CRETACEOUS ERA. 



83 



FIG. 61. 



. -0/77,7- nir 

 ta; B, Teztulama; C, Ver- 



neuilina; D, Cristellaria; E, 

 Dentalina. 



through the bodies of worms and fish, such as feed on the 



corals of the present day, and in whose stomachs he has found 



impure chalk. This, however, 



cannot be a full explanation of 



the production of chalk, if we 



admit some more recent dis- 



coveries of Professor Ehren- 



berg. That master of micro- 



scopic investigation announces, 



that chalk is composed partly 



of "inorganic particles of ir- 



regular elliptical structure and 



granular slaty disposition," 



and partly of shells of incon- 



ceivable minuteness, " varying 



from theone-twelfth to the two 



hundred and eighty-eighth 

 .. L- l 



part of aline a cubic ^ men 



of the substance containing 

 above ten millions of them ! 



The chalk of the north of Europe contains, he says, a large 

 proportion of the inorganic matter ; that of the south, a larger 

 proportion of the organic matter, being in some instances 

 almost entirely composed of it. The shells of some are cal- 

 careous, of others, siliceous. M. Ehrenberg has likewise 

 detected microscopic sea-plants in the chalk. 



The distinctive feature of the uppermost chalk beds in Eng- 

 land is the presence of flint nodules. These are generally 

 disposed in layers parallel to each other. It was readily pre- 

 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 chemi- 

 cally, and aggregated on the common principle of chemical 

 affinity into nodules of flint, probably concentrating, in every 



G2 



