“It appears probable from these observations that glauconite is formed by chemical reactions in the ooze at the bottom of the sea, where dissolved silica comes in contact with iron oxide rendered soluble by organic matter; the resulting silicate deposits itself in the cavities of shells and other vacant spaces. A process analogous to this in its results, has filled the chambers and canals of the Laurentian Foraminifera with other silicates; from the comparative rarity of mechanical impurities in these 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.”
[As stated in the text, the canals of Eozoon are sometimes filled with dolomite, or in part with serpentine and in part with dolomite.]
(B.) Silurian Limestones holding Fossils infiltrated with Hydrous Silicate.
Since my attention has been directed to this subject, many illustrations have come under my notice of Silurian limestones in which the pores of fossils are infiltrated with hydrous silicates akin to glauconite and serpentine. A limestone of this kind, collected by Mr. Robb, at Pole Hill, in New Brunswick, afforded not only beautiful specimens of portions of Crinoids preserved in this way, but a sufficient quantity of the material was collected for an exact analysis, a note on which was published in the Proceedings of the Royal Irish Academy, 1871.
The limestone of Pole Hill is composed almost wholly of organic fragments, cemented by crystalline carbonate of lime, and traversed by slender veins of the same mineral. Among the fragments may be recognised under the microscope portions of Trilobites, and of brachiopod and gastropod shells, and numerous joints and plates of Crinoids. The latter are remarkable for the manner in which their reticulated structure, which is similar to that of modern Crinoids, has been injected with a silicious substance, which is seen distinctly in slices, and still more plainly in decalcified specimens. This filling is precisely similar in appearance to the serpentine filling the canals of Eozoon, the only apparent difference being in the forms of the cells and tubes of the Crinoids, as compared with those of the Laurentian fossil; the same silicious substance also occupies the cavities of some of the small shells, and occurs in mere amorphous pieces, apparently filling interstices. From its mode of occurrence, I have not the slightest doubt that it occupied the cavities of the crinoidal fragments while still recent, and before they had been cemented together by the calcareous paste. This silicious filling is therefore similar on the one hand to that effected by the ancient serpentine of the Laurentian, and on the other to that which results from the depositions of modern glauconite. The analysis of Dr. Hunt, which I give below, fully confirms these analogies.
I may add that I have examined under the microscope portions of the substance prepared by Dr. Hunt for analysis, and find it to retain its form, showing that it is the actual filling of the cavities. I have also examined the small amount of insoluble silica remaining after his treatment with acid and alkaline solvents, and find it to consist of angular and rounded grains of quartzose sand.
The following are Dr. Hunt’s notes:—
"The fossiliferous limestone from Pole Hill, New Brunswick, probably of Upper Silurian age, is light gray and coarsely granular. When treated with dilute hydrochloric acid, it leaves a residue of 5·9 per cent., and the solution gives 1·8 per cent. of alumina and oxide of iron, and magnesia equal to 1·35 of carbonate—the remainder being carbonate of lime. The insoluble matter separated by dilute acid, after washing by decantation from a small amount of fine flocculent matter, consists, apart from an admixture of quartz grains, entirely of casts and moulded forms of a peculiar silicate, which Dr. Dawson has observed in decalcified specimens filling the pores of crinoidal stems; and which when separated by an acid, resembles closely under the microscope the coralloidal forms of arragonite known as flos ferri, the surfaces being somewhat rugose and glistening with crystalline faces. This silicate is sub-translucent, and of a pale green colour, but immediately becomes of a light reddish brown when heated to redness in the air, and gives off water when heated in a tube, without however, changing its form. It is partially decomposed by strong hydrochloric acid, yielding a considerable amount of protosalt of iron. Strong hot sulphuric acid readily and completely decomposes it, showing it to be a silicate of alumina and ferrous oxide, with some magnesia and alkalies, but with no trace of lime. The separated silica, which remains after the action of the acid, is readily dissolved by a dilute solution of soda, leaving behind nothing but angular and partially rounded grains of sand, chiefly of colourless vitreous quartz. An analysis effected in the way just described on 1·187 grammes gave the following results, which give, by calculation, the centesimal composition of the mineral:—
| Silica | ·3290 | 38·93 | = | 20·77 | oxygen. | |||
| Alumina | ·2440 | 28·88 | = | 13·46 | " | |||
| Protoxyd of iron | ·1593 | 18·86 |  | = | 6·29 | " | ||
| Magnesia | ·0360 | 4·25 | ||||||
| Potash | ·0140 | 1·69 | ||||||
| Soda | ·0042 | ·48 | ||||||
| Water | ·0584 | 6·91 | = | 6·14 | " | |||
| Insoluble, quartz | ·3420 | |||||||
| 1·1869 | 100·00 | |||||||
"A previous analysis of a portion of the mixture by fusion with carbonate of soda gave, by calculation, 18·80 p. c. of protoxide of iron, and amounts of alumina and combined silica closely agreeing with those just given.