shells of various kinds are extremely numerous in modern rocks and earth. Still, the earliest fossil known, the Eozoön Canadense, the organic nature of which was formerly questioned, but now seems certain, belongs to the Azoic rocks. This determination of life in what was formerly regarded as the Azoic or lifeless age, has necessitated the establishment of an age of Dawn-life, hence named Eozoic. Life in Azoic time was also inferred from its immense quantities of carbon and of graphite, the most ancient deposits of which might be of organic origin. But aside from the Eozoön fossils, we appear to have further positive evidence of life in rhizopod fossils of Stromatopore structure, as discovered in the so-called green-stones of the Huronian, as well as in the great bog-ore deposits, which were evidently formed, then as now, through the agency of swampy vegetation.
It now seems most likely that flints, called silicic rocks, because they contain so much of the glass-substance known as silica, were largely produced from silicic organic remains, and the correctness of this view is strongly sustained by the microscope discovering in most flinty masses the crystalline needles of sponges, incasements of diatoms, capsules of infusorians, or spheroid frames of rhizopods. The silica which percolates and hardens petrified wood and other fossils may have originated chiefly from organic structures. Also, we find in chalk the molds of the silicic parts of animals, but the silica is dissolved out and gone.
The greatest use of those animalcules which have the body of plasma incased by a cell-membrane, and are called infusorians, will be pointed out further on, yet their influence on the crust of our globe must be noticed here; for a few of these bear shells and hence are found in a petrified state. Their fragmentary shells almost compose the flint rocks at Delitzsch, near Leipsic, Saxony, while some of the living sorts occur as fossils in the coal and chalk formations. Many green-sand rocks, even as far down as the Silurian, consist mainly of similar silicic shells, or the nuclei or molds of their chambers. The whetstones so extensively manufactured from the lower green-sand stone in the Black-Down Hills of England, have probably derived their useful qualities from them. Also, the silicic polishing-stone, called tripoli, or Polirschiefer, in Germany, not only contains such shells, but is entirely composed of them. This substance is used chiefly as a powder for polishing metals and stones. Infusorial formations of similar character are found at Cassel, Planitz, and Bilin. The layer at Bilin, in Bohemia, is fourteen feet thick, and Ehrenberghas estimated that it contains 41,000,000,000 shells in every cubic inch, while all are united and imbedded by an amorphous silicic substance forming compact masses of rock. At Agea, in Bohemia, there is another deposit two miles long, with an average thickness of twenty-eight feet. Its upper layer is about ten feet thick, and consists wholly of such shells; while the lower eighteen feet is a dense
