340 



In Plate I. Fig. 1. is represented a specimen of bituminous wood 

 dug up at Blaekwall ; 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 proporti- 

 onate 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 pel- 

 lucidity in their intervals, will testify the nature of the impregna- 

 tion, where the original arrangement of the fibres is almost lost. 

 Thus may be illustrated the formation of such specimens as are 

 represented at PL 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 PL 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 

 entirely 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 PL III. Fig. 8. where the external surface 



