xxin FORAMINIFERAL LIMESTONES 319 



inclosed organic remains has been mainly replaced by more or less 

 crystalline silica. In others a recrystallisation of the calcitic 

 material is in progress, as described on page 131 ; and the matrix 

 presents in places a mosaic of calcite. 



These rocks have therefore been subject to some degree of 

 alteration, the causes probably lying within the mass. They are, 

 however, far from frequent. They are best represented in the upper 

 valley of the Sarawanga River in the vicinity of Tembe-ni-ndio, 

 where they reach to a height of about 250 feet above the sea. The 

 greatest elevation at which I found them was in the mountainous 

 interior of the Waikawa Promontory, where they occur at a height 

 of 1,100 feet above the .sea. 



As samples, the results of the examination of two rocks from 

 the Tembe-ni-ndio district are here appended : — 



A. 

 Carbonate of lime 46 per cent. 



(Fragments of palagonite and of semi-vitreous 

 basic rocks 30 „ „ 



Minerals 12 ,, „ 

 Secondary silica replacing the carbonate of 

 lime in the organic remains 12 , „ 



100 

 It displays to the naked eye fragments of shells including 

 pteropods, and numerous tests of foraminifera 2 or 3 millimetres in 

 diameter. In the section it displays in addition coral debris and a 

 considerable quantity of rounded and sub-angular pieces of pala- 

 gonite and semi-vitreous basic rocks, usually less than a millimetre 

 across, with smaller fragments of minerals (pyroxene and plagio- 

 clase), and much calcitic material in the matrix. 



B. 



Carbonate of lime 25 per cent. 



Debris of palagonite and of basic volcanic rocks . 45 ,, ,, 



Minerals 10 >, » 



Siliceous casts of foraminifera 20 „ „ 



100 

 The organic remains mainly consist of tests of foraminifera, 

 many of which occur in the residue as colourless siliceous casts. 

 The fragments, whether of minerals or of volcanic rocks, are 

 usually less than half a millimetre across. The foraminifera are 

 mostly small and of the " Globigerina " type ; but there is a cast 

 of a tube of some boring-mollusc, and fragments of shells also 

 occur. 



