“…According to calculations made for me by Dr. Earl Ingerson of the U. S. Geological Survey the smallest angle between a fracture plane in quartz and the base was, according to Anderson (1945, p. 412, foot note 1, his plane d), 32|°; according to Osborne et al (1936, p. 1364), 22°57'. But Engelhardt (1944 found angles between fracture planes and the optic axis ranging from 45° to 75° (45° and 15° with the base).…”