COMPOSITION OF MARINE DEPOSITS. 



631 



ganisms, both animal and vegetable, but that it is entirely or 

 almost wholly composed of such remains. Among these were 

 about 26 species of Diatom acese (siliceous), 8 species of Foramini- 

 fera (calcareous), and a miscellaneous group of objects (Fig. 334), 

 consisting of calcareous and siliceous spicules of Sponges and 

 Gorgoniae, and of fragments of the calcareous skeletons of Echi- 

 noderms and Mollusks. 



445. ~Now almost exactly the same collection of forms, with 

 the exception of the siliceous Diatomacese, 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 Creta- 

 ceous (chalk) formation has a like composition ; for many parts 



FIG 335. 



Microscopic Organisms in Chalk from Gravesend; a, 6, c, d, Textularia globulosa; c, , , 

 Rotalia aspera ; /, Textularia aculeala; g, Plaimlaria hexas: h, Navicula. 



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. 335, 336) ; in other in- 

 stances, the disintegrated prisms of Pinna ( 336) 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 

 the shells of Cytherina, a marine form of Entomostracous Crus- 

 tacean ( 367). Different specimens of Chalk vary greatly in the 



