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 ma- 

 trix 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 re- 

 ceiving 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 decom- 

 position 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 inter- 

 nal 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 Plicatulce attached to them. Their 

 being found adhering to broken surfaces and over shrinkage-cracks 

 shows that the nodules were previously mineralized, and are con- 

 sequently derivative. But such of the nodules as have a light- 

 coloured and dull surface have no Plicatulce upon them. These latter 

 I take to be indigenous to the deposit. "Where nodules adhere to 

 bones, the Plicatulce sometimes 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. 



