160 A NATURALIST IN THE PACIFIC CHAP. 



On the other hand, the tuffs (often foraminiferous) as well as the 

 agglomerate-tuffs of the north and south slopes of the range are in 

 part suggestive of marine erosion. Intrusive masses of basalt are 

 to be observed occasionally, and doubtless to this cause may be 

 attributed the concretionary structure of the tuffs in places, and 

 the alteration of these deposits in one or two localities, where they 

 are penetrated by cracks filled with chalcedony. 



(3) TRAVERSE OF THE KOROTINI RANGE FROM VATU-KAWA 

 TO VANDRANI. On leaving Vatu-kawa 1 , which is not more than 

 50 feet above the sea, the ascent for the first 600 feet up the steep 

 mountain-side lies along the rocky bed of the Wai-ni-ngio River, 

 which from its rapid fall has more the character of a torrent. On 

 its sides are exposed basic agglomerates and agglomerate-tuffs ; 

 whilst the large boulders in its bed are composed of a somewhat 

 altered olivine-basalt. At 600 feet the track abandons the stream- 

 course for the steep mountain slopes, and thence up to 1,100 feet 

 similar agglomerates and tuffs prevail. At this last-named elevation 

 there are displayed fine and coarse indurated palagonite-tuffs, a 

 little altered in character and with little or no lime. A specimen 

 of the former, of which the materials composing it do not exceed 

 2 mm. in size, shows in the slide an occasional " Globigerina " test 

 filled with palagonitic debris. Such a marine deposit is evidently 

 not of shallow-water origin. The coarser tuff is made up of com- 

 pacted sub-angular fragments, not over 2 mm. in size ; but contains 

 no organic remains. The prevailing rocks exposed between 1,100 

 and 1,900 feet, a little below the summit of the range, are somewhat 

 altered compacted non-calcareous breccia-tuffs, composed of 

 sub-angular fragments 5 or 6 mm. in size, of a more or less glassy 

 and often vacuolar basic or basaltic andesite, only in part pala- 

 gonitised, the vacuoles as well as the interstices between the 

 fragments being sometimes filled with a zeolite. 2 



1 On the right side of the river close to Vatu-kawa there are some cliffs 

 displaying a section of the mountainous spur, referred to on p. 151, that separates 

 the valleys of the Mbale-mbale and Vatu-kawa rivers, an exposure quite apart 

 from the rocks exhibited on the adjacent southern slopes of the main range. 

 These cliffs are formed of bedded grey tuffs marked by single layers of blocks 

 6 to 8 inches across and dipping about 30 S.S.W. The tuffs in their texture 

 are not unlike sub-aerial tuff-deposits. They contain no lime and are composed 

 of basic materials with a little palagonite. They seem to indicate some sub- 

 sidiary vent, close to the present village of Vatu-kawa, which may have been 

 active shortly before or during the emergence of this district. 



2 These altered tuffs on the southern slope of this range are described 

 on p. 332. 



