364 S. F. ADAMS 



The later silification develops with this dolomite as a replace- 

 ment of the previously formed quartz and dolomite as a cavity 

 and vein filling, and as an incrustation. That it actually replaces 

 the earlier quartz is not certain, but its replacement of dolomite 

 is manifest in the corroded rhombs separated from their excluded 

 organic halo by silica. Chalcedony and quartz occur in this 

 silification. Quartz predominates. It occurs in subhedral crystals 

 in veinlets, presenting a microscopic comb structure, or as cavity 

 filling, in which case it has developed from the walls inwardly as in 

 a geode. In both of these cases it often shows zonal lines. It 

 replaces the dolomite around the borders, and, especially in the 

 case of the compound dolomite groups, it begins replacing in the 

 center of the group as well as along the edges. The development 

 between parallel rows of dolomite crystals has already been men- 

 tioned. The chalcedony and fibrous silica are found in the centers 

 of filled cavities, in veinlets, and as a replacement of dolomite. 



Another very interesting late development is that of minute 

 patches of an opaque gray metallic mineral, taken to be hematite, but 

 in too minute amounts to test. This appears in a polished surface 

 in veinlets and patches and in one case in a small rhomb as if it 

 were a pseudomorph after dolomite. 



In conclusion: dolomite seems to be the earliest replacing 

 mineral. The small area of large-sized rhombs is taken to be the 

 result of replacement in a partially rotten or injured spot in the 

 wood. The outer part of the specimen was replaced by a finer- 

 grained dolomite, but without retention of much of the cell struc- 

 ture. The curious arrangement of crystals in rows points to 

 initial crystallization along radial channels. What furnished 

 these channels is not clear. The distance between rows is greater 

 than the spaces between medullary rays. Also, the channels have 

 longitudinal extent, which leads to the possible hypothesis that 

 they were caused by closely spaced radial cracks. 



From the general character of the petrification it appears that 

 crystallization took place rather slowly along certain lines, later 

 spreading throughout the wood and destroying most of the cell 

 outlines. For some reason these solutions gave out before complete 

 dolomitization, for the inner part of the specimen shows no evidence 



