150 THE FORMATION OF FLINTS 



fera, coccoliths, and other calcareous constituents of the 

 chalky powder, are being dissolved away. When all 

 action has ceased we may conclude that all the carbonate 

 of lime has been removed, but to make certain we add a 

 little more acid. Assured on this point, we now observe 

 that a white sediment still remains at the bottom of the 

 vessel ; this we might assume to consist of sponge spicules, 

 but not to take anything for granted, we will now pour off 

 the acid, wash with clean water, and place the insoluble 

 residue under the microscope. The sponge spicules, as 

 we had expected, are obvious enough, but, to our surprise, 

 so likewise are numerous foraminifera,* still presenting 

 their characteristic form, even down to details of ornament. 

 The minute coccoliths are also represented, and if now we 

 bring to our aid the effects of polarised light, we shall 

 discover that these objects possess the optical properties 

 of crystalline silica, and indeed differ in no essential respect 

 from flint (Fig. 43). Thus we have before us evidence of a 

 process which is the direct opposite of that we referred 

 to in the case of Stauronema, where carbonate of lime 

 had taken the place of opal. Here, on the contrary, silica 

 in a crystalline form has replaced carbonate of lime and 

 produced siliceous pseudomorphs of the constituents of 

 chalk. This, then, is the first step in the formation of a 

 flint, grain by grain, particle by particle, the substance of 

 the foraminifera and coccoliths is removed and silica 

 deposited in its place. 



The next step is a simple deposition of silica from 

 solution by which the siliceous chalk with the mingled 

 remains of sponge spicules, both opaline and crystalline, 

 are cemented together to form a white porous flint. With 

 continued deposition of silica the pores of the white 

 flint are obliterated, and it passes into black flint. 



* This fact was first observed by Mr. Joseph Wright (Rep. Belfast 

 Field Naturalists Club, Appendix III., vol. i., 1875, p. 74). 



