i 3 4 A NATURALIST IN THE PACIFIC chap. 



second kind, without felspar phenocrysts, belongs to genus 12 of 

 the same class (page 275). A good example of the porphyritic 

 rocks is afforded in the large blocks lying in the stream-beds 

 during the first half of the way from Ndreketi to Sarawanga. 



It may be pointed out here that these doleritic, semi-ophitic 

 basaltic andesites of the Ndreketi plains differ conspicuously from 

 the prevailing type found on the slopes of Seatura, on the 

 Sarawanga and Mbua plains, and on the Wainunu table-land. 

 There we have, as a rule, olivine-basalts, having a specific gravity 

 of 2'86 to 2'90, and showing but scanty interstitial glass, the 

 felspars of the groundmass being on the average not over - 2 mm. 

 in length, whilst the augite granules are, as a rule, only 

 •02 — '03 mm. in diameter, and the ophitic structure is infrequent. 



The submarine deposits, consisting of foraminiferous clays and 

 coarser tuff-sandstones, the former being usually beneath, are 

 found at intervals all over this area. They occur inland as far as 

 Vuinasanga and Nareilangi, near the base of the mountains of 

 Va Lili and Na Raro, reaching as high as 300 feet, their place 

 being taken on the mountain slopes by coarser tuffs and 

 agglomerates. When not weathered they are more or less 

 calcareous, and contain occasionally marine molluscan shells, 

 whilst palagonitic debris enter largely into their composition. 

 The foraminiferous clays, often much bleached by hydration, are 

 well represented around Mbatiri and in the districts between that 

 town and Natua and Nareilangi. They are relatively deep-water 

 deposits, and belong to the type described on page 323. Others, 

 again, as exposed in the banks of the river at Natua, are chocolate 

 coloured and of the kind referred to in detail on page 335. These 

 foraminiferous clays in the region between Natua and Mbatiri are 

 overlain in places by coarse, almost brecciated, tuffs, formed in 

 part of the debris of acid andesites, such as compose the not far 

 distant mountain of Na Raro. 



Since the massive basaltic rocks are exposed in all the deeper 

 rock channels of these plains, it is apparent that the overlying 

 submarine deposits can possess no great thickness. Probably they 

 are never 100 feet thick, and usually far less. In many places, 

 through their denudation, the underlying basaltic rocks are 

 exposed, and in a decomposing condition largely form the surface. 

 These deposits as a rule display bedding, the beds being horizontal 

 or at least only inclined 2 or 3 degrees. This horizontality is a 

 nearly constant feature of these submarine beds, as they overlie 

 the basaltic rocks of the plains ; and it is a feature we should 



