ABSTRACT
I SH O U LD have been glad to accept Mr. Ainsworth’s proffered hospitality for a few days’ rest ; but I was now at the entrance to the country I had especially come to explore, and nothing could tempt me to linger on the threshold. Up to this point the work had not been of special interest. The geology was monotonous and dull, and the geography was fairly well known. But a change in the geological structure of the country was at hand. In the best existing sketch map of the geology of East Africa, a large tract of country to the west of Machakos is coloured as recent alluvium. I expected this would prove to be an old lake basin or a desert of windborne drift. From the hills crossed on the way to Machakos, I had caught through the clouds occasional glimpses of a vast level plain, and these had strengthened this expectation. Great therefore was my surprise, on reaching the summit of the last ridge of the Iveti Mountains, to see to the west an undulating prairie instead of a level plain, and that this was composed not of alluvium or sand, but of a hard, dark-coloured rock. Its extent also was greater than I had expected. Here and there in the foreground, bosses and ridges of gneiss, such as Lokenya and Koma, rose above the surface ; a few dark lines of trees marked the courses of the rivers. Except for these, we could see only a vast expanse of rolling grass-land, extending westward and southward as far as the eye could follow it. The
rock of this prairie ended abruptly at the foot of the old gneiss ridge on which I stood, but it followed its outline, running up the valleys, round the spurs, and into the hollows of the mountains, just as the water of a lake adapts itself to the irregularities of its shore. In this, as well as in other ways, the view reminded me so much of that of the Snake River lava plains of Idaho, as seen from the western flanks of the Rocky Mountains, that I felt sure that this was a plain of lava and not of alluvium. I walked quickly down the slope to the nearest point where the rock could be seen, and found, as I expected, a lava, a coarse trachyte with very large porphyritic crystals of sanidine.
