448 Mr. W. J. Sollas on the Flint 



pseudomorphs we proceed nearer the flint and reach a porous 

 superficial layer, formed by the cementation of the siliceous 

 pseudomorphs together into a siliceous network ; the side of 

 this network next the flint enters half immersed into it as it 

 were ; a step further and we reach the flint itself, the siliceous 

 pseudomorphs being now completely involved and no more 

 distinguishable from one another in the common " fusion- 

 ment" than the separate snow-crystals of a mass of snow 

 which has been frozen by infiltrating water into ice. 



Instances of the deposition of silica after carbonate of lime 

 are so plentiful and well known that it would be superfluous 

 to mention them here ; chalcedonized corals and shells are 

 common in British deposits, and usually, so far as I know, in 

 connexion with the remains of siliceous organisms. 



Finally, the power of a dilute solution of colloid silica to 

 replace carbonate of lime has been experimentally demon- 

 strated by Prof. A. C. Church*, who has actually converted a 

 coral into silica by its means. 



(3) The deposition of Silica in combination as a Silicate. 

 — Since studying the structure of glauconitic grains from the 

 Cambridge Greensand, I have taken a deep interest in the 

 problem of their formation, but have never yet met with a 

 satisfactory solution of it. The occurrence of glauconitic 

 casts in the siliceous chalk of the Trimmingham flints, in the 

 interstitial siliceous chalk of Pharetrosponyia, and along with 

 siliceous spicules on existing sea- floors is a very suggestive 

 fact ; and one sees no difficulty in the supposition that the 

 dissolved silica derived from siliceous organisms should com- 

 bine with the impurities present in the surrounding sediment, 

 and so give rise to glauconitic deposits ; thus, with such 

 matters as iron oxide, alumina, and potash the silica is sup- 

 posed to combine, while carbonate of lime it merely replaces. 

 The case of the- green grains of the Cambridge bed requires a 

 little fuller consideration. In that deposit we meet with the 

 fossil sponge Pharetrospongia, in which the structure of a 

 coarse-fibred Renierid is perfectly preserved to us ; no doubt 

 it owes its preservation to the thickness of its fibres, the spicu- 

 lar components of which, however, no longer consist of silica, 

 but of carbonate of lime. But if one Renierid sponge existed 

 during the deposition of the Greensand, can we suppose that 

 no other species was associated with it? Is it not infinitely 

 more likely that a great number of others lived at the same 

 time and have since disappeared? In the coprolites of 

 the Greensand we have indeed evidence of the existence of 

 * Chem. News, v. 95 ; Joiirn. Cbem. Soc. xv. 107. 



