A REPLACEMENT OF WOOD BY DOLOMITE 363 



anhedral and of a fairly uniform size in any one part of the thin 

 section, although the average size for the different parts of the 

 specimen may vary from very small to a maximum width of about 

 o . 2 mm. In general, it is the rule for one quartz crystal to con- 

 tain several wood-cell outlines. As before stated the average 

 diameter of cells is 0.4mm., with a range of from 0.02 mm. to 

 0.07 mm. In longitudinal sections the quartz is usually elongated 

 in the direction of the wood fiber, showing that the growth of 

 crystals has been influenced by cell structure. Distinct crystal 

 outlines with an occasional hexagonal cross-section are noticeable 

 in several places throughout the anhedral quartz. They are 

 better-developed individuals belonging to the same silification. 

 As may be seen in one of the accompanying photographs (Fig. 3), 

 retention of cell structure is patchy. The best-developed quartz 

 crystals occur on the borders of those places in which no cell 

 structure is evident. They give the appearance of having grown 

 into an open space, or at least into a decayed spot, where there 

 was nothing to resist their assuming idiomorphic form. At their 

 roots these crystals also inclose cell outlines. 



Secondary enlargement of the euhedral and subhedral crystals 

 is seen in several places. A rough radial grouping occurs in some 

 of the quartz. Extinction in sequence often gives these groups 

 the same coarsely spherulitic appearance as in the dolomite. 

 Wavy extinction is common in many crystals. In one thin section 

 cell outlines are well preserved by another form of silica. This 

 silica is very fine grained, fibrous, often spherulitic, with very weak 

 double refraction, about the same index of refraction as quartz, 

 and of negative elongation. 



Late silica and dolomite are found in veinlets and cavities. In 

 several small veinlets which cut both siliceous ground-mass and 

 early dolomite crystals, banding by alternating quartz and dolomite 

 occurs. Dolomite also is found in patches throughout the quartz 

 replacement of heartwood. In cavities it is often in the center 

 as a filling around projecting euhedral quartz crystals. A very 

 prominent occurrence of this late dolomite is as an enlargement, 

 although not in crystallographic orientation, of the older rhombs 

 and dolomite crystals. 



