1 62 A NATURALIST IN THE PACIFIC chap. 



fine and coarse basic tuffs showing secondary calcite, quartz and 

 opal, as alteration products. Others are palagonite-breccias with 

 the vacuoles of the altered glass filled with opal. Others again are 

 massive basic rocks, such as fine-textured augite-andesites, or 

 doleritic basaltic andesites, semi-ophitic in character, the plagioclase 

 phenocrysts being more or less occupied by calcitic and other 

 products. The alteration is not always far advanced, but it is 

 sufficiently marked to give a common character to the rocks of the 

 district. 



Ascending the lower slopes of the range up to 800 feet one finds 

 the altered rocks still exposed in the stream-courses ; but the 

 changes exhibited are not always the same. A specimen from 500 

 feet looks like a tuff, but in the slide it appears as a semi-vitreous 

 augite-andesite, its substance being penetrated by fine veins of 

 chalcedonic quartz and opal, whilst the same material is developed 

 within the larger plagioclase crystals. Another specimen from 800 

 feet, which is apparently a tuff, contains so much lime that it effer- 

 vesces freely with an acid. It was composed originally of fragments 

 of a hemi-crystalline basic rock, of which the plagioclase pheno- 

 crysts have been replaced by calcite ; whilst the augite and inter- 

 stitial glass is now represented by viridite and a chloritic mineral. 

 It is to be inferred that at some time hot springs were very 

 numerous in the district between Nukumbolo and the lower slopes 

 of the range, those at Nukumbolo, as far as I know, alone existing 

 in our time. 



From a height of 1,100 or 1,200 feet the mountain slopes rise 

 steeply to the summit rather over 2,000 feet in elevation. At the 

 foot are exposed in situ aphanitic augite-andesites, 1 which in some 

 specimens show a little alteration in the chalcedonic quartz filling 

 minute cracks, and in one case there is an irregular cavity, f inch 

 across, filled with milk-white opal. Another rock exposed at the 

 foot of the steep ascent is a semi-vitreous basaltic-andesite, doleritic 

 in texture and ophiticin structure, but apparently not much changed. 2 

 At 1,700 feet is displayed a vesicular basic andesite, semi-vitreous 

 in character, and above this I found a porphyritic basaltic andesite. 



The summit of the range is i| or 2 miles in breadth and is 

 relatively level, its undulating surface varying in elevation between 

 1,900 and 2,200 feet. The prevailing rocks exposed on this elevated 

 plateau are vitreous pitchstone-like rocks finely vesicular and 

 scoriaceous, the cavities being filled either with aragonite or with 



1 Referred to genus 16, species A, sub-species 1, of the augite-andesites. 



2 Referred to genus 9, sub-genus A, of the augite-andesites. 



