54 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4,
dition of casts, and in that case always converted into phosphate.
In a few very rare instances the shelly matter has heen replaced by
phosphate. In the Cephalopods also the shelly substance of the septa
is usually so replaced; and so likewise are the internal floors of the
whorls in Pleurotomaria. The fossils in which the carbonate is preserved are mandibles of Nautilus, Belemnites, Hipponyx, Serpula,
Funis, Brachiopoda, Ostreidae, Hippuritidae, Crustacea, Echinoderms
(occasionally), and Crinoidea.
A very large proportion of the fossils, as well as most of the nodules of a dark colour, have Plicatuloe attached to them. But this is not the case with "associated" bones, nor with another class of phosphatic nodules, which, instead of having a dark and polished surface, present a cream-coloured and dull exterior, though internally often similar to the dark variety. It is plain that the "associated" bones must have been enveloped in the matrix before the Plicatuloe had time to fix upon them. And the same must have been the case with the light- coloured nodules.
It is by no means usual to find the impression of the exterior surface of a shell upon a phosphatic nodule. This renders it probable that the animal matter within the shell was the determining cause of the deposit; and in some cases at least it may be analogous to molluskite*. If it was deposited from solution in carbonated water, it is possible that the same condition of the water which enabled it to hold phosphate of lime in solution also caused it to dissolve away the shell; and because the interior walls, like the septa in the Nautili, would be more slowly reached, their subsequent removal might leave void spaces protected from the intrusion of matrix, and ready to be infiltrated with phosphate. Thus we can understand the internal parts of shells being mineralized while the outer parts have been removed.
Casts of the small coral Smilotrochus are common in phosphate. But it is always the interior of the calyx which is modelled. I have never seen a cast of the exterior. This points to the same fact, that it was the animal matter which determined the deposition of the phosphate. The grains of glauconite which abound in the matrix, and also in the indurated matter which occasionally adheres to the nodules, are never present within the matter of the brown nodules.
I have a section of a vertebra of a Plesiosaurus in which the osseous substance appears to be converted into the mineralized phosphate, while the canals of the bone contain glauconitic grains abundantly. This shows that they existed in a medium which was able to penetrate very fine canals, or else that they were segregated from a matrix which could do so; from their appearance I suspect the former. In either case their absence from the interior of the nodules is suggestive.
I now approach the special object I have in view—namely, to discuss the origin of the phosphatic nodules themselves.
On examining any of the heaps of these prepared for sale, it is easy to see that two varieties are especially abundant. The bulk of the collection consists partly of finger-shaped, and partly of amor-
- Mantell's 'Wonders of Geology,' p. 401.
