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LETTER XLI. 
VARIETY OF PETRIFIED WOODS PROCEEDING FROM ORIGINAL 
NATURAL DIFFERENCE FROM THE LABOURS OF MAN. 
The resemblance between the recent and fossil woods is some- 
times very close, with respect to colour. The bituminous willow, 
found by Captain Perry, near Dagenham-breach, he describes, as 
having suffered little or no alteration in its colour ; and, in a speci- 
men resembling mahogany, and in various others, now before me, 
there appears to be the strongest reason for supposing, that, from 
the absence of certain mineral or saline agents, the bitumen has 
retained the colour of the vegetable substance, to which it was 
indebted for its origin. 
So perfectly do the masses of petrified wood sometimes retain 
the characters which belonged to them in a recent state, that it 
frequently happens that, although so considerably changed in their 
nature, it is not difficult to determine to what species of wood the 
petrified mass originally belonged. Thus, in Plate II. Fig. 4, and 
Fig 5 is a specimen which bears an exact resemblance to a piece 
of deal, which, having split at one end, has had the rifts, thereby 
occasioLd, filled up by a dark-coloured bitumen, as is depicted in 
Fig 4 • and which, as is shown in the section of the same piece, at 
Fig. 5,’ has insinuated itself to a considerable depth into the sub- 
stance of the wood. Volkman describes a similar specimen, as 
having a black crust, like resin or pitch; and which he supposes to 
have been the resin of the fir or pine ; which had been caused thus 
to exude, by the influence of subterranean heat. The fir, which in 
a petrified state is known by the terms Elatites, Peucites, and 
