178 A NATURALIST IN THE PACIFIC chap. 



the first contain only a little carbonate of lime. No sections have 

 been made of these deposits ; but when powdered and examined 

 under the microscope they appear to have the same general 

 composition as the deposit described above from an elevation 

 of 950 feet. They are probably foraminiferous though scantily. 

 The tuffs found at the Ndromo Cave (2,100 feet) contain 4 per 

 cent, of carbonate of lime and small tests of foraminifera are 

 visible with a lens. The mineral fragments include plagioclase 

 and rhombic pyroxene, and there are inclosed rounded gravel- 

 fragments, 5 mm. in size, of a semi-vitreous rock. Palagonitic 

 debris make up the mass of these tuffs. A coarse deposit from 

 2,500 feet is non-calcareous, but has the same general composition. 



(2) The agglomerates. — These deposits are best represented 

 in the upper part of the mountain, between 1,500 and 2,200 feet 

 above the sea. Here they often present vertical precipices having 

 a drop varying between 100 and 400 feet, with the submarine tuffs 

 exposed at their base. Such cliffs, however, display no structure. 

 Their vertical faces are to be attributed to joints and to the 

 extensive " slips " that frequently occur on these slopes, when 

 large masses of agglomerate, undermined by the percolation of 

 springs through the tuffs beneath them, roll far down the moun- 

 tain-sides. The blocks of the agglomerates are fairly uniform in 

 size, being usually 4 or 5 inches across. They are composed of a 

 semi-vitreous hypersthene-augite andesite, containing both augite 

 and rhombic pyroxene, but of an unusual type. It is a blackish 

 rock carrying opaque phenocrysts of plagioclase, and is charac- 

 terised by the prismatic form of the pyroxene (monoclinic) of the 

 groundmass. A very similar rock from the Sokena agglomerates 

 has been before described. It is referred to genus 1 8 of the class, 

 and the prismatic sub-order to which that genus belongs is 

 described on page 289. 



(3) The junction of the agglomerates and submarine tuffs. — This 

 is well displayed at the Taloko Cave. Here the agglomerates lie 

 conformably on the sedimentary tuffs ; but the line of junction 

 is sharply defined and the only evidence of transition is afforded 

 by the great diminution in the size of the blocks of the agglo- 

 merates, which are i to 2 inches across. Immediately beneath the 

 agglomerate is a layer 2\ centimetres thick of a rather coarse 

 sedimentary palagonite-tuff having the composition of the deposits 

 above described, but not effervescing with an acid, and showing 

 no foraminiferous tests. The size of its '' grain " is about a milli- 

 metre. This passes downward rather abruptly into a chocolate- 



