VII MBENUTHA CLIFFS iii 



vitreous groundmass.' By clambering up the steep slope on the 

 south side of these cliffs, it wilLbe observed that this thick bed of 

 agglomerate is covered by bedded foraminiferous clays and tuffs 

 similar to those that underlie it. It is therefore vi^ithout doubt 

 submarine, and presents the result of the more violent outbursts 

 of some neighbouring vent. That this vent is now represented by 

 the " plug '' of basic lava forming the peak of Navuningumu is 

 highly probable. It is, however, noteworthy that these beds of 

 agglomerates, tuffs, and clays, as shown in the photograph of the 

 cliffs, are all inclined at an angle of 20° towards the axis of eruption 

 or to the westward. The tuffs and clays underlying the agglome- 

 rates are, as already remarked, much disturbed in places. It would 

 seem that all the beds here exposed were originally horizontal, and 

 were tilted up during the disturbances accompanying the outbursts 

 of volcanic activity. 



The natural section, which the Mbenu-tha cliffs present, is 

 doubtless due to landslips. Similar exposures, displayed by cliffs 

 of basic agglomerate with submarine tuffs and clays at their base, 

 are common on the mountain-slopes of other parts of the island. 

 Water oozes through the underlying soft deposits, and the result 

 is seen in the occurrence of huge masses of agglomerate on the 

 slopes below. 



From the details here given respecting Navuningumu and its 

 surroundings, it is apparent that there have been two stages in the 

 history of this volcanic mountain. The first was submarine and 

 was characterised by the discharge of acid lavas which consolidated 

 around the vent and were afterwards covered over with deposits of 

 foraminiferous clays. The second was in the last part supra- 

 marine. With the renewal of activity, the overlying acid andesites 

 were broken through and basic materials were discharged from the 

 new vent. The bed of acid agglomerates exposed in the Mbenu-tha 

 cliff belongs to that period of the second stage when the explosive 

 agencies were most violent. It represents the extensive destruction 

 of the overlying rocks. The foraminiferous tuff- sandstones are 

 submarine accumulations of the finely comminuted fragments of 

 basic pumice that constituted the dust and fine ash discharged 

 from a supra-marine vent. The scoriaceous and amygdaloidal 

 blocks of the basic agglomerates overlying these tuffs around the 

 base of the mountain have had a similar origin. The original ash- 

 cone that at one time rose above the surface of the sea has long 



' It belongs to the 3rd order of the homblende-hypersthene-andesites 

 described on p. 299. 



