SILIFICAT1ON OF CHALK 151 



Patches of chalk escape to a slight but variable extent 

 complete replacement, and these remain as the grey flint 

 which occurs in irregular layers or scattered blotches in 

 the midst of the black. 



The pores of ordinary chalk amount to somewhat over 

 30 per cent, of its bulk, and consequently 70 per cent, of 

 the silica of a flint, disregarding included sponge spicules, 

 has been deposited as a pseudomorph after carbonate of 

 lime, the remaining 30 per cent, has filled up the pores. * 



In the case of some hollow flints the solution of silica 

 has traversed the whole thickness of the wall, and 



FIG. 43. A Silicified Foraminifer. 



1. Seen in optical section. t. 



The shell, still showing traces 

 of fibrous structure, o. Opal. 



2. The same, seen by polarised 



light, between crossed Nicols 

 (x 100). 



reaching the central cavity, has crystallised out in well- 

 formed crystals of quartz. In this we have evidence of a 

 long-continued permeability which permitted the passage 

 of the solution till the pores of the walls were destroyed. 

 As a rule the grey flint is irregularly distributed, but in 

 some cases it occurs in layers more or less regularly alter- 



* Out of the regular form of these blotches the fertile fancy of the 

 older observers sometimes derived strange shapes. As we sometimes 

 picture faces in the fire, so they saw in these outlines various animal 

 forms, cocks and hens, human faces, and much else, which were 

 solemnly described as fossils, and elaborately illustrated by numerous 

 copper-plate engravings. 



