Still another mode of occurrence is presented by a remarkable specimen from Tudor in Ontario, and from beds probably on the horizon of the Upper Laurentian or Huronian.[Y] It occurs in a rock scarcely at all metamorphic, and the fossil is represented by white carbonate of lime, while the containing matrix is a dark-coloured coarse limestone. In this specimen the material filling the chambers has not penetrated the canals except in a few places, where they appear filled with dark carbonaceous matter. In mode of preservation these Tudor specimens much resemble the ordinary fossils of the Silurian rocks. One of the specimens in the collection of the Geological Survey (fig. 30) presents a clavate form, as if it had been a detached individual supported on one end at the bottom of the sea. It shows, as does also the original Calumet specimen, the septa approaching each other and coalescing at the margin of the form, where there were probably orifices communicating with the exterior. Other specimens of fragmental Eozoon from the Petite Nation localities have their canals filled with dolomite, which probably penetrated them after they were broken up and imbedded in the rock. I have ascertained with respect to these fragments of Eozoon, that they occur abundantly in certain layers of the Laurentian limestone, beds of some thickness being in great part made up of them, and coarse and fine fragments occur in alternate layers, like the broken corals in some Silurian limestones. [Footnote Y: See Note B, Chap. III.] Finally, on this part of the subject, careful observation of many specimens of Laurentian limestone which present no trace of Eozoon when viewed by the naked eye, and no evidence of structure when acted on with acids, are nevertheless organic, and consist of fragments of Eozoon, and possibly of other organisms, not infiltrated with silicates, but only with carbonate of lime, and consequently revealing only obscure indications of their minute structure. I have satisfied myself of this by long and patient investigations, which scarcely admit of any adequate representation, either by words or figures.