360 KRNEST \vai;im;n. 



lignified traclieides and medullary cells, and both previously 

 and subsequently to this infiltration the tissue of the wood 

 may undergo through pressure great contortion and differen- 

 tial movement. Most of the lignified substance ultimately 

 disappears and is entirely replaced by silica. Later the 

 cavities may be partially or almost completely filled with 

 silica in the form of chalcedony. 



The tissue of the wood forms a definite map whereby the 

 differential movement caused by pressure may be accurately 

 traced, the medullary rays being particularly useful in this 

 respect. 



Sheering movement mostly occurs along the lines of the 

 widest tracheides (spring-wood), that is in the region of rela- 

 tive weakness. Very frequently a special infiltration of 

 black oxide of iron occurs along tliese sheering lines and this 

 may produce a very conspicuous ringing to the wood (text- 

 fig. 2, A). A slight dift'erence in the texture of the original 

 lignified wall of the spring tracheides and a more ready cir- 

 culation of water through them have probably caused this 

 curious differential staining with iron oxide. The inner edge 

 of the black ring, which is generally close to (text-fig. 3), or 

 actually in contact with (text-fig. 2, A), the autumn-wood of 

 the previous year, tends to be sharply defined ; while the 

 outer edge, where the spring-wood is gradually passing over 

 into autumn- wood, is more irregular. Grreat pressure acting 

 from different directions may produce remarkable faults and 

 sheering phenomena analogous on a microscopic scale to the 

 great faults and contortions seen in geological strata, the 

 structure of the wood exhibiting very clearly all the move- 

 ments which have occurred (text-fig. '2, A, B, C). 



In fig. A the specimen experienced a great flattening pres- 

 sure, and sheering took place along the six shaded bands. 

 The course of tliree medullary rays has been strongly indicated 

 for the purpose of showing the result of the differential move- 

 ment. The bands are black through iron staining. 



In fig. B the wood has been subjected to enormous pressure 

 which has been applied in different directions at different 



