THEORY OF FORMATION OF FLINT NODULES 393 



nucleus. While the process was probably metasomatic in the sense that 

 the solution of the chalk involYed the deposition of the silica, it does not 

 necessarily follow that the deposition of the silica in any given flint was 

 the result of the solution of the very chalk which that flint replaced. 

 The fact that such crystallization as has taken place in the flint appears 

 to have been subsequent to its coagulation as colloidal silica militates 

 against the idea that the silica was drawn from the surrounding chalk 

 by crystalline attraction and that the cavity was formed by the pressure of 

 crystal growth. The transition from flint to chalk in the outer part of 

 the flints appears much like the result of an absorption of the siliceous 

 solution from the cavity into its walls and is certainly not due to the 

 extension of crystal fibers through the chalk. 



(2) The fossil sea-urchins which occur in the chalk, and were, of 

 course, originally composed of calcium carbonate, are now for the most 

 part replaced by flint, though some are still calcareous. The silicified 

 examples are sometimes free, but often embedded in other flints, of which, 

 however, they do not seem to have been the original nuclei. This seems 

 to prove that the silicification of the fossils took place much later than 

 the original deposition and subsequent to the formation of joints, bedding 

 planes, and other cracks, and was a part of the process which produced 

 the other flint nodules, tabulae, and veins. 



The siliciflcation of the fossils does not seem to have been a slow process 

 of molecular replacement, since the details of the shells are only roughly 

 preserved. This would point to replacement by deposition from moving- 

 waters by a more rapid process. As many sea-urchins while silicified on 

 the surface remain chalky in the center, it seems clear that their silicifica- 

 tion was accomplished by material supplied from outside, applied by 

 percolation of solutions along the plane separating their tests from the 

 surrounding chalk. It is believed that these solutions reached the surface 

 of the fossils from cracks in the rock which were situated close to their 

 surfaces, and this would account for the silicification of some fossils and 

 not of others. It would apply to sponges also. The idea that these 

 acted as concretionary centers in the ordinary sense seems to have 

 little foundation as regards the majority of flints. Acids produced by 

 the decay of animal tissues may well have played a part in the deposition 

 while being circulated through the fissures, as well as in the immediate 

 vicinity of the remains, by assisting in the production of hydrochloric acid. 



Source of the Silica 



The source from which the flint depositing waters obtained their silica 

 can hardly have been siliceous spicules or tests deposited with the chalky 



