54 A NATURALIST IN THE PACIFIC CHAP. 



specific gravity is 2-85 and it is referred to genus 25 of the olivine 

 basalts (page 259). I descended by a gentle slope to the north, 

 coarse basic tuffs and agglomerates containing amygdaloidal 

 fragments being displayed on the surface. In a stream at the 

 foot, close to Koro-ni-solo, were blocks of a heavy compact olivine- 

 basalt with specific gravity 2*96. 



DISTRICT BETWEEN MOUNT SESALEKA, THOMBO-THOMBO 

 POINT, AND VATU-KAROKARO HILL. This is a broken country 

 with several abruptly rising lesser hills. Starting from Koro-vatu 

 and crossing the Thombo-thombo promontory, I reached the coast of 

 Mbua Bay near Navunievu. Basic tuffs and agglomerates pre- 

 vailed on the way, the last containing blocks of a scoriaceous 

 basaltic lava bearing olivine. The massive rocks exposed belong 

 in some cases to genus 13 of the olivine-basalts as described on 

 page 256, being dark grey and having a specific gravity of 2*88, 

 and in other cases to genus 16, species B, of the augite-andesites 

 when they are lighter in colour and have a specific gravity of 277. 

 In both cases the interstitial glass is scanty. 



I ascended Vatui, one of the numerous small hills of the 

 district. It is 450 feet high and is capped by a bare mass of tuff- 

 agglomerate, 40 to 50 feet high and containing fragments of 

 vesicular basic lava. This mass is pierced by a dyke, 18 inches 

 thick, which is inclined to the N.NE. at a high angle of 60 or 65 

 degrees with the horizon. This dyke is composed of a compact 

 olivine-basalt which is remarkable for the prevalence of small 

 augite prisms in the groundmass. It is described on page 265 

 under genus 44 of the olivine-basalts. Hand-specimens are 

 magnetic and display polarity, which is due, as pointed out in 

 Chapter XXVI., to the exposed situation of the peak. 



Vatui in its characters is evidently typical of the other lesser 

 hills around, which, as viewed from below, possess bare tops and 

 precipitous declivities of the same formation. All the hills in the 

 district including Sesaleka are capped by these basic tuffs and tuff- 

 agglomerates ; and doubtless as in the case of Sesaleka these 

 deposits are all submarine. This is true also of Vatu-karokaro, a 

 hill 600 feet high, overlooking Mbua Bay and about two miles 

 east of Sesaleka. In the lower part of this hill is exposed a dark 

 compact basaltic andesite, referred to genus 13, species B, of the 

 augite-andesites (sp. gr. 2*83), whilst blocks of a black olivine-basalt 

 (sp. gr. 2*91) occur in the agglomerate of the summit. These hills 

 may all be regarded as " volcanic necks " or the stumps of volcanic 

 cones, probably submarine. 



