CRETACEOUS ERA. 71 



sumed 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 with flint abounds in the north of Europe ; that 

 without flints in the south ; while in the northern chalk 

 siliceous animalcules are wanting, and in the southern present 

 in great quantities. The conclusion seems natural, that in 

 the one case the siliceous exuviae have been left in their 

 original form ; in the other, dissolved chemically, and aggre- 

 gated on the common principle of chemical affinity 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 ascertained 

 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 in the rock. Singly they are 

 the most unimportant of all animals, but in the mass, forming 

 as they do such enormous strata over a large part of the 

 earth's surface, they have an importance greatly exceeding 

 that of the largest and noblest of the beasts of the field. 

 Moreover, these species have a peculiar interest, as the only 

 specific types of that early age which have survived to the 

 present day. While the specific features of all higher ani- 

 mals have been again and again changed since that period, 

 these humble creatures have preserved the characters they 

 then possessed shall we say, through a continuing unifor- 

 mity in the conditions under which they have lived, while all 

 other animals have been exposed to circumstances productive 

 of change ? 



All the ordinary and more observable orders of the inhabi- 



