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 groundmass contains a little brown interstitial 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 2·91. 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 flow from the old Ndriti crater that the submarine bank was formed off the adjacent coast on which the low Lekumbi promontory has been built up.
In the numerous dykes of the Ndriti basin and in the great alteration which their rocks have frequently undergone, we have evidence in support of the view that this is an old crateral cavity, an opinion that is supported by the indications of lava-flows that have issued, apparently in later times, from the mouth of the basin. Reference has already been made to the locality where a dyke-rock and the rock-mass, into which it has been intruded, are both propylitic; and from this and other facts, such as the varying degrees of alteration in different parts of the basin, it is to be inferred that in the last stage of the activity of this vent its bottom and sides were extensively affected by solfataric influences. Since that period, the configuration of the crater-basin has been greatly modified through the denuding agencies.