THE PRESEBVATION OP EOZOON. 119 



Lower Silurian strata.* This hydrous silicate of protoxide of 

 iron and potash, which sometimes includes a considerable 

 proportion of alumina in its composition, has been observed 

 by Ehrenberg, Mantell, and Bailey, associated with organic 

 forms in a manner which seems identical with that in which 

 pyroxene, serpentine, and loganite occur with the Eozoon in 

 the Laurentian limestones. According to the first of these 

 observers, the grains of green-sand, or glauconite, from the 

 Tertiary limestone of Alabama, are casts of the interior of 

 Polythalamia, the glauconite having filled them by ' a species 

 of natural injection, which is often so perfect that not only the 

 large and coarse cells, but also the very finest canals of the 

 cell- walls and all their connecting tubes, are thus petrified and 

 separately exhibited.' Bailey confirmed these observations, 

 and extended them. He found in various Cretaceous and 

 Tertiary limestones of the United States, casts in glauconite, 

 not only of Foraminifera, but of spines of Echinus, and of the 

 cavities of corals. Besides, there were numerous red, green, 

 and white casts of minute anastomosing tubuli, which, accord- 

 ing to Bailey, resemble the casts of the holes made by bur- 

 rowing sponges (Cliona) and worms. These forms are seen 

 after the dissolving of the carbonate of lime by a dilute acid. 

 He found, moreover, similar casts of Foraminifera, of minute 

 mollusks, and of branching tubuli, in mud obtained from 

 soundings in the Gulf Stream, and concluded that the deposi- 

 tion of glauconite is still going on in the depths of the sea.f 

 Pourtales has followed up these investigations on the recent 

 formation of glauconite in the Gulf Stream waters. He has 

 observed its deposition also in the cavities of Millepores, and 

 in the canals in the shells of Balanus. According to him, the 

 glauconite grains formed in Foraminifera lose after a time 

 their calcareous envelopes, and finally become ' conglomerated 

 into small black pebbles,' sections of which still show under a 

 microscope the characteristic spiral arrangement of the cells.J 



* Silliman's Journal [2] , xxxiii., p. 277. Geology of Canada, 

 p. 487. 



t Sillimari's Journal [2] , xxii., p. 280. 



J Report of United States Coast-Survey, 1858, p. 248. 



