SPECIMENS OF FOSSIL WOODS IN NATAL MUSEUM. 371 



wood exhibits the effects of the pressure, and the medullary 

 rays generally show a remarkably zig-zag course. This mostly 

 occurs without any -obvious fracture of the walls of the tissue 

 elements of the wood. It is probable that partial impreg- 

 nation of the walls of the elements with silica would not 

 necessitate fracture under differential movement, but it 

 is extraordinary to note the amount of such movement which is 

 possible without any great dislocation of the tissue. In all 

 probability most of the contortion occurred before the process 

 of fossilization had proceeded very far. 



(5) FOSSILIZATION AND ALTERATION OF THE WOOD. 



The least altered w^ood is shown in transverse section in 

 PI. XX YI, fig. 10. 



There appear to be two different directions in which changes 

 took place iu the lignite seam : 



(1) ordinary fossilization or silicification of the tissues,, 

 comparable to that which produced the well-silicified coni- 

 ferous woods described above ; 



(2) changes in the direction of bituminous coal formation. 

 According to Dr. Aitken,^ bituminous coal consists of : 



(a) an opaque black substance which is insoluble in acids, 

 and ma}^ be free carbon (?) ; 



(&)'a yellow or reddish resin-like substance which is trans- 

 lucent or transpai'ent, volatile by heat, and insoluble in naphtha 

 and acids, and possibly referable to the Succinite group; 



{<•) earthy matter. 



In the specimens which exhibit the structure of wood in the 

 least altered condition, the formation of a yellowish-red 

 substance had begun. The cavities of the wood fibres had 

 become filled, and the medullary cells were also partially 

 occupied. The outlines of the wood fibres in many parts 

 of the section had become lost ; while in some parts this 

 substance formed a thin layer in between the fibres, thus 

 marking them out with great clearness (fig. 11, i/i). 



' Dana, Dwiglit James, 'A System of Mineralogy,' p. 756. New York, 

 1889. 



