8 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, ", 6 1.88 
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. 

