140 WHAT MAY BE LEARNED FROM EOZOON 



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. ^ 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 or- 

 ganic 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 trans- 

 parent 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 colour- 

 less as that of the shell itself, and distinguishable only by careful 

 management of the light. 



In the case of recent and fossil Foraminifers, these very 

 frequently have their chambers filled solid with calcareous 

 matter, and as Dr. Carpenter well remarks, even well preserved 

 Tertiary Nummulites in this state often fail greatly in showing 

 their structures, though in the same condition they occasionally 

 show these in great perfection. Among the finest I have seen 

 are specimens from the Mount of Olives, and Dr. Carpenter 



^ Canadian Naturalist^ i859 : "Microscopic Structure of Canadian 

 Limestones." 



