Fig. 24.—Cast of Cavities of Polystomella in Glauconite (magnified).
After a photograph from Dr. Carpenter, and mounted specimens from his collection.
When we examine oceanic sediments of older date, we find similar fillings in limestones, chalks, and sandstones of various ages, some of the latter containing glauconite so abundantly as to bear the name of green-sands, from their colour; and in these older examples we more frequently find alumina and magnesia occupying a large place in the mineralizing silicate. [Fig. 24A] gives two illustrations of this—one a crinoidal stem from the Silurian of New Brunswick, injected with a silicate of alumina, iron, magnesia and potash; the other a spiral shell from more ancient perhaps Cambrian rocks in Wales, filled with a silicate apparently more nearly related to serpentine. Further examples will be referred to in an appended note.
Fig. 24A.—(a) Joint of Crinoid injected with a Hydrous Silicate, Silurian, Pole Hill, New Brunswick. (× 25.)
(b) Spiral Shell injected with a Hydrous Silicate allied to Serpentine, near Llangwyllog, North Wales, (× 25.)
We may now consider shortly the relation of dolomite, or the mixed carbonates of lime and magnesia, to the preservation of fossils. The presence of dolomite or magnesian limestone in these beds does not affect the conclusion as to their probable organic origin. This form of limestone occurs abundantly in later formations, and is even forming in connection with coral deposits in the modern ocean.
Dana has shown this by his observations on the occurrence of dolomite in the elevated coral island of Matea in Polynesia,[20] under circumstances which show that it was formed in the lagoon of an ancient coral atoll, or ring-shaped island, while he finds that coral and coral sands of the same elevated reef contain very little magnesia. He concludes that the introduction of magnesia into the consolidating under-water coral sand or mud has apparently taken place—"(1) In sea-water at the ordinary temperature; and (2) without the agency of any other mineral water except that of the ocean"; but the sand and mud were those of a lagoon in which the saline matter was in process of concentration by evaporation under the solar heat. Klement has more recently taken up this fact in the way of experiment, and finds that, while in the case of ordinary calcite this action is slow and imperfect, with the aragonite which constitutes the calcareous framework of certain corals,[21] and at temperatures of 60° or over, it is very rapid and complete, producing a mixture of calcium and magnesium carbonates, from which a pure dolomite more or less mixed with calcite may subsequently result.[22]
[20] "Corals and Coral Islands," p. 356, etc.
[21] Aragonite, like ordinary limestone, is calcium carbonate, but its atoms seem to be differently arranged, so as to make it a less stable compound, and it has a different crystalline form. Some calcareous organisms are composed of aragonite, others of ordinary calcite.
[22]"Bulletin Geol. Soc. Belgium," vol. ix. (1895, p. 3). Also notice in Geol. Mag., July, 1895, p. 329.