§ 32. PETRIFACTION OF VEGETABLES. 711 



large a proportion of calcareous earth, to be silicified ; they 

 seem to have been changed into spar, by water charged 

 with carbonic acid gas having insensibly effected the 

 crystallization of their molecules.* 



Some specimens of silicified wood, collected from the 

 interior of Australia, by Sir T. Mitchell, and recently trans- 

 mitted to the British Museum, are entirely permeated by 

 silex ; but on the external surface of these stems, there are 

 some circular spots of chalcedony, that appear to have origi- 

 nated from the exudation of the liquid silex from the interior, 

 in viscid globules distended with air, which burst, collapsed, 

 and became solidified in their present form. 



That silicification is induced under circumstances con- 

 nected with a high temperature, we have a remarkable 

 instance in the petrified wood observed in Kerguelen Island, 

 by Captain Sir James Ross. Seams of coal, varying in 

 thickness from a few inches to four feet, are imbedded in trap 

 rock ; and numerous fossil trees were found lying under a 

 bed of shale, which was covered by a mass of basalt several 

 hundred feet thick. Some portions of the wood were so 

 little altered that it was necessary to take them in the hand 

 to be convinced of their fossil state; and the wood was 

 found passing from that condition into charcoal which 

 would burn freely ; while other portions were so completely 

 silicified as to scratch glass. "j* In fact, the permeation of 

 vegetable tissues, by aqueous solutions of silex of a high 

 temperature, appears to be the necessary condition under 

 which silicification takes place. J 



* See an interesting Essay on this subject, by M. Alexandre Brong- 

 niart, " Essai sur les Orbicules Silicieux, <L-c." Paris, 1831. 



t Voyage of Discovery in the Southern and Antarctic Regions, in 

 1839-43. 



J The experiments and observations on the structure of plants, by 

 my friend, Mr. Reade, afford some interesting facts in illustration of 

 this subject. Mr. Reade states that "by the agency of heat the 

 surrounding siliceous matter may be, liquefied, and the carbon and 



