54 PROCEEDINGS OE 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 pre- 

 served are mandibles of Nautilus, Belemnites, Hipponyx, Serpula, 

 Funis, Brachiopoda, Ostreidae, Hippuritidas, Crustacea, Echinoderms 

 (occasionally), and Crinoidea. 



A very large proportion of the fossils, as well as most of the nodules 

 of a dark colour, have Plicatulce 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 Plicatulce 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 sur- 

 face 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 in- 

 filtrated 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 Smilotroclius 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 pene- 

 trate 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- 

 h * Mantell's ' Wonders of Geology,' p. 401. 



