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divided into those, in which the bituminized fibres, having re- 

 tained their solidity at the time of being impregnated with silex, 

 have not allowed any diffusion of the matter, of which they were 

 constituted, in the surrounding silicious fluid, which has merely 

 penetrated and involved the bituminous fibres, and filled up all the 

 interstices, without having been itself affected by the bitumen ;and 

 into those, in which the bituminous mass has been in so fluid a 

 state, as to have allowed its intimate mixture and union, with the 

 permeating lapidific fluid, which has thereby assumed, with other 

 peculiar properties, a lustre similar to what might be expected, 

 from a mixture of silex and of bitumen. The species into which 

 silicious woods may be divided, appear to be, 1st, CALCEDONIC 

 FOSSIL WOOD; 2dly, AGATIZED; 3dly, JASPERINE; and 4thly, 

 OPALINE FOSSIL WOOD. These several species we will now proceed 

 to notice, in the order here observed. 



Yours, &c. 



LETTER XXXIV. 



CALCEDONIC WOOD...AGATINE....JASPERIZED. 



fossil wood appears to be the most simple form, in 

 which silicized bituminous wood is found to exist ; and, therefore, 

 serves to show, most clearly, the nature of the impregnation of 

 solid bituminous wood with a solution of flint. In this wood, the 



