MICROSCOPIC GEOLOGY I — CHALK. 



70/ 



Mud, with the exception of the siliceous Diatomaceje, is found in 

 many parts of the ' Calcaire Grossier' of the Paris basin, as well 

 as in other extensive deposits of the same early Tertiary period. 

 And there is little doubt that a large proportion of the great Cre- 

 taceous (Chalk) formation has a like composition ; for many parts 

 of it consist in great part of the minuter kinds of Foraminifera, 

 whose shells are imbedded in a mass of apparently amorphous 

 particles, many of which, nevertheless, present indications of 

 being the worn fragments of similar shells, or of larger calcareous 

 organisms. In the Chalk of some localities, Foraminifera consti- 

 tute the principal part of the minute organisms which can be 

 recognized with the .Microscope (Figs. 404, 4u5) ; in other in- 



Fig. 405. 



Microscopic Organisms in Chalk from Meudon ; seen partly as 

 Opaque, and partly as Transparent objects. 



stances, the disintegrated prisms of Pinna (§ 45S), or other large 

 Shells of the like structure (as Inoceramus) constitute the great 

 bulk ; whilst in other cases, again, the chief part is made up of 



