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LETTER XXXVI. 



EVIDENT AFFINITY BETWEEN SILICIZED BITUMINOUS, OR OPA* 

 LINE WOOD, AND PITCH-STONE. ...ANALYSIS OF OPALINE WOOD 

 ....OF PITCH-STONE. ...SIMILARITY INFERRED. 



A.S that particular character, a waxy, or resinous lustre, is evi- 

 dent in those substances which, though now chiefly formed of 

 silex, display the most evident marks of their having been wood ; 

 and as nature does not multiply her agents unnecessarily ; it does 

 not appear to be unreasonable to suppose, that particular appear- 

 ance has been derived, in every other stone which possesses it, from 

 the same source, as that which has furnished it to the fossil wood. 

 You will already have perceived, that I am leading you, with as 

 little abruptness as I can, to the consideration of an opinion, which 

 I suspect must demand some little management to secure it a 

 favourable reception. That many of the pitch-stones, the semi- 

 opals, and perhaps the noble opal itself, derive their peculiar cha- 

 racters from certain portions of the vegetable matter, which, having 

 been buried at very considerable depths, have undergone a parti- 

 cular change in the combination of their constituent principles ; 

 and in that state, has been intimately united with silex. We will 

 now proceed to examine, how far this position is supported, by the 

 physical and chemical properties of these substances. 



Among the most beautiful and frequent specimens of the opalized 

 woods are those which, from their near resemblance to pitch-stones, 

 mineralogists have termed LIGNIFORM PITCH-STONES, the QUARTS 



VOL. I. Y Y 



