44 A NATURALIST IN THE PACIFIC chap. 



rocky faces having a drop of from lOO to 300 feet, the highest 

 peaks ranging from 1500 to 1658 feet above the sea. 



All around the mountain, except on the upper steep portion 

 on the south side where it is well-wooded, the slopes have the 

 usual character of the " talasinga " districts, being occupied only 

 by grass, ferns, cycads, and the ordinary scanty vegetation of such 

 regions. Whilst on most sides the surface configuration is fairly 

 regular and the ascent to the summit is more or less regular, on 

 the south side bold spurs with valleys between them descend to 

 the coast, and the central mass rises abruptly in the middle of the 

 peninsula from a height between 300 and 500 feet above the sea. 

 It is on this side that Naivaka has the appearance of having been 

 originally a crateral mountain, of which, however, only the north 

 segment in a much degraded condition now remains, whilst the 

 other two-thirds have disappeared. 



The prevailing rocks are a blackish compact olivine-basalt, 

 having as a rule much smoky glass in the ground-mass and 

 possessing a specific gravity of 2*92 — 2 '94. They are referred to 

 in the description of genus 25 of the olivine-basalts given on 

 page 259. 



These rocks compose the agglomerate and the agglomerate- 

 tuffs that form the eastern portion of the summit and probably 

 most of the elevated part of the mountain. Similar agglomerates 

 occur along most of the north coast, the rock being in a few 

 places scoriaceous or amygdaloidal ; and they occur in huge fallen 

 masses on the south side near the foot of the precipitous portion. 

 The blocks in the agglomerate of the summit are usually six to 

 eight inches across. 



On the south-west side the massive rocks exposed are less 

 basic with a specific gravity of 276 to 279. They are also more 

 altered, the olivine being infrequent and the interstitial glass 

 scanty. They differ besides in the parallel arrangement and in 

 the length of the felspar-lathes ('iS mm.), which are on the average 

 half as long again as those of the prevailing olivine basalts 

 (•12 mm.). They are placed in a different order of these rocks 

 and belong to genus 37 described on page 262. 



Tuffs did not come frequently under my notice. At one part 

 of the north coast the cliffs are formed of a palagonitic tuff-sand- 

 stone, effervescing with an acid, which is described on page 330. 

 Although no organic remains are to be noticed, it is probably a 

 submarine deposit. 



On a spur on the south-west side, at an elevation of 600 feet. 



