Fig. 4.8 — A broken cap of quartz is viewed along the optic axis in polarized light. Few, 

 broad thickness-contours show good alignment with the optic axis. The fine TOOTHED- 

 PATTERNS are due to optical twinning. 



Fig. 4.9 — The same cap as in Figure 4.8, but viewed slightly off the optic axis. Note 

 the increased number of thickness-contours, and their fineness, also that some twinning 

 regions are enhanced. 



354 



