CRETACEOUS PERIOD. 



471 



through the soft mass are found very characteristic nodules of pure 

 flint. These nodules are of various sizes and shapes, sometimes 

 scattered irregularly, 

 sometimes arranged in 

 layers. Often some 

 fossil, especially a 

 sponge, forms the nu- 

 cleus around which 

 the aggregation of the 

 siliceous matter takes 

 place. On account of 

 its extreme softness, 

 chalk is often sculpt- 

 ured by erosive agen- 

 cies into fantastic cliffs 

 and needles (Fig. 756). 



Examined with the microscope, chalk is found to be composed largely 

 of Ehizopod shells, and of Coccoliths and Coccospheres (supposed shells 

 of unicelled plants), some perfect, more broken, most of all completely 

 disintegrated (Fig. 757). The flint-nodules, similarly examined by sec- 

 tion, show spicules of sponge and siliceous shells of Diatoms. Chalk 



such as described was supposed to be 

 found nowhere except in Europe, but 

 recently good chalk composed of Fora- 

 miniferal shells, and containing flints, 



Figs. 



Chalk-Cliffs with Flint-Noaules. 



has been found in Texas (Hill). 



Fig. 760. Fig. 761. 



Figs. 757-761.— For aminifer a of Chalk: 757. Chalk as seen under the Microscope (after Nichol- 

 son). 758. Cuneolina pavonia. 759. Flabellina rugosa. ?60. Lituola nautiloides. 761. Chrysa- 

 lidina gradata (after D'Orbigny). 



758-761 represent some of the more common Ehizopods found in 

 chalk. 



