Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/103

1872.] FISHER — CRETACEOUS PHOSPHATIC NODULES. 61 The segregation of minerals is, I suppose, an obscure subject; and I cannot explain the process in the case before us. Dialysis seems to require a passage of the depositing fluid across the dialyzer from a region of greater to one of less saturation; and it is not easy to see how this could happen in the case of an organism immersed in ooze, or in the ocean. Subaerial water, percolating through strata, and charged with mineral matters in its progress, might perhaps more easily act in this manner. But there does not seem to be any cause to doubt that the phosphate of lime was precipitated from solution in water charged with carbonic acid, it having been previously secreted by animal agency. Even yet it subsists in the chalky matrix to so great an extent as to be appreciable, as I am informed by Mr. Liversedge, of Christ's College, who has examined it for me.

Analyses of the glauconite grains by Professor Liveing, and of the phosphatic nodules by Dr. A. Voelcker, will be found in Mr. Seeley's paper before referred to*.

When the coprolitic bed is within three feet, or thereabouts, of the top of the ground, the nodules become weathered, eventually receiving a uniform greyish colour upon the surface. This weathering commences with dendritic whitish markings slightly eating into the substance of the phosphate; probably they are caused by the decomposition of rootlets in contact with the nodules, which have given off carbonic acid, and dissolved the mineral where they have touched it. That these curious markings are not the result of any peculiar internal organization of the substance of the nodules, is shown by the fact that exactly similar markings occur on sharks' teeth under like conditions.

The phosphatic nodules of a dark and somewhat lustrous surface, as already stated, usually have Plicatuloe attached to them. Their being found adhering to broken surfaces and over shrinkage-cracks shows that the nodules were previously mineralized, and are consequently derivative. But such of the nodules as have a light-coloured and dull surface have no Plicatuloeupon them. These latter I take to be indigenous to the deposit. "Where nodules adhere to bones, the Plicatuloesometimes occur attached partly to each.

Many of the nodules occur in a fragmentary state, broken up into small angular fragments, evidently by shrinkage-cracks having formed in them as in septaria; and these cracks do not appear to have been occupied by crystallized calcite, but by indurated matrix. These changes must all have occurred before the formation of the deposit.

The indurated matrix which fills the axes of the cylindrical nodules was in all probability introduced while the fossils were in their original gisement.

Hence we gain a clue to the derivation of the fossils of the thin crowded layer of the so-called Upper Greensand of Cambridgeshire. They seem to have been washed out of a calcareous marl similar in character to the marl which lies above it. In short, the nodule-bed is a condensation of the "Chalk-marl with glauconite grains."


 * Geol. Mag. vol. iii. pp. 305, 306.