80 TERTIARY FORMATION PARIS BASIN. 



10. Above tYs plastic clay 

 \\w find thick deposits of marine 

 limestones, more or less arena- 

 ceous in structure, the different 

 beds of which may be easily 

 distinguished by their characters. 

 These limestones contain a pro- 

 digious quantity of mil'lioliles 

 (Jlg> 147) extremely small 



Fig. W. Mil' Halites (greatly mag. 



nified). 



shells the most of which do 

 not attain .03937 of an inch in 

 size, and yet they constitute a 

 great number of genera. These 

 serve, in a manner, as paste to 

 an immense number of shells 

 of different genera, which are 

 more analogous to creatures now 

 living than any we have hither- 

 to mentioned : three per cent, 

 of them are even identical with 

 species now existing in the 

 neighbouring seas. The cerithia 

 are here so abundant that the 

 formation is sometimes known 

 by the name of cerithia lime- 

 stone, although these same fos- 

 sils are found in many other de- 

 posits. There are certain spe- 

 cies which are characteristic, 

 that is, they are always found 

 vherever these deposits exist : 

 such, for example, is the Ceri'- p ig . US.Ceri'thium giga ntevm 

 t/lium giga'nteum (JJg> 148), (very much reduced). 



10. What lies above the plastic clay? What are rnil'liolites ? What 

 proportion of fossil shells found in eocene strata resemble living species ? 

 What is Cerithia limestone ? 



