ON THE MICROSCOPIC STRUCTURE OF FLINTS, &C. 237 



was surrounded having been removed by the acid. The structure 

 thus revealed is shown in this drawing. (Fig. 4.) 



I must not be understood to say that because in the instances I 

 have described the nodules are undoubtedly organic, that therefore 

 all nodules are organic. I wish particularly to guard against this 

 inference. I have shown an organic structure in certain cases, but 

 it would be rash in the extreme to draw from these a hasty 

 generalization as to the nature of the whole group. Nevertheless, 

 the evidence adduced certainly points in that direction. 



How these structures have been preserved is a subject upon 

 which I have treated in a paper lately read before the Geologists' 

 Association, but, as it can scarcely be considered microscopical, 

 and I have already taken up a good deal of your time, I will only 

 venture to say that they were silicified by the substitution of 

 silicon for carbon, and the subsequent elimination of the other 

 constituents; and that afterwards the interstices were filled in, 

 either with the same or with different materials, until they were 

 converted into solid compact masses. 



I have one other drawing (Fig. 2) to which I wish to call your 

 attention. It represents the microscopic appearance of a piece of 

 flint that has been subjected to a sort of natural injection. The 

 organic structure has been infiltrated with some ferruginous 

 solution, which has subsequently been converted into hydrated 

 sesquioxide of iron. This is by no means uncommon, but, ac- 

 cording to my experience, the specimens best adapted for exhibit- 

 ing the structure are the green-coated flints from the Bull Head 

 bed, between the Thanet Sand and the Chalk. When a very thin 

 poKshed slice of such a flint is viewed as an opaque object the 

 oxide of iron appears of a light brown or buff tint, in striking 

 contrast to the transparent silica not infiltrated, which of course 

 appears black. The structure thus displayed is the complement of 

 that developed by hydrofluoric acid, the latter being only a mould 

 or impression, while the former may be said to be a natural in- 

 jection of the organic structure itself. 



