174 



ANATOMY OF THE GYMNOSPERMS 



from the Cretaceous of the Queen Charlotte Islands, which, as 

 shown on a former occasion (56), contains at least 70 per cent 

 of calcium carbonate. The general effect of this form of pres- 

 ervation upon the structure is substantially the same as in 

 silicification. 



Pyritization. A still less common form of preservation is that 

 which involves a replacement of the silica or calcite of the pre- 

 vious forms by crystalline sulphide of iron. This is a feature 

 more or less common to fossils from the older formations, which 

 always involves a complete obliteration of structural details, 

 though in rare cases the more general features may be seen 

 when viewed by reflected light. Plants presenting this form 

 of preservation are among the least valuable for purposes of 

 scientific study. 



