362 5. F. ADAMS 



to the replaced wood as a whole. Although these "chains of 

 cells" include varied sizes and odd shapes, it is safe to say that 

 they are a relic of the original vegetable fiber. The size of the 

 cells in these chains varies from o .02 mm. to o .07 mm. in diameter 

 and averages 0.04mm., which is a range in size and an average 

 also true of cell outlines well preserved by silica. Occasional 

 cells are replaced by individual dolomite crystals which do not 

 extend beyond the cell walls, and still other cells show evidence 

 of secondary enlargement of dolomite, proceeding outward from 

 the cell as center. Cell outlines in dolomite are not often retained 

 except in or near the cell chains. The parallel row arrangement is 

 repeated throughout much of some of the thin sections. It is this 

 arrangement which causes the radial lines of buff and black as 

 seen in the hand specimen. The fine black lines are composed of a 

 late silica which develops most readily between the abutting ends 

 of the dolomite crystals. An explanation for this phenomenon 

 which seems tenable is that the dolomite started crystallization 

 simultaneously along somewhat widely spaced radial lines; that 

 in the first crystallization some of the cell outlines were preserved; 

 but that in the subsequent growth wood structure was largely 

 obliterated. Judging from the secondary enlargement of dolomite 

 with a cell as a center, the initial crystallization occurred within the 

 cell, in some cases at least. 



Often the dolomite crystals are grouped radially with a small 

 center of finely crystalline silica of the same late type as just 

 mentioned. The silica, of course, is a development subsequent 

 to the radial growth of the dolomite. The siliceous cores of these 

 groups are seen as fine black dots in the hand specimen. Extinc- 

 tion in sequence often occurs in the radially grouped dolomite, 

 and when combined with simultaneous extinction in opposite 

 sectors, gives the group a rough spherulitic appearance. 



Thin sections of what has been called the heartwood show 

 the best retention of cell structure and annular rings. For the 

 most part, the replacing mineral is quartz.^ Crystals are usually 



^ The exact relations of quartz, chalcedony, fibrous, and other forms of silica 

 have not been settled. In this article the identification of quartz as such is made 

 easy by its occasional euhedral form. The chalcedony to be mentioned later was so 

 called on account of its lesser refractive index than quartz and negative elongation. 



