In the Grenvillian system, as represented in the vicinity of the Ottawa River, perfect specimens of Eozoon are found in one only of the principal limestones there exposed, and in certain layers of that limestone, and they are associated with concretions and grains of the greenish mineral serpentine, which, as we shall see, has much to do with their preservation. As exposed on broken surfaces, the specimens consist of concentric layers of greenish serpentine and white calcite, not, however, even or uniform, as in ordinary concretions having concentric structure, but often approaching and uniting with each other, so as to constitute wide flat chambers, and forming patches from an inch to nearly a foot in diameter, while some of the larger patches seem to coalesce or to become confluent. On weathered surfaces the serpentine laminæ often become brown, owing to the rusting of the iron contained in them, and project above the general surface, in this case resembling very much the appearance of the layer-corals so plentiful in some limestones of later date. The external forms of Eozoon are at first sight not very obvious, as they adhere very closely to the containing rock; but the smaller specimens, when entirely weathered out or disengaged by the solution of the limestone in an acid, usually present the form of a broad inverted cone, like some modern sponges or the broader turbinate fossil corals (Fig. 32). The limestone having, like the other beds of the formation, been much compressed and folded, the specimens of Eozoon are sometimes crumpled in these folds or broken across by small cracks or faults, which shift the laminæ slightly out of their places. The cracks thus formed are also sometimes filled with a fibrous variety of serpentine, known to mineralogists as chrysotile and popularly as "rock cotton" or "asbestus." It is finely fibrous, and of a silky lustre, and must have been deposited by water in the cracks and fissures formed by the fracturing of the rock and the contained fossils, by movements taking place after the whole was hardened. Accordingly these veins often cross not only the rock, but also the serpentine and calcite layers of the contained masses of Eozoon, without regard to the direction of their laminæ, though sometimes they run parallel to the structure, the rock having broken more easily in that direction. [Illustration: Fig. 32.--_Entire specimen of Eozoon, disengaged from the matrix and showing its turbinate form, enclosed in the outline of a larger specimen of similar form._ Both natural size, Côte St. Pierre. (Specimens in Peter Redpath Museum.)]