114 MICROSCOPICAL OBJECTS FOUND 



been completed, exhibiting either a crystalline and 

 quartzose, or the botryoidal aspect so common 

 to the Chalcedonies deposited by the Geysers. 

 In neither of these is any trace of the original 

 structure of the shell preserved, and I cannot 

 but think that the removal of the lime has in 

 such cases been rendered more or less complete 

 before the introduction of the silica; consequently, 

 instead of being deposited in such a way as to 

 preserve the original structure, atom replacing 

 atom, the cavities are more or less perfectly filled 

 vs^ith crystalline chalcedony, according apparently 

 to the duration of the process, and the size of the 

 cavity ; the smaller organisms being in general 

 completely solid, whilst the larger ones are merely 

 lined with the siliceous matter. Similar phenomena 

 occur amongst the silicified organisms of the harder 

 portions of the Calcareous Grit in Yorkshire. 



What has taken place in the flints of the Chalk ? 

 This is difficult to answer, as we have evidence 

 of a much more complicated agency. I have 

 already remarked, that the Rotaliae from the 

 chalky surfaces of flints, though unattached to 

 the flint itself, are nevertheless often changed 

 into silica, and it would appear that the Rotaliae 

 embedded in the solid flint have also become 



