COMPOSITION OF FLINT 139 



acts but very slowly upon it ; hence its enduring quality 

 as a building material. These properties are largely 



FIG. 37. Coccoliths, after H. Lohmann. 1. Cocco- 

 lithophora pelagica, Wallich : seen en face. 

 2 and 3. Coccolithophora wallichi, Lohmann : 

 2 seen en face, 3 in optical section, longi- 

 tudinal. 4. Pontosphcsra huxleyi, Lohmann, 

 a single complete individual. co'. Outer 

 coccoliths. co". Inner coccoliths. 5 and 6. 

 Pontosphcera hczckeli, Lohmann, coccoliths. 

 7. Syracosphcera pulchra, Lohmann, complete 

 individual, chr. Chromatophore. 8. Cocco- 

 lithophora wallichi, complete individual, n. 

 The nucleus, o. Opening in the cyst. Ik. 

 Kefractile body. All figures magnified 2,000 

 diameters, except Figs. 5 and 6, which are 

 more highly magnified. 



dependent on its chemical composition. Flint, unlike 

 chalk, does not consist of carbonate of lime, but of 

 silica, which is known to us in another form as rock- 



