Introduction to Optical Mineralogy and Petrography #265323

Following Iddings, Winchell, and other American petrographers, the symbols X, Y, and Z, are here em ployed in referring to the axes of ether elasticity, instead of the German a, b, and c, used in many text and refer ence books. This is done to avoid confusion, especially in conversation or discussion, with the crystallographic axes.