33? 



LETTER XXXV. 



OPALINE WOOD... .THE RESULT OF THE UNION OF SILICIOUS AND 



SOFT BITUMINOUS MATTER. 



\V E shall now proceed to the examination of that kind of sili- 

 cious wood, in which the influence, and, indeed, the presence, of 

 bitumen appears to be still more apparent. 



Opaline wood, Holz Opal of the Germans, Quartz-resinite xyloide 

 of Hauy, or silicized bituminous wood, is characterized by a peculiar 

 waxy, or rather, resinous lustre. Its fracture is conchoidal, and it 

 generally shines with the particular lustre just described. Sometimes, 

 the fibrous structure of the wood which remains will so direct the 

 fracture, that the fragments will be linear, or laminated. Its specific 

 gravity is from 2.045 to 2.675 ; and its hardness is such, that it will 

 frequently yield sparks freely, on collision with steel. 



In this, as well as the other kinds of petrified wood, we are asto- 

 nished at discovering the nice state of preservation, in which the 

 fibrous structure of the wood exists, whilst it has undergone so vast 

 a change in the nature of its substance'. This, it has been already 

 remarked, appears to have been accomplished by the bituminiza- 

 tion of the wood itself; and in this kind the bituminization appears 

 to have been more perfect than in the preceding kind. By this 

 process, although the disposition of the fibres is frequently not at 

 all altered, its nature is changed ; and it is rendered susceptible of 

 an impregnation, with a fluid holding earth in solution. 



The specimen depicted at PL II. Fig. 7- appears strongly to cor- 

 roborate this opinion respecting the conversion of the ligneous into a 

 bituminous substance, and its subsequent impregnation with silici- 

 ous particles. In that specimen is a knot of the wood, which differs, 



VOL. i. xx 



