© CRETACEOUS GROUP 



these beds as of the same age as the white chalk of Europe : 

 but as the American strata contain, as will be shown here- 

 after, some Tertiary shells, a positive decision must be 

 deferred, until sanctioned by further observation. 



MINERALOGICAL CHARACTERS. 



1. Ferruginous Sand. The mineralogical characters 

 of this deposit are extremely variable, consisting however, 

 for the most part, of minute grains collected into friable 

 masses of a dull bluish, or greenish colour, often with a 

 shade of gray. A hundred grains of the green variety 

 gave Mr. Seybert the following constituents : — 



Silex, 



49.83 



Alumine, 



6.00 



Magnesia, 



1.83 



Potash, 



10.12 



Water, 



9.80 



Protox. of iron, 



21.53 



Loss, 



.89 



100.00 

 In a less cautious analysis by Mr. J. P. Wetherill and 

 myself, of an apparently similar marl from another locali- 

 ty, we obtained silex 49.00, protoxide of iron 30.00, 

 alumine 5.50, and lime 4.70. Hence it appears that the 

 predominant constituents of these marls are silex and iron. 

 They often contain beds of dark blue, tenacious clay, 

 which is sometimes mixed with the marl, forming the 

 marly clay of Mr. Peirce : in other instances the two 

 are disposed in alternate layers. 



