362 ERNEST WAI.'REN. 



only a portion of a traclieide wall, ;is shown in one of tlie 

 n])per traclieides of the figure. The silica which subsequently 

 fills the cavities of the tracheide may in a similar way be 

 either colourless or blackened. 



It is particularly observable that in the sheering bands 

 (PI. XXY, fig. 2) the silica is generally blackened. 



In these bands it may be very usually seen that the medul- 

 lary rays (fig. 2, )n.r.), although of course bent, do not iu 

 themselves exhibit so much flattening and distortion as the 

 surrounding traclieides. This appears to be only explicable 

 by supposing that the sheering took place after partial silici- 

 fication, the walls of the traclieides being only partially im- 

 pregnated Avhile the medullary cell cavities were already 

 filled and supported by solid silica Subsequently, on pressure 

 being applied, the surrounding incompletely silicified traclieides 

 would be flattened and contorted, while the medullary rays 

 supported by silica would be less affected. 



It is, indeed, quite likely that, owing to the difference in 

 the tissue of the medullary rays and wood proper, the rays 

 might become filled with silica before the tracheides, and 

 the cracks that would necessarily arise in the silica through 

 differential movement of the solid mass would be re-cemented 

 together by silica. 



An examination of the specimens of calcified wood demon- 

 strates that Ciilcium salts are far less satisfactory than silica as 

 fossilizing agents. 



In the case of the phosphatized wood from the beds at 

 Weenen the chief mineral constituent is calcium phosphate. 

 This substance does not appear to be able to actually enter 

 the lignified substance of the tracheides like silica, since in 

 section (PI. XXV, fig. 3) remains of the lignified walls can be 

 seen in an apparently little altered condition (o.t.iv.) between 

 the mineral walls. It would clearly appear that the mineral 

 salts are laid down on the inner side of the lignified walls 

 to form a kind of cast of the cavities, and that it is gradu- 

 ally added on to from the outside as the lignified walls 

 slowly disappear. A secondary deposition of mineral matter 



