328 A NATURALIST IN THE PACIFIC chap. 



amounting to 12 or 13 per cent. It often happens in the case of a 

 series of tuffs, apparently non-calcareous, that an occasional thin 

 band of a fine clay-like rock contains a good percentage of lime. 

 It is pointed out below, however, that the absence of effervescence 

 does not necessarily imply the absence of foraminiferous tests. 



Tests of foraminifera, often macroscopic bottom forms, together 

 with shells of small gasteropods, are displayed at times ; but they 

 are as a rule in such cases not frequent. I found foraminiferous 

 tuffs at considerable heights in some localities, as for instance 

 between 2,000 and 2,500 feet on the slopes of Mount Thambeyu 

 (page 178), at an elevation of 1,850 feet on the south slope of the 

 Korotini Range above Vatu-kawa (page 158), and between 2,000 

 and 2,400 feet on the summit of the range between Waisali and 

 Sealevu (page 154). In the last-named locality, where the tuffs are 

 coarse and often of the nature of agglomerate-tuffs, they are 

 highly fossiliferous ; but such a character is exceptional. 



The submarine origin of the tuffs can often be demonstrated in 

 the absence of evidence of organic remains, as by their inter- 

 stratification with foraminiferous clay rocks, such as we find at an 

 elevation of 1,000 to 1,100 feet on the top of the "divide" between 

 the Ndreketi and Lambasa basins. A single seam of marl-like 

 rock displaying only a solitary test of a foraminifer in the slide 

 may throw light on the origin of the coarser tuffs with which it is 

 associated. The use of the microscope is essential in the case of 

 some of the harder tuffs, where there has been a little alteration. 

 Here casts of foraminifera may be observed, although no carbonate 

 of lime is indicated by an acid. In some localities where no 

 organic remains are evident in the tuff, fine waterworn gravel is to 

 be noticed. 



These deposits are composed as a rule of sub-angular fragments 

 of semi-vitreous basic or basaltic rocks and of palagonite, together 

 with fragments of plagioclase and pyroxene, the interspaces being 

 filled with fine debris of the same materials. The relative propor- 

 tion, however, of the three principal constituents varies considerably, 

 the palagonite, for instance, being sometimes scanty and sometimes 

 abundant. The size of the larger fragments in a tuff of the most 

 common kind is about a millimetre ; but deposits rather finer and 

 rather coarser are also frequent. In the very coarse tuffs and in 

 the breccia-tuffs, where the larger materials are mostly of pala- 

 gonite, the larger fragments may be a centimetre in size and even 

 more, the interspaces being filled with fine debris of the same 

 character cemented together often by carbonate of lime. 



