159 



On Silicified Structure in Pyritized Wood. 



By M. Hawkins Johnson, F.G.S. 

 (Read March 2ith, 1867.) 



On the north coast of the Isle of Sheppey, when the tide is low, 

 a great quantity of fossil wood may be found, which is washed out 

 of the London Clay, of which the cliffs consist, by the action of 

 the waves. Lying on the shore the fragments appear, at first 

 sight, exactly like broken twigs and pieces of recent wood in a 

 state of decay, but their weight, their bronze tint, and their 

 glittering fracture soon dispel the illusion. They consist mainly of 

 iron pyrites, and are used at Queenborough in the manufacture of 

 sulphuric acid. 



If a piece of one of these twigs be carefully ground to a smooth 

 surface, and then submitted to the action of strong nitric acid for 

 about an hour, the iron pyrites will be dissolved to a greater or 

 less depth, and the woody structure left standing in relief; that is 

 to say, the carbonaceous walls of the wood cells, which appear to 

 have been silicified, are not dissolved, while the pyritous infiltra- 

 tion which subsequently filled the pores of the structure has been 

 removed. 



This structure is of so definite a character that it is impossible 

 to confound it with any accidental arrangement of particles of 

 sulphur, as was suggested by some, to account for the beautiful 

 forms revealed by similar treatment of the pyritous nodules in the 

 Chalk ; while, supposing the structure revealed by nitric acid in the 

 pyritous nodules from the Chalk, really to represent the original 

 organic structure of the thing fossilized, and to be due, as I 

 suppose, to the replacement of the carbon of that structure by 

 silicon, much more might we reasonably expect to find so highly 

 carbonaceous a structure as wood similarly preserved — which agrees 

 with the fact. 



