XXIII VOLCANIC MUD-ROCKS 323 



The organic remains consist mainly of tests of minute foramini- 

 fera of the " Globigerina " type, casts of which, both glauconitic 

 and chalcedonic, occur in the residue. About 88 per cent, of the 

 residue consist of fine clayey materials less than "25 mm. in size. 

 The mineral fragments, which average about "i mm. in diameter, are 

 mostly of felspar with a little pyroxene and brown hornblende. 



B. From the Mbenutha Cliffs, 1,100 feet above the sea. 



Carbonate of lime 15 per cent. 



/Fine material mainly derived from the degra- 



„ . , I dation of palagonite 60 „ „ 



R^^^due-^ Minerals 23 ,, ," 



leasts of foraminifera 2 „ „ 



100 



This rock is somewhat hard, so that the proportion of 

 fine clayey material, which is however large, cannot be ac- 

 curately determined. It shows in places dark streaks composed 

 of an abundance of minute and often perfect tabular crystals of 

 zoned plagioclase and prisms of rhombic pyroxene, the size in 

 neither case exceeding half a millimetre, both of them being 

 derived from the acid andesites of the neighbourhood. In the 

 slide it displays minute tests of foraminifera of the " Globigerina "" 

 type in a matrix formed mainly of palagonitic debris, fragments; 

 of minerals and semi-vitreous basic rocks. The larger fragments 

 of the minerals and of the volcanic rocks do not exceed '15 mm. ; 

 but most of the material is very fine. The tests of the foramini- 

 fera are sometimes filled with the matrix, but often they are 

 entirely of calcite and exhibit in polarised light a dark cross. 



C. From between Natua and Mbatiri, about 290 feet above the 

 sea. 



Carbonate of lime 25 per cent. 



[■Fine material derived from the degradation of 



palagonite and of semi-vitreous basic rocks 62 „ „ 



Residue 



Minerals 2 



^ Casts of foraminifera 11 



» )> 



100 



This is a relatively deep-water deposit, the foraminifera being 

 ninute and of the *' Globigerina " type. About 90 per cent, of the 

 •esidue consists of fine clayey material, with which the calcite is 

 ;o intimately mixed that each particle is highly refractive and 

 effervesces freely in an acid. The mineral fragments (pyroxene 



Y 2 



