78 OUTLINES': FIELD-GEOLO PARTI 



quite in their original condition. As a rule, however, even 

 the harder parts have undergone some change ; they have 

 lost some portions of their original substance, more par- 

 ticularly the animal matter, and have had mineral m 

 infiltrated instead. And the process of replacement has 

 often continued until the whole of the original bone or shell 

 or stem has been removed and has been at the same time 

 replaced, particle for particle, by carbonate of lime, 



a b 



FIG. 14. Common Cockle (Cardiunt cdule)\ (a) side view of both valves; (ft) 

 mould of the external form of one valve taken in plaster of Paris ; (c) side 

 view of cast in plaster of Paris of interior of the united valves. 



spathic iron, or some other mineral substance, whereby the 

 minute structure of the organism has been more or less 

 perfectly, sometimes indeed exquisitely, preserved. In 

 other cases, the whole of the material of the animal or plant 

 has disappeared, and has been replaced by a cast which 

 retains the external form of the original, but is internally 

 entirely structureless; or the cast, if there ever was one, has 

 been destroyed, and only an empty cavity remains to mark 

 where the organism once lay. In the case of the mollusca 

 we may have a cast of either the external or internal form 

 of the shell. The observer will often be puzzled at first 

 by such internal casts, as he will at once understand if he 



