Gori'espondence — C. Cams -Wilson. 475 



and opaline silica. On Chesil Beach there are pebbles which appear 

 to be nothing more or less than indurated colloid silica. 



Again, we have at Rottingdean, Walmer, and other places, 

 examples of flint filling up cracks and fissures in chalk that was 

 evidently ex-marine at the time. In some places fractured flints 

 have been repaired by such secondary precipitations of silica, in 

 others (Freshwater) there has been no such reparation in the crushed 

 flints, but at Alum Bay silicate of iron has acted as a cementing 

 medium. At Corfe, though the conditions appear to be the same, 

 I have found the cracks of crushed flints filled with calcite. 



Near Faversham there are compound flint nodules, flints within 

 flints — as many as four or five, formed one around the other, with 

 evidence of a periodic cessation of the process of aggregation between 

 each.^ It is diflicult to understand how such processes could operate 

 either in ex-marine chalk or submarine ooze. Again, why are so many 

 flint nodules hollow when they contain fossils ? Did the gases of 

 decomposition cause expansion before the silica became completely 

 " set " ? Why are cavities so' often lined with crystaUine quartz 

 when fossils, or parts, are present, and with chalcedony when they 

 are absent ? I have frequently noticed this. 



Our ignorance of the chemical and physical conditions prevailing 

 at the bottom of the old chalk sea renders the work of the flintist 

 very difficult. What do we know in reference to the pressure, 

 temperature, and composition of the water at that tim^e 1 Supposing 

 flint existed originally in a colloid state on the sea-bed, then the 

 sj)onges and other organisms, so often associated with flint, must 

 have existed there too. But this postulates a hard bottom upon 

 which such things would rest, not a soft one into which they would 

 all sink and become buried in ooze, A friend of mine has a flint 

 nodule from the chalk which has been formed around a piece of 

 teredo-bored wood, and another example of a nodule with a nucleus 

 of silicified wood may be seen in the Museum at Dover. Now, did 

 these pieces of wood sink some 2,000 fathoms to the chalk ooze, 

 and then become surrounded with colloid silica, which subsequently 

 became flint ? If not, did they, then, sink some depth into the ooze 

 and become enveloped by it ? If the latter, was the ooze surrounding 

 the wood replaced by colloid silica, molecule for molecule, or was the 

 ooze mechanically displaced by the colloid during its accrescence 

 on the wood ? If we cannot admit either of the foregoing we are 

 driven to the conclusion that the silica was formed around the wood 

 while suspended in the water. If objects suspended in water sink 

 more slowly as the pressure increases, there might be time for the 

 deposition of colloid silica around organisms before they reached 

 their final resting-place in or on the ooze. 



C. Carus- Wilson. 



Strawberry Hill. 

 September V: , 1920. 



1 Nature, June 28, 1917. 



