7 66 THE POPULAR SCIENCE MONTHLY. 



shells of various kinds are extremely numerous in modern rocks and 

 earth. Still, the earliest fossil known, the Eozoon 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 necessi- 

 tated 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 Eozoon fossils, we appear to 

 have further positive evidence of life in rhizopod fossils of Stromato- 

 pore structure, as discovered in the so-called green-stones of the Hu- 

 ronian, 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 dis- 

 solved 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 poUshing-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 Bo- 

 hemia, there is another deposit two miles long, with an average thick- 

 ness 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 



