The following work, now offered in a further revised edition, has been written to serve as a guide to the study of rocks in thin slices, and is of course assumed to be supplemented throughout by demonstrations on actual specimens. Since it is designed primarily for the use of English-speaking students, examples are chosen, so far as is possible, from British and North American rocks; and a like remark applies to the numerous references to original authorities which arc inserted in foot-notes.<br><br>No systematic account is given of the crystallographic and optical properties of minerals. This is rendered unnecessary by such books as Iddings' translation of Rosenbusch's well-known work and Hatch's translation of the same author's tables. In particular, I have made no explicit reference to the use of convergent light.