28 INTRODUCTION. 



beautiful quartz-crystals. This is very frequently seen, for example, in the 

 larger seeds of Gymnosperms which are found in the often-mentioned 

 black pebbles of Grand' Croix. 



On the other hand, I have in my possession a piece of wood of a milk- 

 white colour from the Habichtswald near Cassel, and coming probably 

 from the sands of the Drusenthal, which can be broken up into single 

 spiculae answering to the casts of the tracheides, exactly in the manner 

 of the silicified and calcified woods mentioned above. The substance of 

 the membranes has entirely disappeared, the spiculae are suspended in the 

 interspaces which answer to the walls and are connected together only by 

 a fragile froth-like network of very thin lamellae of silica. Schimper l 

 mentions an exactly similar wood, coming indeed from Tasmania, which 

 was given to him by Robert Brown. I have seen another piece of the 

 same kind, said to have been brought from Texas, in the botanical depart- 

 ment of the British Museum. From these specimens to the silicified woods 

 of Autun, Charles, and other places in the departments of the Sa6ne et 

 Loire and Allier mentioned by Renault 2 there is obviously but a single 

 step. The latter are certainly solid and coherent, but in the spots which 

 correspond to the membranes they contain a system of very fine pores, 

 which readily imbibe drops of water placed upon them. These again are 

 merely the small interstices from which the organic substance has dis- 

 appeared when exposed to the air. Since preparations from these woods 

 in canada-balsam are too transparent, Renault puts them first of all into 

 coloured solutions, which remain in these fine pores and show the direction 

 of the membranes. 



From the accounts which we possess it would appear that the silicifica- 

 tion is accomplished in two ways, a circumstance which Renault 3 would 

 connect with the difference in the compactness or porousness of the wood 

 in different cases. First there is the usual process in petrifactions ; after 

 the parts of the plants were buried, either the whole of the environment 

 stiffened into a hard encasing mass of silica, or the remains only were 

 silicified and thus served as centres of dispersion for the silicic acid, which 

 either entered as free acid in solution, or was extracted from alkaline com- 

 pounds by the carbonic acid, the humus-acids of the decaying organic 

 substance. Proofs of this mode of formation may be obtained from 

 the woods found in the district of Zobten in Silesia and described by 

 Comventz 4 , which externally have the appearance of lignite, but show 

 centres of petrifaction in their interior in the form of hard silicified nuclei. 

 Felix 5 supposes that the wood of the lignite of Grobers near Halle under- 

 went the same process ; but in it the periphery only is silicified, while the 

 central portions can be cut, and burn when lighted. 



1 Schimper (1), Introd. 3 Renault (2), Introd. Renault (2). 4 Conwentz (1). s Felix (1). 



