A REPLACEMENT OF WOOD BY DOLOMITE 



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dolomite before the surrounding silification. In general, cell 

 outlines are not evident near the borders of dolomite rhombs. 

 Had the dolomite been a replacement of silica, one would expect 

 to find the cell outlines in the silica as well preserved in the immedi- 

 ate neighborhood of the rhombs as elsewhere. The exclusion of 

 woody material is also evidence of the early crystallization of this 

 dolomite. 



Fig. 4. — High power view of rhomb in Figure 3, showing dolomite core, organic 

 rim, and silica fringe separating the two. Also shows distorted cells. X76I. 



In the main mass of buff dolomite the crystals are slightly 

 smaller than the rhombs just mentioned, and they rarely show 

 crystal outlines. With magnification the thin section appears to 

 be a compact mass of rather even fine-grained crystals, usually 

 about twice as long as wide. Many crystals appear to be twinned. 

 No polysynthetic twinning was noticed either in the thin section 

 or in fragments. An approach to cyclic twinning is occasionally 

 seen, but commonly the apparent twinning is of a simple contact 

 nature. However, the presence of twinned crystals was not 

 proved. 



The dolomite crystals are sometimes arranged side by side in 

 long rows; opposite them is a similar row; and along the center 

 where the rows touch are two imperfect chains of distorted cell 

 outlines. The rows are always arranged radially with respect 



