The blocks in the agglomerate-tuff are either angular or sub-angular, and are less than a foot across. They are all composed of more or less vitreous porphyritic olivine-basalts, showing large crystals of plagioclase a fifth of an inch (5 or 6 mm.) in length. But they vary somewhat in character. Some of them, that are vesicular and almost scoriaceous, may be termed from their glassy nature porphyritic pitchstones. Others again, where the groundmass is hemi-crystalline, may be designated porphyritic compact basalts, and are referred to genus 37 of the olivine-basalts.
The matrix of the agglomerate-tuff is made up of angular fragments, up to 5 mm. in size, of singular vitreous and semi-vitreous olivine-basalts, in part palagonitised. There is evidence of crushing _in situ_ of some of the porphyritic felspar crystals; but it is not so marked as elsewhere noticed. The palagonite is also in part interstitial, a character that goes to support the view advanced on page 342, that the palagonite may be connected in its origin with the heat developed during crushing, only a moderate temperature being required for the partial fusion of the glass.
In crossing the range by this route from Mbale-mbale one first ascends, as above observed, the spur of the Koro-tini Bluff up to a height of 1,200 feet. The track then descends into the valley-gorge of the Natoarau river on the east, the bottom of which is 750 feet above the sea, and from here the climb begins. One ascends the bed of the stream course, clambering over slippery rock surfaces up to 1,200 or 1,300 feet, where the stream is left, and the mountain-slopes, often steep and precipitous, are then followed to the summit, 2,000 feet in height. Coarse and fine palagonite-tuffs and agglomerate-tuffs of the same character are exposed on the surface from the commencement of the ascent up to 1,850 feet; but they are displayed much more extensively in the stream-course than in the soil-covered upper slopes.
The tuffs are grey except when hydrated, when they turn yellowish-brown. Some of them contain lime, as much at times as 10 or 12 per cent.; whilst others possess little or none. Tests of foraminifera are not infrequently inclosed, even as high as 1,850 feet. A description of one of these tuffs containing a few tests of Globigerina, which was obtained at 1,200 feet, is given on page 331, under sample D. It will be there seen that they are derived from different basic rocks, some containing but little glass, others mainly vitreous, only the more glassy constituents being palagonitised. The palagonite-tuff sandstones exposed in large blocks on a bare spur at 1,850 feet contain 12 per cent. of lime, the largest tests of foraminifera being not over half a millimetre.[67] These tuffs occasionally show bedding. At 1,000 feet they dip gently to the S.S.W., and at 750 feet they are inclined about 15° in the same direction. In this last locality they consist of alternating layers, 1 to 4 inches in thickness, of fine and coarse tuffs, the coarser looking like sandstone.... The blocks in the agglomerate-tuff are sub-angular, and of an olivine-basalt with hemi-crystalline groundmass,[68] their size ranging from 2 feet to an inch. I noticed one large block of this rock, measuring 2 × 1½ × 1 feet, imbedded alone in the tuffs at 1,200 feet. At one place a tuff containing small fragments of basalt displayed a concretionary structure, indicating probably the proximity of a dyke, the globular masses being 4 feet across. A little lime occurs in the matrix of the agglomerate-tuff.