PETRIFACTION OF PLANTS 1081 



thus saturated may be dissolved in concentrated hydrofluoric acid, 

 when the woody tissue will be left behind unattacked. This shows, 

 according to Goppert, a cellular structure which in most cases 

 is sufficient for the generic determination of the wood. 



Replacement of the cell walls themselves generally follows 

 impregnation, and thus the wood becomes wholly changed to silica. 

 Under these circumstances the finer structure is often destroyed 

 and the mass becomes uniform and breaks with a conchoidal frac- 

 ture. Illustrations of this are found in the brilliantly colored, 

 agatized woods of Arizona, fragments of which are hardly dis- 

 tinguishable from agates of wholly inorganic origin. 



Opalized woods are not uncommon. Here, as in the case of 

 woods replaced by quartz, the structure of the wood is generally 

 retained, and in some cases the interior has been found to be but 

 slightly impregnated with the^opal, or even to be unaltered wood, 

 thus showing the progress of opalization from without inward. 

 (Blum-4:/07.) 



Calcified woods are not uncommon, occurring in all formations, 

 from the Devonic up. They have been found in limestones, sand- 

 stones, shales, basaltic conglomerates, volcanic ashes and tufifs, and 

 other deposits. Daubree found at Bourbonne-les-Bains. in the de- 

 partment of Haute-Marne, France, piles of red beechwood, in 

 places so completely impregnated with transparent calcium car- 

 bonate that on solution in hydrochloric acid only 3.1 per cent, of 

 insoluble matter, showing plant structure, remained. The piles 

 supported an ancient Roman canal, and when found were buried 

 about 8 meters below the surface. 



Aragonite is also known to have replaced wood. Even gypsum 

 has been found replacing wood in some Tertiary beds, and phosphate, 

 as well as fluorite of lime, is likewise known in this connection. 

 Barite also has replaced wood in some limestones of the Lias, and 

 a talc-like, complex silicate, probably pyrophyllite, has been found 

 replacing fronds of Neuropteris and Pecopteris and the leaves of 

 Annularia in Carbonic rocks in the Piedmont district. Chlorite has 

 been found occurring in a similar manner. So delicate is the replace- 

 ment that the venation is easily recognizable, although no part of the 

 original organic matter remains. A\'ood largely replaced by sulphur 

 and devoid of structure has been found in Cesena, Italy, and 

 plant remains replaced by sulphur have also been obtained from 

 the Tertiary beds of Aragon. (Blum-7 :/7o.) 



In the Carbonic and later coal-bearing horizons wood replaced 

 by siderite, often with considerable iron oxide, or wholly by limonite 

 or hematite, is not uncommon in various parts of the earth ; while 



