v MOUNT SEATURA 71 



dyke, four inches wide, has penetrated the lava, acquiring at the 

 same time a more glassy texture. The small size of the felspar- 

 lathes of both rocks distinguishes them from the dyke rocks of the 

 basin, where the felspars are twice as long. Both rocks show some 

 degree of alteration. 1 



In following the valley of the Ndama River from Ndriti to 

 Telana, about three miles farther down, one traverses a picturesque 

 region. Emerging from the great basin the river flows through 

 the rolling plains of the " talasinga " district. Near Ndriti, and 

 occasionally on the way to Telana, is exposed a scoriaceous grey 

 basaltic rock ; and between two and three miles below Ndriti 

 there is to be observed in the river-bed evidence of a comparatively 

 recent flow of a highly basic scoriaceous lava from the ancient 

 crater of the Ndriti basin. The rock, which is dark and fresh- 

 looking, shows large porphyritic crystals of augite and olivine but 

 no plagioclase, whilst the ground mass contains a little brown inter- 

 stitial glass. Its characters will be found described under genus 3 

 of the olivine-basalts (p. 255). Its specific gravity, notwithstanding 

 its large empty steam-pores, is 2~gi. It differs markedly from 

 the basaltic rocks of the Seatura slopes and the Mbua and Ndama 

 plains, in the great porphyritic development of augite and olivine, 

 in the large size of the felspars and augite of the groundmass, and 

 in its numerous steam-holes. But in the coarseness of its small 

 felspars it belongs to the same type as ,the altered or propylitic 

 basic rocks of the Ndriti basin. It is probably by some such lava 



1 The dyke-rock has a specific gravity of 27 ; but is slightly vesicular. It 

 shows a few small plagioclase phenocrysts in a groundmass of felspar-lathes, 

 augite grains and prisms, magnetite, and a little brown interstitial glass. The 

 felspar-lathes average "14 mm. in length and are for the most part not parallel. 

 Secondary calcite occurs in the groundmass, and the powdered rock effervesces 

 a little in an acid. 



The rock forming the offshoot of the dyke differs only from the parent rock 

 in its more vitreous character. Although the felspars and augites of the 

 groundmass are fairly developed, the residual glass is much more copious, and 

 in places where it has segregated, forming "lakelets," it has been subjected to 

 an alteration often observed in palagonite when there are concentric alternating 

 zones of a tan-coloured fairly refractive material and calcite. 



The reddish scoriaceous lava in contact with the dyke shows no pheno- 

 crysts. The groundmass displays more or less parallel felspar-lathes, "i mm. 

 long, augite grains, and much magnetite, The residual glass is fair in quantity ; 

 but is mostly gathered into "lakelets" of brown altered glass with sometimes 

 calcite in the centre. 



The vitreous border of the dyke is composed of a dark glass quite opaque 

 in the outer portion, but clearer and showing incipient crystallisation in the 

 nner portion. 



