1864.] STERRY HTTNT EOZOON. 71 



silicate of protoxide of iron and potash, which sometimes includes a 

 considerable proportion of alumina in its composition, has been ob- 

 served by Ehrenberg, Mantell, and Bailey, associated with organic 

 forms in a manner which seems identical with that in which pyrox- 

 ene, serpentiae, and loganite occur with the Eozoon in the Lauren- 

 tian limestones. According to the first of these observers, the grains 

 of greensand, 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 obser- 

 vations, 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, according to Bailey, resemble 

 casts of the holes made by burrowing Sponges (Oliona) and Worms. 

 These forms are seen after dissolving the carbonate of lime by a 

 dilute acid. He found, moreover, similar casts of Foraminifera, of 

 minute MoUusks, and of branching tubuli, in mud obtained from 

 soundings in the Gulf-stream, and concluded that the deposition of 

 giaticonite is still going on in the depths of the sea*. Pourtales has 

 followed up these investigations on the recent formation of glauco- 

 nite in the Gulf-stream waters. He has observed its deposition also 

 in the cavities of Millepores, and in the canals in the shells of Bala- 

 nus. According to him the glauconite-grains formed in Foraminifera 

 lose after a time their calcareous envelopes, and finally become " con- 

 glomerated into small black pebbles," sections of which stiU show 

 under a microscope the characteristic spiral arrangement of the cells f. 

 It appears probable from these observations that glauconite is 

 formed by chemical reactions in the ooze at the bottom of the sea, 

 where dissolved siUca comes in contact with iron-oxide rendered 

 soluble by organic matter ; the resulting sihcate deposits itself in the 

 cavities of shells and other vacant spaces. A process analogous to 

 this, in its results, has filled the ch?,mbers and canals of the Lauren- 

 tian Foraminifera with other silicates ; from the comparative rarity 

 of mechanical impurities in the silicates, however, it would appear that 

 they were deposited in clear water. Alumina and oxide of iron enter 

 into the composition of loganite as well as of glauconite ; but in the 

 other replacing minerals, pyroxene and serpentine, we have only 

 silicates of lime and magnesia, which were probably formed by the 

 direct action of alkaline silicates, either dissolved in surface-waters 

 or in those of submarine springs, upon the calcareous and magnesian 

 salts of the sea-water. Experiments undertaken with the view of 

 determining the precise conditions under which these and similar 

 silicates may thus be formed are now in progress. 



* Amer. Journ. Science, 2nd ser. vol. xxii. p. 280. 

 t Eep. Amer. Coast-Survey, 1858, p. 248. 



