382 “THE MEDALS OF OREATION. Cuar. XI 
sands of the Crag in Norfolk and Suffolk, and in the Eocene — 
beds at Grignon, near Paris, and the Pliocene of Palermo, in — 
Sicily, are as perfect as if collected from the sea-shore, having — 
suffered no loss but that of colour. In some instances, even 
the varied markings on the surface remain ; but in general 
the shells are bleached, or have a ferruginous stain. 
2dly. The form preserved, but the constituent substance — 
mineralized. This state is very common in shells that are 
imbedded in hard rock, whatever may be the age of the 
deposit. In calcareous strata the testaceous substance is 
generally transmuted into calcareous spar, as in most of the 
specimens from the chalk, oolite, mountain limestone, &c. 
In sands abounding in silex, the shell is changed into flint, 
as in the exquisite fossils from the Green Sand of Blackdown, 
Devonshire ; in deposits permeated with sulphuret of iron, 
the shells are often metamorphosed into pyrites, as in the 
Ammonites in the Lias, Galt, &e. 
ddly. In the state of casts and impressions. Although in 
loose sand the shells are either empty, or filled with detritus 
easily removable by washing ; in clay, limestone, and sand- 
stone, the cavities are generally occupied by consolidated 
materials, which had entered when in a soft or fluid state ; 
and frequently the substance of the shell has disappeared, 
and the stony cast of the interior alone remains. In many 
instances, the spaces left by the dissolution of the shells are 
filled with spar, or the casts are closely invested by the 
surrounding stone, from long-continued superincumbent — 
pressure while the matrix was in a plastic state ; and in such 
cases the casts are often distorted and flattened. But the 
vacancy is occasionally empty, and on its walls is found an 
impress of the external surface of the shell, with all the lines 
and ornaments of the original as sharp as if cast in plaster 
of Paris. | 
The specimen, Lign, 122, fig. 2, from the tertiary strata 
at Bracklesham Bay, Sussex, is a polished slice of indurated 
