98 MICROSCOPICAL OBJECTS FOUND 



we have only siliceous organisms. It becomes 

 probable that many of our European Green-sands, 

 and other siliceous strata, however barren of 

 such structures they appear, may have once con- 

 tained multitudes of calcareous microscopic organ- 

 isms, some of which have been removed after the 

 consolidation of the strata, leaving either hollow 

 casts, or having had the cavities subsequently 

 filled with Silica, as in the case of the shells at 

 Blackdown; in other instances the change may have 

 taken place whilst the deposits were soft, when the 

 siliceous sand would be pressed down into the 

 spaces previously occupied by the animal body, 

 and thus all traces of the organised structure 

 might permanently disappear. 



Nature furnishes us with an agent quite equal to 

 the production of such effects as we are at present 

 acquainted with. This is carbonic acid gas, 

 in solution in water. Mr. Lyell has already 

 availed himself of this instrument to account for 

 the subtraction of calcareous matter from imbedded 

 shells, as well as for some of the changes that 

 have taken place in the structure and composition 

 of stratified rocks.* He has also recorded one 



* Principles of Geol. Vol. i. p. 318. Second Edition. 



