304 THE FOEAMINIFEEA 



glauconitic casts in abundance. Similar glauconitic 

 ■casts are forming within the shells of Foraminifera at 

 the present day, as, for example, in the greensands 

 and glauconitic muds of the Agulhas Shoal, off the 

 South African coast. 



Casts of Foraminifera in iron pyrites, or, more 

 strictly speaking, in Marcasite, are frequent in many 

 clays which are strongly impregnated with sulphides, 

 as the Lias Clay, Gault Clay^ and London Clay, and 

 the rapid change from the disulphide into the 

 sulphate is frequently seen in the fracture and 

 ultimate destruction of the shell. 



Calcite infillings are often observed in the forami- 

 niferal tests of certain chalk strata, such as that of 

 Southerham, near Lewes, but, owing to the ease with 

 which such casts are broken up, on account of the 

 cleavage in the mineral, they are not often seen in 

 the separated condition. From certain of the 

 Egyptian limestones the writer has described Num- 

 mulites and other Foraminifera in which the internal 

 septation is broken down and the whole of the test in 

 some instances occupied by a mould of Calcite. 



Phosphatig casts and replacements of Foraminifera 

 are met with abundantly in certain limestone deposits 

 where the strata have been impregnated with phos- 

 phoric acid, due to the decomposition of fish remains 

 in many cases, and giving rise to the phosphate of 

 lime or phosphatite. A notable instance of phos^^ha- 

 tised Foraminifera is that occurring in the Phos- 

 phatic Chalk of Taplow and Twyford. One may 

 •easily obtain very beautiful casts of the Foraminifera 



