i88 A NATURALIST IN THE PACIFIC chap 



•down the south slope at an elevation of 1,800 feet, but non-scoria- 

 ceous.) The matrix of the agglomerate principally consists of fine 

 palagonitic material with small fragments of plagioclase and 

 pyroxene but apparently no lime. 



At heights of about 2,800 feet on the south side of the peak, and 

 of 1,600 feet on the north flank of the range, are exposed non- 

 calcareous greyish tuffs remarkable for the quantity of crystals of 

 rhombic pyroxene, entire and in fragments, that they contain. 

 This is a characteristic feature of the more acid andesitic tuffs of the 

 island, and it is to these deposits that the Mariko tuffs in question 

 make a near approach. They contain at times subangular 

 fragments of more basic rocks ; and are true tuffs in the sense that 

 although perhaps deposited on a sea-bottom they represent the 

 ejected materials of a subaerial vent. 



The crest of the range, where it is crossed by the road from 

 Vunimbua to Nukumbolo and for 200 feet below, is formed of a 

 decomposed rock, perhaps a breccia. A fragment of the rock ob- 

 tained from the crest is a grey somewhat altered hypersthene-augite 

 andesite(sp. gr. 275) with an orthophyric groundmass, and referred to 

 the order described on page 290. This rock may be connected with 

 the tuffs above alluded to ... . Reference may here be made to a 

 black basaltic rock (sp. gr. 2*88) of which, at an elevation of 2,500 feet 

 at the foot of the peak, I found a portion of a columnar block about 

 18 inches across. It may prove to be an olivine-basalt ; but no 

 section has been made of it. 



It is apparent from the foregoing description of the Mariko Range 

 that in general structure it does not differ materially from the other 

 mountain-ridges of the island, although in the types of the rocks it 

 presents some variety. Here also we have agglomerates prevailing 

 on the flanks and forming the summit. As far as the characters of 

 the rocks can guide us, we cannot determine whether the range has 

 been built up by a number of vents on a great fissure, or whether 

 it represents the remains of a huge crater. In this uncertainty we can 

 only appeal to the contrast between the gentle gradient of the north 

 slopes and the precipitous descent of the south slopes as favouring the 

 last supposition. We cannot, however, doubt that the agglomerates 

 of the upper portion of the range are the products of an eruptive vent 

 or of vents that rose above the surface of the sea, since the blocks 

 are all of one kind of andesite and are often scoriaceous. We can 

 be fairly certain that at such a time the lower slopes were in part 

 submerged, seeing that foraminiferous tuffs underlying the agglo- 

 merates are now exposed. But we have to distinguish between these 



