LETTER XXX. 



THEORY OF THE PETRIFACTION OF WOOD PROPOSED. . ..PETRIFI EI> 

 WOOD, SILICIOUS, CALCAREOUS, AND ALUMINOUS. 



VV HILST endeavouring to ascertain the nature of the processes, 

 by which the several bituminous substances have been formed, it 

 was suggested, that the ligneous parts of vegetables, in detached 

 pieces, as well as in large masses, when placed in subterranean 

 situations, and pervaded by moisture, pass through certain spon- 

 taneous changes, from the intestine motion of their particles, and 

 the new arrangement of their principles^ by which they gradually 

 acquire a bituminous nature. In this manner is formed bituminous 

 wood ; which, if the process is not interrupted, passes into the state 

 of pure bitumen; or if exposed to the influence of certain circum- 

 stances, already particularized, a different modification of its par- 

 ticles ensues ; and jet, or some of the different varieties of coal, are 

 produced. But if the superincumbent strata be not sufficiently 

 compact, to confine the more volatile matters ; if the water be in 

 too large, or in too small a quantity, or not sufficiently stagnant ; 

 or if any other circumstance prevent the farther progress of the 

 bituminous fermentation the bituminous wood, being now ren- 

 dered unfit for the food of insects, and having acquired a consider- 

 able degree of durability, may remain without any farther change 

 for ages. This fact is rendered evident in many parts of this king- 

 dom, where trees have been thus preserved from a period beyond 

 the tradition of man. 



In this bituminized wood, two circumstances are observable, which 

 are particularly deserving notice. The first is, that, although ren : - 



