394 E. M. BUR WASH CHALK AXD FLINTS OF XOETHEEX FEAXCE 



since the chalk: is itself much the more soluble : and in fact the deposition 

 of the flint was probably due in part to the fact that the silica-laden 

 water had reached supersaturation on coming into contact with the 

 more soluble chalk, when the least soluble of the solutes would be deposited 

 as colloidal silica and its coagulation aided by the sodium carbonate. The 

 writer ventures this as a suggestion in face of a considerable weight of 

 authority for a concretionary theory by which siHceous tests and spicules 

 in the chalk are thought to be the source of the silica. This seems to the 

 writer to fail in accounting for the removal of the dissolved chalk which 

 was metasomatically replaced by the silica. If the silica came out of 

 the Joint-blocks of chalk, the replaced chalk must have penetrated into 

 them or been carried off through the fissures by moving waters. Such an 

 exchange of molecules between points separated by some distances as the 

 first hypothesis supposes must be made to depend entirely on the forces 

 of diffusion and molecular segregation acting in a substance which was 

 already compact and solid enough to support crevices under considerable 

 pressure of superincumbent rock. The waters through which these forces 

 could act must have been stationary and the theory does not account for 

 the presence of organic matter in the fiint. On the other hand, it is 

 easy to suppose that meteoric waters percolating through fissures might 

 introduce organic matter into them and would also be charged with 

 enough carbon dioxide to render the chalk easily soluble. If carbonic 

 acid were wanting, its production might easily be accoimted for by the 

 presence of other acids along with the silica, especially in the presence 

 of sodium chloride, with which the chalk must have been charged at the 

 time of its deposition. A small amount of hydrosulphuric or sulphuric 

 acid would be sufficient, while the presence of humous acids must be 

 posited. The reaction would then be 



H,.S — 2XaCl == 2HC1 — Xa.S 

 2HC1 ^ CaCOs == CaCL + HXO3 



or perhaps more directly 



HoS — CaCOc = HoCO. — CaS. 



Geouxd-watee 



This discussion as to the origin of the flints may serve a further pur- 

 pose, in view of the conditions which undoubtedly prevail at present as 

 to the passage of meteoric waters through the fissures of the chalt, which 

 takes place with great freedom, and also in view of the presence of carbon 

 dioxide, which is commonly found in tunnels and other excavations. 



