recent silicified woods and corals, inasmuch as the portion believed to have been the original calcareous test has not usually been silicified, but still remains in the state of calcium carbonate. [Illustration: Fig. 13.--Diagram showing different States of Fossilization of a cell of a Tabulate Coral, (_a_) Natural condition walls calcite, cell empty. (_b_) Walls calcite, cell filled with the same, (_c_) Walls calcite, cell filled with silica or silicate, (_d_) Walls silicified, cell filled with calcite. (_e_) Walls silicified, cell filled with silica or silicate.] With regard, then, to the calcareous organisms with which we have now more especially to do, when these are embedded in pure limestone and filled with the same, so that the whole rock, fossils and cavities, is one in composition, and when metamorphic action has caused the whole to become crystalline, and has perhaps removed the remains of carbonaceous matter, it may be very difficult to detect any traces of structure. But even in this case careful management of light may reveal some indications. In many instances, however, even where the limestones have become perfectly crystalline, and the cleavage planes cut freely across the fossils, these exhibit their forms and minute structures in great perfection. This is the case in many of the Lower Silurian limestones of Canada, as I have elsewhere shown.[62] The grey crystalline Trenton limestone of Montreal, used as a building stone, is an excellent illustration. To the naked eye it is a grey marble composed of cleavable crystals; but when examined in thin slices, it shows its organic fragments in the greatest beauty, and all their minute parts are perfectly marked out by delicate carbonaceous lines. The only exception in this limestone is in the case of the crinoids, in which the cellular structure is filled with transparent calc-spar, perfectly identical with the original solid matter, so that they appear solid and homogeneous, but there are examples in which even the minute meshes of these become' apparent. The specimen represented in Fig. 14 is a mass of Corals, Polyzoa, and Crinoids, and shows these under a low power, as represented in the figure. The specimen in Fig. 15 shows the Laurentian Eozoon in a similar state of preservation. It is from a sketch by Dr. Carpenter, and exhibits the delicate canals partly filled with calcite or dolomite, as clear and colourless as that of the shell itself, and distinguishable only by careful management of the light. [62] _Canadian Naturalist_, 1859: "Microscopic Structure of Canadian Limestones." [Illustration: Fig. 14.--Slice of Crystalline Lower Silurian Limestone; showing Crinoids, Bryozoa, and Corals in fragments.] [Illustration: Fig. 15.--Walls of Eozoon penetrated with Canals. The unshaded portions filled with Calcite. (After Carpenter.)]