the Suva " soapstone " contain- 

 ing 5 or 6 per cent, of lime and displaying shells of foraminifera, 

 pteropods, and other molluscs, were obtained from different heights 

 up to 100 feet above the sea. Since 87 out of the 92 species of 

 foraminifera represented in the deposits are known to be living 

 now in the Pacific, Mr. Brady had no hesitation in assigning the 

 beds to the post-tertiary epoch. 1 



Samples of the Volcanic Mud-rocks 

 A. From districts west of Ndranimako, 100 f eet above the sea. 



Carbonate of lime 20 per cent. 



fFine debris of palagonite and semi-vitreous 



basic rocks 62 „ „ 



Residue 1 Minerals I4 fj )5 



I Casts of foraminifera 4 » » 



100 

 1 Quart. Journ. Geolog. Soc. xliv. 1888. 



