SPECIMENS OF FOSSlf. WOODS IN NATAL MUSEUM. 357 



filling the cavities, and the amount of the original carbon of 

 the wood remaining in the fossils, Mr. Neville Nevill, F.R.S., 

 Government Chemist, kindly caused a number of the samples 

 to be carefully analysed. The result of the analyses is given 

 above. 



These analyses at once show that the amount of carbon 

 remaining from the wood is very small. Ordinary wood con- 

 tains about 50 per cent, of carbon, while in the silicified 

 specimens Nos. 219, 221 there is a little over 2 per cent., and in 

 Nos. 160, 174 practically nil. 



The amount of oxide of iron varies from about 1-3 per cent. 

 In all the specimens except in the phosphatized Avood (Mus. 

 No. 220) and in the calcified wood (Mus. No. 12) the great 

 mass of the fossil consists of silica. 



The silica is largely in the crystallized form, since under 

 polarized light the field as a whole remains equally illuminated 

 during the revolution of the analyser, except that the traclieide 

 walls show much less brilliance, and sometimes they are 

 isotropic. It would thus appear that the silica, which pene- 

 trates the walls of the tracheides and cells and ultimately for 

 the most part replaces them, may retain the amorphous form^ 

 while that which fills the cavities mostly becomes crystallized. 



By ascertaining the specific gravity of the fossil wood and 

 knowing that of the mineral constituents and the amount of 

 such constituents, it would at first sight appear possible to 

 approximately calculate the amount of space left unoccupied 

 by solid matter. But the specific gravities of minerals, such 

 as silica, vary greatly according to the physical condition of 

 the mineral, whether crystallized or amorphous, and the method 

 would therefore be wholly unreliable. 



The following table gives the specific gravities of the seven 

 samples, and a rough estimation of the amount of unoccupied 

 space deduced from microscopic examination, and from the 

 relative amount of water absorbed after an immersion for 

 half an hour, taking the Weenen fossil wood as the standard. 



