XI THE KOROTINI RANGE 157 



southern edge of the level-topped mountain range behind, and like 

 the Sealevu cliffs on the north side it affords a natural section of 

 its mass. It is shown in the plate facing page 15 3, where it rises at 

 the back of the lagoon. 



Approaching the bluff from Mbale-mbale, one crosses a low-lying 

 district less than 100 feet above the sea before striking the spur. 

 Here and in the lower few hundred feet of the spur are exposed 

 basic agglomerates, and occasionally in the mass a semi-vitreous 

 vesicular olivine-basalt, almost like a pitchstone, and displaying 

 large porphyritic crystals of plagioclase, 5 or 6 millimetres long, the 

 agglomerates being made up of the same material. Higher up, at 

 elevations between 1,000 and 1,500 feet, are exposed coarse pala- 

 gonite-tuffs made up of fragments, usually i to 3 mm. in size, 

 of extensively palagonitised basic vitreous rocks, such as occur in 

 the cliffs above. These tuffs become coarser as one approaches 

 the precipitous bluff, the base of which lies about 1,650 feet above 

 the sea. Here the cliffs present a bare rocky face, some 200 feet 

 high. The lower portion is composed of an agglomerate-tuff, and 

 the upper portion mainly of agglomerates. These deposits dis- 

 play no bedding excepting a single plane of division inclined 

 steeply to the north at an angle of perhaps 40°. 



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 frag- 

 ments, up to S 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 sa 

 marked as elsewhere noticed. The palagonite is also in part inter- 

 stitial, 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- 



