io6 A NATURALIST IN THE PACIFIC CHAP. 



gorge-like channel of the river between the village of Nambuna 

 and the foot of Ndrandramea, and in fact in all places in this 

 district where the streams have worn deeply into the surface. 



They have a coarse felsitic groundmass, and are described under 

 the felsitic order of the hypersthene-andesites on page 297. They 

 present all degrees of change from the hard dark grey mottled 

 rocks, in which the phenocrysts of plagioclase and rhombic 

 pyroxene are in part replaced by calcitic, viriditic, and chloritic 

 materials, to those where the pseudomorphism and alteration is 

 complete, when the decomposition products give their character 

 to a pale yellowish rock, which sparkles with pyrites and often 

 effervesces briskly with an acid. After this comes the final stage 

 of disintegration, and we get a whitish rotten stone, often full of 

 pyrites, the last condition of which is shown in a kaolin-like 

 material exposed in the river-side. 



The extensive alteration of these rocks is also indicated by the 

 occurrence amongst the gravel of the river-bed and small stream 

 courses near Nambuna of fragments of clear quartz prisms, half an 

 inch across, and of nodules, three inches in size and sometimes 

 hollow in the centre, formed of radiating quartz crystals that once 

 filled cavities in the altered rock. Small masses of vein-quartz also 

 occur in these streams, formed in a fissure by the growth of the 

 crystals from the sides towards the centre. I was unable to find 

 the source of the quartz ; but it is probable that it was produced 

 near the line of contact between the basaltic flows to the eastward 

 and the older felsitic rocks of the district. The great alteration of 

 the acid andesitic rocks exposed as the bed-rocks in this region 

 may in all probability be attributed to the vicinity of these basaltic 

 rocks. The two formations apparently come into contact about a 

 mile east of Nambuna. In traversing this district on the road to 

 Ndrawa one first observes in situ in the streams the decomposed 

 felsitic bed-rock with occasional loose blocks of a quartzitic rock that 

 displays in the thin section a mosaic of irregular grains of quartz. 

 Afterwards, as one rises gradually to the top of the basaltic plateau, 

 basaltic rocks are alone exposed in position. 



In the character of the fine river sand a clue may be found to 

 the exact locality of the contact. In the midst of the andesitic 

 area between Nambuna and Ndrandramea, the sand, besides 

 containing much magnetic iron, is also composed to a large extent 

 of rhombic pyroxene prisms, clear quartz grains, and fragments of 

 plagioclase, all derived from the porphyritic crystals of the dacites, 

 &c. Near the basaltic district we find that the quartz and rhombic 



