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, according to Bailey, resemble the casts of the holes made by burrowing 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 deposition of glauconite is still going on in the depths of the sea.[AB] 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.[AC] [Footnote Z: _Silliman's Journal_ [2], xxix., p. 284; xxxii., p. 286. _Geology of Canada_, p. 577.] [Footnote AA: _Silliman's Journal_ [2], xxxiii., p. 277. _Geology of Canada_, p. 487.] [Footnote AB: _Silliman's Journal_ [2], xxii., p. 280.] [Footnote AC: _Report of United States Coast-Survey_, 1858, p. 248.]