72 MICROSCOPICAL OBJECTS FOUND 



D'Orbigny, for abstracts of whose masterly papers 

 we are indebted to Mr. Weaver.* The general 

 result at which all have arrived is, that Chalk 

 contains vast numbers of minute organisms, espe- 

 cially Foraminifera, to which the deposit ])rinci- 

 pally owes its origin. 



The Cretaceous strata in which the structure of 

 these organisms appears to be the least altered, 

 are those of the Upper Missouri, U.S., in which 

 some of the species are identical with, and others 

 have a close affinity to those from the Chalk of 

 our own island; Rotalia globulosa(Ehr.)and Tex- 

 tillaria Americana (Ehr.) being the most common, 

 the latter replacing the T. globulosa (Ehr.) 

 of Europe. They still present the vitreous 

 and transparent appearance, which characterises 

 similar forms of Rosalinse and Rotalia3 in a 

 recent state ; along with these, the same deposit 

 also contains a large amount of opaque amorphous 

 calcareous matter, very different in its character 

 from what would be presented by the hyaline 

 fragments of Rotaliae, in an unaltered state. 



Chalk from Dover, for which I am indebted 



* Phil. Mag., London and Edinburgh. Vol. xviii, p. 37.'>, 

 and Annals of Nat. Hist. June and July, 1841. 



