152 Mr. E. J. QUEKETT on some Fossil Woods. 
stance, it can be easily imagined how silica held in solution in the water 
would pervade it, and the intercellular spaces and the interior of the woody 
fibres would be cemented together into one mass of silica. 
The reason why some woods break down more easily than others after being 
fossilized, I have not yet been able to determine; but it is certain that coni- 
ferous woods are found to be the most frequent examples in which the tissue 
is not cemented, and I imagine that in those woods there is great power of 
resisting decomposition when immersed in water, or there exists little or no 
silica as an organized part of their skeleton, so that no points in the mem- 
brane for the commencement of deposits are offered; whereas, where silica 
does exist, the molecules form the first centres, and the whole become cemented 
together. 
It is thus, I am induced to believe, that silicification in the above instances 
proceeded so far as to fill the fibres, vessels and cells, and the Spaces on their 
exterior; but as the vegetable membrane was interposed, the complete co- 
hesion of the parts was prevented, and consequently they are now capable of 
being separated, and the frustules of silica when examined prove to be casts 
of the interior of the tissues and of the interspaces external to them, thus 
appearing to offer the most satisfactory evidence respecting the nature of the 
organs in question. 
