THE MICROSCOPE IN GEOLOGICAL INVESTIGATION. 



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the " coccoliths ' and ' coccospheres ' formerly described (409). Now 

 the resemblance which this Globigerina-mud, when dried, bears to 

 Chalk, is so close as at once to suggest the similar origin of the latter, 

 and this is fully confirmed by Microscopic examination. For many sam- 

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

 especially Globigerince (Figs. 489, 490), 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, the disin- 

 tegrated prisms of Pinna ( 563), or of other large shells of the like struc- 

 ture (as Iiioceramus}, form the great bulk of the recognizable compo- 

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

 shells of Cytherina, a marine form of Entomostracous Crustacean ( 604). 

 Different specimens of Chalk vary greatly in the proportion which the 

 distinctly organic remains bear to the amorphous residuum, and which 



FIG. 490. 



Microscopic Organisms in Chalk from Meudon; seen partly as opaque, and partly as transpa- 

 rent objects. 



the different kinds of the former bear to each other; and this is quite 

 what might be anticipated, when we bear in mind the predominance of one 

 or another tribe of Animals in the several parts of a large area; but it 

 may be fairly concluded from what has been already stated of the amor- 

 phous component of the Globigerina-mud, that the amorphous constitu- 

 ent of Chalk likewise is the disintegrated residuum of Foraminiferal shells. 

 But further, the Globigerina-mud now in process of formation is in 

 some places literally crowded with Sponges having a complete siliceous 

 skeleton ( 511); and some of them bear such an extraordinarily close 

 resemblance, alike in structure and in external form, to the Ventriculites 

 which are well known as Chalk-fossils, as to leave no reasonable doubt 

 that these also lived as siliceous sponges on the bottom of the Cretace- 

 ous sea. Other sponges, also, are found in the Globigerina-mud, the 

 structure of whose horny skeleton corresponds so closely with the sponge- 



