Nodules of the Trimmingham Chalk. 447 



of hollow casts, while the interstices of the network are filled 

 up by a chalcedonic deposit surrounding the exterior of the 

 casts. The chalcedony has a fibrous structure, the fibres 

 radiating from the incrusted surface ; where one group of fibres 

 meets another a sharp line of demarcation is produced ; and 

 intersecting lines of demarcation make with one another an 

 angle of 120° (" Structure of Siphonia" he. cit. p. 816) . The 

 chalcedony so constantly appears as a growth upon the sili- 

 ceous skeleton that it looks very much as if the latter had 

 exerted some special attraction upon the silica in solution, 

 leading to its deposition. The idea finds support in an ob- 

 servation of Carter's, who asserts that in the Haldon Green- 

 sand a chalcedonic deposit frequently occurs on the imbedded 

 sponge-spicules, but never on the clastic grains of quartz. 

 On the other hand the deposition of the chalcedony on a sili- 

 ceous skeleton may be explained without invoking the aid 

 of any specific attraction ; for if a solution of silica were to 

 exert a solvent action on the siliceous skeletons bathed by it, 

 it is quite possible that deposition might by the very act of 

 solution be brought about, a molecule of mineral silica being 

 deposited for every molecule of organic silica removed ; and 

 the process of crystallization over any surface once set up, 

 would continue in the same place in preference to beginning 

 afresh on some new one. 



(2) Deposition of Silica as a Pseudomorph after Carbonate of 

 Lime. — It is a curious fact that the action of siliceous solutions 

 on carbonate of lime is not to displace the carbonic anhydride 

 from the latter, but to replace the molecule of carbonate of lime 

 as a whole ; it is a fact, however, that has long been well 

 known, though it is only lately that it has been shown to 

 have been concerned in the formation of chert and flints. 

 The valuable observations of Prof. Rupert Jones, the inves- 

 tigation of Hull and Hardman, and the elaborately careful 

 study of Prof. Renard prove conclusively that flint and 

 chert are to a certain extent pseudomorphs after carbonate 

 of lime ; and of this the Trimmingham flints furnish us with 

 a fresh demonstration. Thus some of the nodules consist 

 within of ordinary flint, black, translucent, and compact, but 

 exteriorly simply of ordinary chalk with a few siliceous re- 

 mains scattered through it. Between these two we find every 

 intermediate stage of silicification. Passing from the chalk to 

 the flint, one finds first the coccoliths, Foraminifera, and other 

 calcareous constituents of the chalk converted into silex, the sili- 

 ceous pseudomorphs retaining all the details of their original 

 form, down to the delicate striae on some of the foraminiferal 



