xxm SUBMARINE PALAGONITE TUFFS 327 



2,000 and 2,500 feet above the sea. In the interior of the island 

 they are generally to be observed underlying the basic agglomer- 

 ates. Wherever an extensive exposure of the agglomerates exists 

 in the mountainous districts, these tuffs are as a rule to be found at 

 the base of the cliffs. The precipitous bluff of agglomerate, that 

 so often gives a character to the mountainous interior, and the line 

 of cliff of the same formation that runs along the slopes, represent the 

 work of landslips, as is shown by the huge masses of agglomerate 

 lying on the ground below. These " slips " are not uncommon, and 

 are due to the undermining influence of the springs that percolate 

 through the tuffs and clays underlying the agglomerates. 



When the tuffs are well displayed they as a rule show stratifi- 

 cation. The bedding may be indicated either by distinct parting- 

 lines or by alternating bands of varying degrees of coarseness. 

 That these deposits, when occurring in mass in the upland regions, 

 are often horizontal or but slightly inclined, is evidenced by the 

 Nganga-turuturu Cliffs, 1,200 feet above the sea, which are described 

 in Chapter VIII., in the line of cliffs behind Sealevu (Chapter XL), 

 and high up the slopes of Mount Thambeyu (Chapter XII.) as high 

 as 1,500 feet. This is also shown in the circumstance that the line of 

 junction with the overlying agglomerate, except in rare cases, as in 

 that of the Mbenutha Cliffs, is generally horizontal. It is, however, 

 not uncommon to find the beds exposed on the mountain-flanks 

 dipping away from the axis of the range at a small angle, as on the 

 slopes behind Mbale-mbale and in the Sokena Cliffs. In the 

 lower regions, where these deposits are associated with the volcanic 

 mud-rocks on the basaltic plains, they are but slightly inclined- 

 On the other hand, as in the Kumbulau district, the sea-cliffs for 

 some miles may be composed of tuffs more or less steeply tilted. 



These tuffs are generally more or less compacted and have a 

 greyish colour ; but as usually exposed in a weathered condition 

 they are often pale brown or yellowish and are more friable. They 

 may be uniform in structure, or they may display thin seams of a 

 marl-like clay, or they may contain numerous lapilli of vesicular 

 basic glass extensively palagonitised. Not uncommonly they 

 contain larger fragments of basic rocks, and when these arc at all 

 frequent the terms " agglomerate-tuff" or " tuff-agglomerate " have 

 been employed according to the preponderance of either material. 



Many of these tuffs show no effervescence with an acid ; and 

 this is especially the case with specimens at all weathered. On the 

 other hand there are just as many that contain a little carbonate of 

 lime, not usually more than 3 or 4 per cent., but sometimes 



