126 A NATURALIST IN THE PACIFIC chap. 



down the mountain, separates the two peaks. The southern one, 

 which appears to be inaccessible, is evidently formed of the same 

 acid andesite. 



These hornblende-andesites, with or without porphyritic 

 quartz, appear to be for the most part restricted to the immediate 

 vicinity of Na Raro, except to the south-west, where at a distance 

 of about a mile and a half from the mountain at an elevation of 

 500 feet occur a rubbly hornblende-andesite and agglomerates of 

 the same materials. Though the rock is of the Na Raro type, its 

 presence here is suggestive of a distinct vent of small size, of 

 which most of the traces have been swept away during the 

 emergence of the island. About half a mile south-east of this 

 locality at an elevation of 450 feet occur some singular banded 

 palagonite-tuffs which, although they do not show foraminifera in 

 the section examined, contain a little calcite and are probably of 



submarine origin In this locality I found a large white 



mass, measuring 4x4x5 feet, formed of a siliceous rock appear- 

 ing in thin sections as granular chalcedonic quartz (see page 355). 



The hornblende-andesite of Na Raro, as in the case of the rocks 

 of most of the other peaks of acid andesites, has its peculiar 

 characters. It differs, for instance, from that of Vatu Kaisia in 

 the larger grain of the felsitic groundmass (N. R. -021 mm. ; V. K. 

 ■013 mm.), in the absence or rarity of phenocrysts of rhombic 

 pyroxene, in its lower specific gravity (2'6 N. R. : 27 V. K.), in the 

 presence of a little interstitial glass, and in other particulars. 

 Both, however, belong to the sub-class of hornblende-hypersthene- 

 andesites, and are described on page 301. In the Na Raro rock the 

 rhombic pyroxene is represented in the groundmass. 



With regard to the relative age of Na Raro I am inclined to 

 think that it is the most recent of the acid andesite peaks of the 

 island. Neither vitreous nor vesicular rocks came under my notice 

 in its vicinity ; whilst the tuffs that clothe its lower slopes are 

 non-pumiceous, though of dacitic origin, but containing also a few 

 fragments of a semivitreous basic andesite showing tiny felspar 

 lathes and augite-granules. Since the everywhere prevailing sub- 

 marine palagonite-tuffs and foraminiferous clays do not extend 

 over its area, we may assign to it a later date. It is evidently 

 also posterior in time to the basaltic andesites and aphanitic 

 augite-andesites around, which are covered by these submarine 

 deposits. Relatively recent as it apparently is, this mountain 

 bears the impress of a high antiquity. There is nothing to 

 indicate that this " core " of a volcanic mountain belonged to a 



