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LETTER XXXVI. 
EVIDENT AFFINITY BETWEEN SILICIZED BITUMINOUS, OR OPALINE 
WOOD, AND PITCH-STONE ANALYSIS OF OPALINE WOOD 
OF PITCH-STONE SIMILARITY INFERRED. 
As that particular character, a waxy, or resinous lustre, is evident 
in those substances which, though now chiefly formed of silex, dis- 
play 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 appearance 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 abrupt- 
ness 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 characters from 
certain portions of the vegetable matter, which, having been buried 
at very considerable depths, have undergone a particular 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 che- 
mical 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 quartz 
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VOL. I. 
