The limestone of Pole Hill is composed almost wholly of organic fragments, cemented by crystalline carbonate of lime, and traversed by slender veins of the same mineral. Among the fragments may be recognised under the microscope portions of Trilobites, and of brachiopod and gastropod shells, and numerous joints and plates of Crinoids. The latter are remarkable for the manner in which their reticulated structure, which is similar to that of modern Crinoids, has been injected with a silicious substance, which is seen distinctly in slices, and still more plainly in decalcified specimens. This filling is precisely similar in appearance to the serpentine filling the canals of Eozoon, the only apparent difference being in the forms of the cells and tubes of the Crinoids, as compared with those of the Laurentian fossil; the same silicious substance also occupies the cavities of some of the small shells, and occurs in mere amorphous pieces, apparently filling interstices. From its mode of occurrence, I have not the slightest doubt that it occupied the cavities of the crinoidal fragments while still recent, and before they had been cemented together by the calcareous paste. This silicious filling is therefore similar on the one hand to that effected by the ancient serpentine of the Laurentian, and on the other to that which results from the depositions of modern glauconite. The analysis of Dr. Hunt, which I give below, fully confirms these analogies. I may add that I have examined under the microscope portions of the substance prepared by Dr. Hunt for analysis, and find it to retain its form, showing that it is the actual filling of the cavities. I have also examined the small amount of insoluble silica remaining after his treatment with acid and alkaline solvents, and find it to consist of angular and rounded grains of quartzose sand. The following are Dr. Hunt's notes:--