112 



RELICS OF PRIMEVAL LIFE 



however, if the whole is reduced to a thin transparent 

 sHce and examined microscopically, some traces may 

 be found of the external and internal limiting lines of 

 the fossil, and even of its minute structures, which 

 often cause it to present an appearance granular, 



Fig. 22. — Section of ^^ Trenton Limestone'' {magnified). 

 Showing its composition of fragments of calcareous fossils. 



cellular, or otherwise different from that of the en- 

 closing matrix. It requires, however, both skill and 

 care to detect organic remains in such circumstances, 

 and they may often escape observation, except when, 

 as in many old crystalline limestones, the fossils are 

 darkened in whole or in part with coaly matter 



