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LETTER XXX. 
THEORY OP THE PETRIFACTION OF WOOD PROPOSED PETRIFIED 
WOOD, SILICIOUS, CALCAREOUS, AND ALUMINOUS. 
Whilst endeavouring to ascertain the nature of the processes, 
by which the several bituminous substances have been formed, it 
\vas suggested, that the ligneous parts of vegetables, in detached 
pieces, as well as m large masses, when placed in subterranean 
situations, and pervaded by moisture, pass through certain spon- 
taneous changes, from the intestine motion of their particles, and 
the new arrangement of their principles, by which they gradually 
acquire a bituminous nature. In this manner is formed bituminous 
wood ; which, if the process is not interrupted, passes into the state 
of pure bitumen ; or, if exposed to the influence of certain circum- 
stances, already particularized, a different modification of its par- 
ticles ensues ; and jet, or some of the different varieties of coal, are 
produced. But if the superincumbent strata be not sufficiently 
compact, to confine the more volatile matters; if the water be in 
too large, or in too small a quantity, or not sufficiently stagnant; 
or if any other circumstance prevent the farther progress of the 
bituminous fermentation — the bituminous wood, being now ren- 
dered unfit for the food of insects, and having acquired a considerable 
degree of durability, may remain without any farther change for 
ages. This fact is rendered evident in many parts of this kingdom, 
where trees have been thus preserved from a period beyond the 
tradition of man. 
In this bituminized wood, two circumstances are observable, which 
are particularly deserving notice. The first is, that, although ren- 
