So long as the imbedding mass continues soft and incoherent, shells, corals, etc., can be recovered in a condition similar to that of recent specimens, except that they may have become bleached in colour and brittle in texture, owing to the removal of organic matter intimately associated with the lime, and that their cavities may have been filled with sand or silt washed into them, or with calcite or calcareous spar introduced in solution in water. But if the containing mass has become a hard stone, the material filling the interior of our shell or coral has experienced a similar change; and when we break open the stone, we may obtain the specimen, now hard, solid, and heavy, but still showing more or less of its outer surface and markings, and possibly to some extent also its internal structure when it is sliced and studied under the microscope. But if the whole mass has been metamorphosed, and has become crystalline, the contained fossil and its contents may have experienced a similar change, and may have so coalesced with the containing matrix that it is no longer separable from it. Even in this case, however, if the whole is reduced to a thin transparent slice 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, cellular, or otherwise different from that of the enclosing 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 derived from the decay of their own organic substance. The crystalline Trenton limestone of Montreal, used there as a building stone, is an excellent example (Fig. 22). [Illustration: Fig. 22.--_Section of "Trenton Limestone" (magnified)._ Showing its composition of fragments of calcareous fossils.] [Illustration: Fig. 23.--_Diagram 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.]