CRETACEOUS FORMATION". 



G3 



line" — a cubic inch of the substance containing above ten 

 millions of them ! The chalk of the north of Europe con- 

 tains, he says, a larger proportion of the inorganic matter ; 

 that ot the south, a larger proportion of the organic mat- 

 ter, 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 nautilli, numuli, cyprides, 

 &c. The shells of some are calcareous, of others silice- 

 ous. M. Ehrenbe/g 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 ge- 

 nerally 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 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 quanti- 

 ties. 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 affinity into nodules ot 

 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 ascertain- 

 ed that at least fifty-seven species of the microscopic ani- 

 mals of the chalk, being infusoria and calcareous-shell- 

 ed 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 ani- 

 mals, 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 no- 

 blest of the beasts of the field; Moreover, these species* 

 have a peculiar interest, as the only specific types of that 

 early age which are reproduced in the present day. Spe- 



