25 



this character, may be had recourse to in marking its 

 different varieties, and their approximation to other mineral 

 substances. 



The varieties of siHceous wood depend not only on the 

 nature of the combinations forming the lapideous matter of 

 which it is chiefly constituted, but also, as has been already 

 observed, on the state of the wood previous to its petri- 

 faction. When the fossil is light coloured and of a shivery 

 texture, the wood may be presumed to have been previously 

 in a decayed state, or, as it is termed, rotten wood ; and 

 when close, compact, and dark coloured, it may be sup- 

 posed to have suffered previous bituminization. • 



A. Chalcedonic wood, — In the most common form in 

 which this variet}^ appears, the colour is of a yellowish 

 white, the substance resembling that of withered wood. 

 The surface rough and splintery, the splinters frequently 

 so minute as to be wafted with the slightest breath. The 

 internal part solid, chiefly formed of the translucent 

 siliceous matter, which fills the interstices and such ca- 

 vities as may have been formed by the teredines and other 

 insects, and also sometimes invests the ends of the spe- 

 cimen in a mammillated or stalagmitic form. Specimens 

 occur in which previous bituminization also appears to have 

 taken place, and in which the clear siliceous substance 

 appears as if it had transuded into the cavities, and had 

 exuded at the ends of the specimens. 



Here must be referred those amorphous specimens which 

 possess a rough surface, scarcely any lustre, with patches 

 of apple green colour and of a quartzose hardness, inter- 

 mixed with others of a light or light grey colour, con- 

 siderably softer. When cut and polished, the white parts 

 display evident marks of vegetable texture ; either that of 

 very fine-grained woods, or of some of the palms or reeds, 

 the spaces between being filled with siliceous matter, either 

 translucent, or of an apple green colour. 



E. 



