With regard to the calcareous organisms with which we have now to do, when these are imbedded in pure limestone and filled with the same, so that the whole rock, fossils and all, is identical in composition, and when metamorphic action has caused the whole to become crystalline, and perhaps removed the remains of carbonaceous matter, it may be very difficult to detect any traces of fossils. But even in this case careful management of light may reveal indications of structure, as in some specimens of Eozoon described by the writer and Dr. Carpenter. In many cases, however, even where the limestones have become perfectly crystalline, and the cleavage planes cut freely across the fossils, these exhibit their forms and minute structure in great perfection. This is the case in many of the Lower Silurian limestones of Canada, as I have elsewhere shown.[X] The gray crystalline Trenton limestone of Montreal, used as a building stone, is an excellent illustration of this. To the naked eye it is a gray marble composed of cleavable crystals; but when examined in thin slices, it shows its organic fragments in the greatest perfection, and all the minute structures 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, and can be recognised only by their external forms. The specimen represented in fig. 23, is a mass of Corals, Bryozoa, and Crinoids, and shows these under a low power, as represented in the figure; but to the naked eye it is merely a gray crystalline limestone. The specimen represented in fig. 24 shows the Laurentian Eozoon in a similar state of preservation. It is from a sketch by Dr. Carpenter, and shows the delicate canals partly filled with calcite as clear and colourless as that of the shell itself, and distinguishable only by careful management of the light. [Footnote X: _Canadian Naturalist_, 1859; Microscopic Structure of Canadian Limestones.] [Illustration: Fig. 23. _Slice of Crystalline Lower Silurian Limestone; showing Crinoids, Bryozoa, and Corals in fragments._] [Illustration: Fig. 24. _Wall of Eozoon penetrated with Canals. The unshaded portions filled with Calcite._ (_After Carpenter._)]