340 
In Plate 1. Fig. 1, is represented a specimen of bituminous wood 
dug up at Blackwall, whilst digging for the purpose of forming the 
East-India docks. This specimen, as is often the case, displays no 
very decided traces of its original form : the fibres are irregularly 
grouped, and seemingly agglutinated together ; leaving proportionate 
vacuities between them. Allowing imagination to fully imbue this 
mass with silicious matter, a substance resembling the common 
silicious wood will be formed. The collected bundles of fibres will 
manifest its origin ; and the silicious matter preserving its pellucidity 
in their intervals, will testify the nature of the impregnation, where 
the original arrangement of the fibres is almost lost. Thus may be 
illustrated the formation of such specimens as are represented at 
PI. II. Fig. 8. and PI. III. Fig. 9. 
In many beautiful specimens of opaline fossil wood, the fibrous 
substance may be seen passing through various gradations of change. 
On the external surface, as at PI. II. Fig. 9. and at PI. III. Fig. 8. 
may frequently be seen the silicized fibre, nicely preserved, still 
retaining its natural colour, and not partaking at all of the resinous 
gloss, observable on the other parts of the same mass ; whilst, as the 
same fibrous appearance enters into the substance of the mass, its 
lines are softened down, by its assuming this resinous-like appear- 
ance, and a much greater mellowness of colour ; and it is at last en- 
tirely lost in the prevalent colour of the whole mass. Instances of 
this kind are frequently met with, and may be particularly noticed in 
the two specimens to which you have been last referred. 
This appearance of the ligneous structure, on the external part 
of a specimen, and its gradual disappearance, on the internal part, 
is frequently to be perceived : sometimes also, whilst the external 
part thus preserves the appearance of wood, this appearance will 
almost suddenly be lost in the internal part, which will have ac- 
quired a very high degree of transparency. This circumstance is 
particularly observable in PI. III. Fig. 8. where the external surface 
