NATURAL HISTORY OF CREATION. 95 



pi^oportion of the organic matter, being in some instances 

 almost entirely composed of it. He has been able to 

 classify many of these creatures, some of them being 

 allied to the nautili, nummuli, cyprides, &c. The shells 

 of some are calcareous, of others siliceous. M. Ehrenberg 

 has likewise detected microscopic sea-plants in the chalk. 



The distinctive feature of the uppermost chalk beds 

 in England is the presence of flint nodules. These are 

 generally disposed in layers parallel to each other. It 

 was readily presumed by geologists that these masses were 

 formed by a chemical aggregation of particles of silica, 

 originally held in solution in the mass of the chalk. But 

 whence the silica in a substance so different from it ? 

 Ehrenberg suggests that it is composed of the siliceous 

 coverings of a portion of the microscopic creatures, whose 

 shells he has in other instances detected in their original 

 condition. It is remarkable that the chalk loith flint 

 abounds in the north of Europe ; that ivithout flints in 

 the south ; while in the northern chalk siliceous animal- 

 cules are wanting, and in the southern present in great 

 quantities. The conclusion seems but natural, that in 

 the one case the siliceous exuviae have been left in their 

 original form; in the other dissolved chemically, and 

 aggregated on the common principle of chemical afiinity 

 into nodules of flint, probably concentrating, in every 

 instance, upon a piece of decaying organic matter, as has 

 been the case with the nodules of ironstone in the earlier 

 rocks, and the spherules of the oolite. 



What is more remarkable, M. Ehrenberg has ascer- 

 tained that at least fifty-seven species of the microscopic 

 animals of the chalk, being infusoria and calcareous- 

 shelled polythalamia, are still found living in various 

 parts of the earth. These species are the most abundant 



