72 A NATURALIST IN THE PACIFIC chap. 



flow from the old Ndriti crater that the submarine bank was 

 formed off the adjacent coast on which the low Lekumbi promontory 

 has been built up. 



In the numerous dykes of the Ndriti basin and in the great 

 alteration which their rocks have frequently undergone, we have 

 evidence in support of the view that this is an old crateral cavity, 

 an opinion that is supported by the indications of lava-flows that 

 have issued, apparently in later times, from the mouth of the 

 basin. Reference has already been made to the locality where 

 a dyke-rock and the rock-mass, into which it has been intruded, 

 are both propylitic ; and from this and other facts, such as the 

 varying degrees of alteration in different parts of the basin, it is to 

 be inferred that in the last stage of the activity of this vent its 

 bottom and sides were extensively affected bysolfataric influences. 

 Since that period, the configuration of the crater-basin has been 

 greatly modified through the denuding agencies. 



The absence, or at least the great rarity, of tuff's and agglo- 

 merates in the case of Seatura is remarkable. The mountain has 

 evidently been built up in the mass by flows of basic lava ; and 

 from this source have no doubt in an important degree been 

 derived the basaltic flows of the Ndama, Mbua, and Sarawanga 

 plains, great streams of basalt that further seaward have helped 

 to form the submarine platform extending several miles from the 

 coast. The submarine tuffs and agglomerates that occur at 

 various elevations, reaching as high as 1,200 feet above the sea, in 

 the Sesaleka, Lekutu, Sarawanga, and Ndrandramea districts lying 

 to the north-west, north, and east, did not come under my notice 

 on the Seatura slopes. On the other hand, except in the few 

 localities, where scoriaceous rocks occur, the general type of the 

 basalts is such as we would expect to find in submarine flows. 

 In no part of the island, however, is the antiquity of the land- 

 surface so well attested by the disintegration of the basaltic flows> 

 which extends here to depths of ten and even twenty feet. This 

 is in favour not only of the sufficiency of time, but also of 

 the ability of the denuding agencies to strip off" the surface- 

 deposits. 



However this may be, it is evident that the mountain of 

 Seatura possesses a history quite independent of that of the rest 

 of the island. I have pointed out in Chapter I. that it represents 

 a mountain of the Tahitian type. In its radiating valleys and in 

 its basaltic character it much resembles the mountainous island of 

 Tahiti, which Dana describes as a gently sloping cone of the 



