LAURENTIAN LIFE 



113 



derived from the decay of their own organic 

 substance. The crystalline Trenton limestone of 

 Montreal, used there as a building stone, is an excel- 

 lent example (Fig. 22). 



It is otherwise, however, when the calcareous fossils 

 have been filled or injected with some mineral 

 matter different from the matrix, as, for example, 

 silica or some silicate, oxide or sulphide of iron. 



D 



^nd 



5 







f 

 1 



f 



m 







Fig. 23. — Diagratn of different States of Fossilization of the Cell of a 

 Tubulate Coral. 



(a) Natural condition, {b) Cell filled with calcite. (c) Walls calcite, filling silica. 

 (d) Walls silica, filling calcite. {e) Both walls and calcite silica. All these 

 conditions are found in the fossil corals of the corniferous Limestone of Canada 

 — Middle Permian. 



In this case the texture, colour, or hardness of the 

 filling appear different from those of the limestone, 

 and may be seen in a fresh fracture or polished 

 slice ; or when the rock is weathered, the hard mine- 

 ralizing substance may project from the surface of 

 the specimens, or may be disclosed by treating the 

 surface with a weak acid. The figures here given 

 may suffice to show some of these conditions of 



8 



