57 
tion, the sepulchre of an innumerable race of marine beings, whose 
remains are occasionally observed in strata of a very different mineral 
condition ; that its white appearance is not essential to its geological 
period ; and that it is not confined to England, but is distributed under 
different forms over Europe, Asia and America*. 
The strata which have been called the Cretaceous group are, in the 
south of England, divided into the following order by Mr. Lyell, 
Dr. Mantell, Dr. Fitton and the Rev. Mr. Conybeare :— 
ing flints. 
( a. Upper soft white chalk, contain- 
Chalk b. Lower hard grey chalk, without ae thickness from 600 
formation. < to 1000 feet. 
flints. 
Cretarcous < ec. Chalk marl. J 
oe | a. Upper greensand . . . . . Thickness from 30 to 100 ft. 
Greensand } %. Gault or blue marl. . . . . Thickness from 10 to 150 ft. 
| formation. ‘\ ¢ Lower greensand and ironsand, 
K with occasional limestone. . . Thickness 250 ft. 
My illustrations are entirely confined to that portion of the Cre- 
taceous group more especially called the Chalk formation ; and for the 
investigation of the greensand, I beg to refer my readers to the ex- 
cellent researches and observations of Dr. Fitton and Dr. Mantell. 
Upper white chalk in its purest state contains nothing but lime 
and carbonic acid; it loses half its weight when burnt, the carbonic 
acid being expelled from it in the form of gas. It yields by analysis 
the following results :— 
Vii uataae rt Shy to en cd py mele PREG 50 
Warbonicsacidue eae eine 1 anes een A3:00 
IWiatererni es baie <4)? aden ee) (0250 
100-00 
* The name Cretaceous group designates rocks in all parts of the world which contain the 
same paleontological characters. 
I 
