l66 RELICS OF PRIMEVAL LIFE 



the nature and origin of the substances constituting 

 the specimens. Reference has already been made 

 to these in our fifth chapter, but they may be 

 more particularly noticed here in connection with 

 the forms as above described. 



The calcareous laminae are usually composed of 

 clear translucent calcite or calcium carbonate, 

 though, as in the case of many later fossils, some- 

 times replaced by dolomite. It often has the fine 

 granular appearance above referred to, but is 

 nearly always crystalline, and traversed by cleavage 

 planes visible under the microscope.^ This cry- 

 stalline structure, as every student of fossils knows, 

 is very common in calcareous fossils of all geo- 

 logical ages. In the thicker laminae the canals 

 traversing them and branching out in their sub- 

 stance are usually visible under a low power, 

 except when they are filled with calcite similar to 

 that of the laminae themselves. In this case they 

 can be seen only by very careful management of 

 an oblique and subdued light. When occupied 

 with serpentine, this presents, in a thin slice under 



^ Especially when the specimen has been heated or jarred 

 in the process of grinding or polishing. 



