360 5. F. ADAMS 



cavities with inwardly projecting quartz crystals. Small euhedral 

 quartz crystals have developed on the outside of the siliceous por- 

 tion of the material. 



Under the microscope the individual dolomite crystals are first 

 to attract attention (Fig. 2). The well-developed rhombs usually 

 average about one millimeter on the long diagonal. The domi- 

 nant form is the unit rhombohedron r{ioi i) , sometimes modified by 



■^•■&s'-26;-' ;siiKVi'"„-.-».i(«t'iX.^<.x->viMi .^tav3 A>."a»Aa»t--.iii.. 



Fig. 3. — Thin section of corroded dolomite rhomb in quartz ground-mass. Also 

 showing patchy retention of cell outlines, especially along annual rings. Notice 

 apparent distortion of cells bordering the larger dolomite rhomb. X2i|. 



e(oii2). These crystals neither include nor preserve any cell 

 outlines. A dark-brown rim around the edges indicates that they 

 have excluded the woody material. Replacement of the crystals 

 along borders by finely crystalline silica leaves a corroded dolomite 

 core surrounded at a little distance by a brown organic halo, as 

 the development of the silica does not affect the position of the 

 organic matter (Fig. 3). In a few cases, surrounding groups of 

 cells seem to have been squeezed by the growth of the rhombs. 

 Cells are usually misshapen near the annual rings. However, 

 one or two occurrences can be seen in which cells, bordering dolo- 

 mite crystals which lie entirely within the customarily well- 

 developed spring growth, are distorted as if by pressure from the 

 crystal (Fig. 4). This is an indication of the formation of the 



