Excerpt from Introduction to Optical Mineralogy and Petrography 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. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.