ANIMALCULES WITH SHELLS. 81 



taneous fissuration, gemmation, and ova — extensive de- 

 posits or strata of their cases are constantly forming at 

 the bottom of lakes, rivers, and seas. Hence have 

 originated the layers of white calcareous earth common 

 in peat-bogs and morasses, — the tripoli, or polishing 

 slate of BiHn, consisting wholly of the siliceous cases of 

 animalcules, — and the bog-iron, composed of the ferru- 

 ginous shields of other forms of Polygastria. In short, 

 the extensive and important changes that have been 

 produced on the earth's surface by tliis agency, in the 

 earlier ages of the physical history of our planet, and 

 those of a like nature which are going on at the present 

 tmie, are in the highest degree interesting, and have but 

 lately become the subject of scientific investigation*. 



* For an account of fossil animalcules, see ''Wonders of Geology," 

 vol. ii, p. 798 ; " Medals of Creation," p. 215 ; and " Thoughts on a 

 Pebble," 7th edition. Vide Appendix, 



