§ 30. ' PETRIFACTION OF VEGETABLES. 707 



but in England it is seldom employed except in the fur- 

 naces of our manufactories. This kind of coal is however 

 largely developed in many districts ; and the anthracite of 

 South Wales extends from the Vale of Neath on the east, 

 to St. Bride's Bay on the west. Some of this coal is in the 

 state of charcoal, and requires a degree of heat of 531° 

 Fahrenheit, for its ignition ; but when ignited, it burns 

 with a bright flame, and is the most durable of fuel. 



30. Petrifaction of Vegetables. — As in the sand- 

 stones and other strata, the stems of trees and plants are 

 often found, not in the state of coal, but converted into 

 stone — in some instances calcareous, in others siliceous — 

 I shall in this place offer a few remarks in illustration of 

 the process by which such a change has been effected. 



In true petrifactions a transmutation of the parts of an 

 organized body into mineral matter takes place. Patrin, 

 Brongniart, and other philosophers, suppose that petri- 

 faction has frequently been effected suddenly, by the com- 

 bination of gaseous fluids with the constituent principles 

 of organic structure. It appears, indeed, certain, that the 

 conversion of animal and vegetable substances into silex, 

 must, in many instances, have been almost instantaneous, 

 for the most delicate parts, those which would undergo 

 decomposition with the greatest rapidity, are preserved. 

 The fact of the silicification of trees in loose sand, and of 

 the soft bodies of mollusca in their shells, as in the fossil 

 oysters found in the chalk at Brighton,* while neither the 

 sand nor the shells are impregnated with silex, cannot be 

 explained by the infiltration of a siliceous fluid into cavi- 

 ties left by the decomposition and removal of the animal 

 substance. Von Buch has shown that the silicifying 

 process never immediately attacks the calcareous shell, 

 but developes itself only upon the organic matter, and 

 that where this substance is not present, no silicification 

 * Medals of Creation, p. 363. 



VOL. II. 3 A 



