S8 A NATURALIST IN THE PACIFIC chap. 



Coming to the characters of the basaltic rocks of the Mbua and 

 Ndama plains, it may be remarked that the prevailing rocks are the 

 porphyritic basaltic andesites, having a specific gravity of 277 to 

 2"8i, which are in most cases to be referred to genus 13 (porphyritic 

 sub-genus) of the augite-andesites described on page 278. They 

 possess large phenocrysts of plagioclase and but little interstitial 

 glass. The other rocks are olivine-basalts with specific gravity 

 2'88 to 2'90 and showing only a few small plagioclase-phenocrysts. 

 They display a little residual glass and belong for the most part to 

 genus 37 of the olivine basalts described on page 262. In both 

 these basaltic rocks the felspar-lathes are in flow-arrangement ; but 

 in the basaltic andesites they average 'ii mm. in length, whilst in 

 the olivine-basalts they average 'iS mm. 



The low mound-like " rises " in these plains, to which previous 

 reference has been made, are not usually elevated more than 50 

 feet above the general surface. One of these hillocks that lies near 

 the track from Mbua to Navunievu, about two miles from the 

 Wesleyan Station, is composed of a remarkable semi-vitreous 

 pyroxene-andesite passing upward into a rubbly rock of the same 

 nature. The rock of this old volcanic neck is of an unusual type 

 and is referred to the prismatic order of the hypersthene-augite 

 andesites described on page 289. Both the felspar and pyroxene 

 prisms of the groundmass are in flow-arrangement. One of these 

 mounds near the Mbua Wesleyan Station is apparently formed of 

 the decomposing basaltic andesite of the district. On its surface 

 are fragments of earthy limonite and siliceous rocks. 



The rarity of submarine tuffs and clays on these plains is some- 

 what singular ; but in the occurrence of foraminiferous tuffs high up 

 the slopes of Sesaleka and on the crest of the Mbua-Lekutu divid- 

 ing ridge we have evidence of the original submergence of all these 

 lower regions. It is probable enough that the ages of exposure 

 that have since witnessed the reduction of the solid basaltic rock to 

 a crumbling mass several feet in depth were more than sufficient 

 for the stripping off" of most of the overlying submarine deposits. 

 Such deposits are, however, common on the surface of the extensive 

 " talasinga " plains that constitute much of the north side of the 

 island. 



The Shell-bed of the Mbua River. — Rather curious 

 evidence of an emergence of a few feet and of a considerable 

 advance of the delta of the Mbua river in comparatively recent 

 times is afforded by a bed of marine shells exposed in the right 

 bank of this river, about 200 yards below the boat-shed of the 



