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LETTER XXV11L 



PYRITOUS WOODS. ...OPINIONS OF DR. HUTTON AND MR. PLAYFAlR 

 ....IGNEOUS ORIGIN. ...AQUEOUS ORIGIN EXAMINED. 



1 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 opi- 

 nion, to consider as extraneous ; or, as I term them, secondary fos- 

 sils; and, therefore, must refer you to the arguments which I have 

 adduced, in support of that opinion, for my justification, in detain* 

 ing 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 accom- 

 plished. I have also endeavoured, at the same time, to ascertain 

 the most general basis, or substratum, of vegetable fossils ; my suc- 

 cess in this respect, however, still remains to be determined. 



The subject of the present Letter cannot but prove highly inte- 

 resting, being the conversion of wood to a splendid metallic sub- 

 stance ; in which, although sufficient traces of its original mode of 

 existence are discoverable, the transmutation is such that its supe- 

 rior 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 histpry of these bodies, as far as respects the various appear- 



