A REPLACEMENT OF WOOD BY DOLOMITE 365 



of having been attacked by dolomite. Blum describes a petrifac- 

 tion by opal in which the outer part is opal, whereas the core is 

 unaltered wood, showing that replacement progressed from the 

 outside toward the center. It may be that in the case in hand 

 petrifaction occurred while at least a complete cross-sectional 

 fragment of the wood was intact, and that dolomitization started 

 from the outside and developed inwardly. The solutions contain- 

 ing the carbonate then gave place to others with silicic acid which 

 permeated the heartwood and caused its silification. Cavity 

 and crack fillings, and rearrangements, followed at some time later. 



