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LETTER XXVIII. 
PYRITOUS WOODS.... OPINIONS OF DR. HUTTON AND MR. PLAYFAIR 
....IGNEOUS ORIGIN.... AQUEOUS ORIGIN EXAMINED. 
I CONSIDER your claim as too well founded to allow me to impute 
to you impatience. You requested from me a history of the sub- 
stances generally termed extraneous fossils ; and during a long cor- 
respondence, carried on in consequence of that request, I have not 
yet given you the history of a single substance, which general opi- 
nion allows to be thus classed. The substances on which I have 
hitherto dwelt, I have taken the liberty, contrary to general opinion, 
to consider as extraneous ; or, as I term them, secondary fossils ; 
and, therefore, must refer you to the arguments which I have ad- 
duced, in support of that opinion, for my justification, in detaining 
you so long in their examination. 
The establishing of the right of these to the rank of secondary fos- 
sils, is not, however, the whole of what I hope to have accomplished. 
I have also endeavoured, at the same time, to ascertain the most 
general basis, or substratum, of vegetable fossils ; my success in this 
respect, however, still remains to be determined. 
The subject of the present Letter cannot but prove highly interest- 
ing, being the conversion of wood to a splendid metallic substance ; 
in which, although sufficient traces of its original mode of existence 
are discoverable, the transmutation is such that its superior gravity, 
as well as, often, its lustre, proves that it now contains a considerable 
portion of metallic matter. To give a slight sketch of the history of 
these bodies, as far as respects the various appearances they yield, 
