SPKCI.AIKXS OF FOSSIL WOODS IN NATAL MUSEITM. 363 



is frequently seen forming a more or less complete inner layer 

 to the cast. In some cases this mineral appears to be silica. 

 Sometimes the whole tracheide cast is solid and homogeneous 

 throughout as at s.c.c, fig. 3. At other times a perfectly opaque 

 black mass [o] occupies the cavity. This is probably iron oxide. 

 The casts frequently exhibit cracks which were subsequently 

 filled in by secondaiy deposition of mineral matter (fig. 3,^./.), 



A comparison of figs. 5 and 6 is of much interest in showing 

 that sheering took place at two diiferent times during fossili- 

 sation. At fig. 6 the tracheides were bent out of shape before 

 the cavities were filled with the mineral substance, and at 

 fig. o the sheering took place subsequently, and the mineral 

 casts (s'.S.t.) of the tracheide cavities were shattered. 



With crossed prisms of polarizing apparatus a dark reti- 

 culum is seen. In fig. 4 this dark portion includes any of the 

 original tracheide wall that persists, and also the outer portion 

 of the mineral cast. The remainder of the wall of the cast 

 exhibits irregular radial dark and light bands or patches. 

 The inner secondary layer is generally brighter than the 

 pi'imary cast. It is suggested that the different optical 

 reaction of the mineral substance of the outer portion of the 

 cast may be due to slight impregnation with organic matter 

 from the decomposing lignified walls of the tracheides. 



In the case of the calcified wood from Ingwavuma, which 

 is possil)ly of Cretaceous origin, the main mineral substance 

 is calcium carbonate (see table of analyses, p. 356). This 

 appears to be a very poor fossilizing agent for the woody 

 tissue. The structure of the wood is badly pi-eserved ; but a 

 careful examination of transverse and longitudinal sections 

 proved that the wood was undoubtedly coniferous, similar in 

 chai-acter to the silicified specimens ; but as the pits were not 

 distinguishable a reliable determination could not be made. 



PI. XXV, fig. 7 represents a transverse section. On the 

 left-hand side the elements of the wood, consisting of 

 tracheides {r.t.t.v.) and medullary rays {f.m.r.), are visible 

 in the shadowest outline only. In the middle of the section 

 the tissue is clearly seen (o.t.ir.). The walls are of the 



