72 OUR COMMON BRITISH FOSSILS. 



slabs are hard, as if the soft organic matter of the 

 molluscs and corals, whose hard parts almost wholly 

 make up the rocky mass, had thoroughly permeated 



Fig. 55. Clisiophyllum, a single fossil Coral, characteristic of the Carboniferous 

 limestone formation. 



it, and thus produced a similar induration to that 

 effected by sculptors, when they boil their porous 

 plaster casts in oil to render them tougher and more 

 durable. But hard as the Dudley limestone is, the 



