600 The American Naturalist: [July, 
right and left handed quartz. When they are bent or disturbed they 
furnish biaxial interference figures. Many crystals are composed of 
both stable and unstable quartz, the relative positions of which show 
some relation to the symmetry of the crystal. ‘Such crystals, or crys- 
tals composed entirely of unstable quartz, have the lamelle induced 
by great mechanical stresses. The fact that the structure is only 
faintly induced and that very near the fracture in artificially crushed 
crystals, is explained by the short time during which the stress is 
applied, permanent structure being produced only after a long applica- 
tion of the stress. 
