IX THE NDREKETI PLAINS 133 



Casuarina are most conspicuous amongst the trees, bushes, herbs, 

 grasses and ferns predominate. Here the native Ginger and the 

 native Turmeric with species of Tacca are frequently to be recog- 

 nised, and the waste-land bushes of Dodonaea viscosa and Mus- 

 saenda frondosa are abundantly to be found. 



As in the Sarawanga plains, the basaltic rocks are here often 

 overlain or incrusted by submarine deposits, the former exposed in 

 all the deeper river-beds, the latter frequently displayed in the 

 sides of their tributaries. 



I will deal first with the basaltic rocks. In the places where 

 the surface deposits have been stripped off, these rocks are 

 generally exposed as decomposing boulders, the spheroidal 

 structure being well developed in the weathering process. Not 

 infrequently, however, a rudely columnar structure is exhibited 

 where the rivers have cut deeply into the basalt. The columns 

 that I observed were usually vertical. In the river-bed at the 

 landing-place at Mbatiri, for instance, the columns are from 2^ to 

 3 feet across and vertical. As exposed in the river-crossing about 

 a mile above this town they are 12 to 15 inches in diameter and 

 also vertical. However, at Na Kalou, a coast village about i^ miles 

 east of the mouth of the Ndreketi, where there is an unexpected 

 exposure of basalt, the columns, about a foot in diameter, are 

 inclined at an angle of about 20° from the vertical and face to the 

 north. 



These rocks are, as a rule, compact, only showing a typical 

 scoriaceous structure in the case of specimens obtained near the 

 foot of Nakambuta, an isolated hill about three miles to the 

 southward of Natua, which probably represents a vent of more 

 recent times. Often, however, they have a pseudo-vesicular 

 appearance, from the occurrence in the midst of the patches of 

 interstitial glass of minute irregular cavities that seem to have been 

 formed during the last stage of consolidation of the magma. 



The prevailing type of basalts is a blackish, doleritic, semi- 

 ophitic rock without olivine, with specific gravity 278 to 2"8o. 

 They are characterised by the length of the felspars of the ground- 

 mass ("22 — "35 mm.), by the large size of the augite granules 

 (•I — '3 mm.), and by the quantity of dark interstitial glass. They 

 present two forms, one with and the other without plagioclase 

 phenocrysts. The first kind is referred to genus 9 of the augite- 

 andesites (page 272), some of the specimens being referred to the 

 porphyritic sub-genus, and others to the non-porphyritic sub- 

 genus, according to the size of the plagioclase phenocrysts. The 



