_Summary._ In this brief retrospect, giving only the barest outlines and omitting from necessity much of importance, we have seen petrology grow from occasional crude experiments into a fully organized science in the last half century. It has to-day a well-perfected technique, a large volume of literature, texts treating of general principles, of methods of work, descriptive handbooks on the morphological side, and has attained general recognition as a field, which, though not large, is worthy of the concentration of intellectual endeavor. Like other healthy growing organisms it has given rise to offshoots, and the sciences of metallography and of the micro-study of ore deposits, which are rapidly assuming form, have branched from it. What of the future? The old days of mostly descriptive work, and of theorizing purely from observed results, have passed. The science has entered upon the stage where work and theory must be continually brought into agreement with chemical, physical and mathematical laws and data, and in the application of these new problems present themselves. As we climb, in fact, new horizons open to our view indicating fresh regions for exploration, for acquiring human knowledge and for our satisfaction. _Bibliography._ Footnote 107: W. Cross, Jour. Geology, =10=, 451, 1902. Footnote 108: _Ibid._, p. 45. Footnote 109: Sorby, Quart. Jour. Geol. Soc., =14=, 453, 1858. Footnote 110: Zirkel, Einführung des Mikroskops in das mineralogisch-geologische Studium, 1881. Footnote 111: Williams, G. H., Modern Petrography, 1886. Footnote 112: Zirkel, Mikroskopische Beschaffenheit der Mineralien und Gesteine. Footnote 113: Rosenbusch, Mikroskopische Physiographie der petrographisch wichtigen Mineralien. Footnote 114: Zirkel, Mikroskopische Gesteinstudien, Sitzung vom 12 März, 1863. Footnote 115: Julien and Wright, Geol. Surv. of Michigan, 2, 1873. Appendices A and C. Footnote 116: Dana, E. S., the Journal, =8=, 390–392, 1874. Footnote 117: Zirkel, Geological Exploration of the 40th Parallel; vol. VI, Microscopical Petrography. Footnote 118: Caswell, Microscopical Petrography of the Black Hills. U. S. Geog. and Geol. Surv. Rocky Mts. Rep. on Black Hills of Dakota, 469–527. The separate copies issued bear the imprint 1876; the complete report 1880. Footnote 119: Dana, E. S., Igneous Rocks in the Judith Mts. Rep. of Reconnaissance Carroll, Mont., to Yellowstone Park in 1875. Col. Wm. Ludlow, War Dept., Washington, 105–106. Footnote 120: Hawes, G. W., Rocks of the Chlorite Formation, etc., the Journal, =11=, 122–126, 1876. Greenstones of New Hampshire, etc, ibid., =12=, 129–137, 1876. Footnote 121: Hawes, G. W., the Journal, =21=, 21–32, 1881. Footnote 122: Brögger, Die silurischen Etagen 2 und 3, Kristiania, 1882. Footnote 123: