X NAKAMBUTA 149 



Va-Hli, these rocks are often more or less glassy and take the form 

 of pitchstones ; whilst the agglomerates have the same character. 

 The first probably represent submarine flows of basaltic lava which 

 have spread far and wide over the Ndreketi plains. The inland 

 rocks are, as is pointed out below, the products of vents that, as in 

 the case no doubt of Mount Nakambuta, rose out of a shallow sea. 

 The palagonite-tuffs and clays, often foraminiferous, which cover 

 the Ndreketi plains, are extensively represented in the lower levels 

 up to 4(X) feet or more. 



Between one and two miles to the westward of the Narengali 

 valley, and immediately north of Va-lili, the agglomerates, over- 

 lying palagonite-tuffs, form lofty precipices. The agglomerates 

 are composed of blocks of more or less vitreous basaltic-andesites, 

 some of them semi-vitreous and amygdaloidal, some in the form of 

 pitchstone, and others again as tachylyte that fuses in the lamp- 

 flame. The underlying palagonite-tuffs are bedded, and are 

 composed of fragments of basic glass that originally inclosed 

 porphyritic crystals of plagioclase. In the slide it is observed that 

 the glass and mineral fragments have often been re-fractured as 

 they lie in the tuff and that the former have rounded angles and 

 eroded edges. The interstices are filled with a more or less 

 palagonitised magma. Similar rocks occur in other localities, and 

 they will all be found described on page 334. It may, however, be 

 remarked here that in all cases these rocks would seem to have 

 undergone some crushing, the heat developed in the process being 

 sufficient to partially remelt the glass. A high temperature was 

 not required to effect this fusion, since splinters of the tachylyte 

 occurring in the overlying agglomerate fuse in an ordinary lamp- 

 flame. It is pointed out on page 341 that tuffs of this character 

 differ in origin and in characters from the prevailing foraminiferous 

 palagonite-tuffs. 



The road from Narengali to Natua traverses the length of this 

 district. At and near the mouth of the Narengali valley there are 

 exposed basic tuffs and agglomerates, the blocks in the last case 

 being formed of a semi-vitreous, vesicular or almost scoriaceous 

 basaltic andesite. In this neighbourhood the track passes across 

 the top of a waterfall which is the result of the existence of a huge 

 dyke-like mass of a compact basaltic andesite showing a little 

 interstitial glass and referred to genus 13 of the augite-andesites. 

 It lies in a district of tuffs and agglomerates. Farther on, about 

 two miles north-west of Narengali, the track crosses some rounded 

 hills, elevated about 600 feet, on the top of which is displayed a 



