In the Journal, for example, we may refer to papers[123] by Emerson on the Deerfield dike and its minerals, and on the occurrence of nephelite syenite at Beemersville, N. J.; to various interesting articles by Cross on lavas from Colorado and the pneumatolytic and other minerals associated with them; to important papers by Iddings on the rocks of the volcanoes of the Northwest, and those of the Great Basin, to primary quartz in basalt, and the origin of lithophysæ; to the results of researches by G. H. Williams on the rocks of the Cortlandt series, and on peridotite near Syracuse, N. Y.; to papers by Diller on the peridotites of Kentucky, and recent volcanic eruptions in California; to articles by R. D. Irving on the copper-bearing and other rocks of the Lake Superior region, and to Kemp on dikes and other eruptives in southern New York and northern New Jersey. Other publications would greatly extend this list. _The Petrologic Era._ As the chief facts regarding rocks, especially igneous rocks, as to their mineral and chemical composition, their structure and texture and the limits within which these are enclosed, became better known; and the relations, which these bear to the associations of rocks and their modes of occurrence, began to be perceived, the science assumed a broader aspect. The perception that rocks were no longer to be regarded merely as interesting assemblages of minerals, but as entities whose characters and associations had a meaning, increased. More and better rock analyses stimulated interest on the chemical side and this and the genesis of their minerals led to a consideration of the magmas and their functions in rock-making. The fact that the different kinds of rocks were not scattered indiscriminately, but that different regions exhibited certain groupings with common characters, was noticed. These features led to attempts to classify igneous rocks on different lines from those hitherto employed, and to account for their origin on broad principles. In other words, the descriptive science of petrography merged into the broader one of petrology. No exact time can be set which marks this passage, since the evolution was gradual. Yet for this country, in reviewing the literature, for which the successive issues of the “Bibliography of North American Geology” published by the U. S. Geological Survey has been of the greatest value; the writer has been struck by the fact that in the first volume containing the index of papers down to and including 1891, the articles on subjects of this nature are listed under the heading of _petrography_, whereas in the second volume (1892–1900) they are grouped under _petrology_ and the former heading is omitted. A justification for this is found in examining the list of publications and noting their character. With some reason, therefore, the beginning of this period may be placed as in the early years of this decade. Furthermore, it was at this time that the