130 FIELD GEOLOGY. 



and the internal cast formed of the material which filled 

 up the empty shell. 



Fossils therefore may be considered as occurring 

 chiefly in four different states or conditions, viz. : 



1. As unchanged shells or tests. 



2. As replaced shells or " pseudomorphs." 



3. As internal casts. 



4. As external impressions. 



1. Unchanged Fossils. These may at once be distin- 

 guished from those that have been replaced by their 

 mode of disintegration ; they peel off in concentric 

 layers, and disclose the original lamellar or cellular 

 structure of the shell. Crag fossils and those of many 

 sands and clays occur in this state of preservation, and 

 are quite unaltered save by the abstraction of all the 

 organic matter, for which reason they fall to pieces very 

 easily. 



2. Replaced Fossils. Those that have undergone re- 

 placement split with a definite angular cleavage quite 

 through the shell. The fossils of the chalk, for instance, 

 though still calcareous, split in this way, and the bivalves 

 have moreover entirely lost their internal nacreous layer, 

 so that no traces of the hinge or muscular impressions 

 are, as a rule, visible. In other limestones this calcitic 

 cleavage is still more marked, rhombohedral fragments 

 may be chipped out of the thicker shells or from the 

 tests and spines of Echinoderms, while inside perfect 

 crystals may often be found. This is the case even 

 with the" fossils of some argillaceous beds, and every 

 one is familiar with Lias Ammonites which have been 

 cut and polished to show the chambers full of calcite 



