92 A NATURALIST IN THE PACIFIC CHAP. 



rocks and are more or less palagonitic. The coarser deposits are 

 described as sample A on page 330. At times these tuffs are com- 

 posed of much coarser fragments of the same materials, some of 

 them a centimetre in size. A type of tuff intermediate in character 

 is not uncommon. 



The promontory that lies between Kiombo and Soni-soni 

 Island has been formed by a remarkable basaltic flow. The low 

 tongue, about 50 feet high and 200 to 300 yards across, in which 

 it terminates, was originally severed by a passage worn by the sea 

 from the main portion ; but it is now joined by a low tract only 

 2 or 3 feet above the beach and partly occupied by mangroves. 



The structure of the flow is well exhibited in the shore-flat and 

 coast-cliffs west of Kiombo, and extending to the end of the point. 

 The waves have here cut into its mass and exposed its structure. 

 Its lower part, as exposed in the shore-flat, is made of a compact 

 hemicrystalline basalt ; whilst its upper portion, as displayed in 

 the cliffs, 30 or 35 feet in height, is composed of vitreous and semi- 

 vitreous forms of the same rock looking like pitchstone. The 

 upper vitreous part is sometimes massive ; but usually it is rubbly, 

 with a tendency to form spheroidal masses. All transitions can 

 there be traced between the hemicrystalline rock of the shore-flat 

 and the vitreous rock of the cliffs. 



The rock of the shore-flat, which has a specific gravity of 2*83, 

 is a blackish porphyritic basalt with scanty olivine, and on account 

 of the semi-ophitic character of the augites of the groundmass it 

 is placed in genus 33 of the olivine class. The plagioclase pheno- 

 crysts are 3 to 5 mm. in size. About half of the groundmass 

 is made up of felspar-lathes ('17 mm. long) and large augites ('ii 

 mm.), the rest consisting of a smoky devitrified glass containing a 

 few irregular " lacunae " filled with the residual magma in the form 

 of a reddish-brown opaque palagonite-like material. The rock 

 intermediate between the lower and upper portions of the flow is 

 also intermediate in character, having a specific gravity of 277, 

 whilst quite three-fourths of the groundmass are of smoky glass. 



The vitreous rocks of the cliffs, though usually rubbly in 

 appearance, have also the aspect in places of brecciated pitchstone 

 tuffs with the interstices filled with waxy palagonite ; but the 

 microscopical examination shows that we have not to deal with a 

 rock of detrital origin. We have here the effects of the breaking 

 up and crushing in situ of a dark-brown isotropic basic glass x 



1 The unaltered glass, which incloses a few plagioclase phenocrysts, has a 

 specific gravity of 27, and is readily fusible. 



