Professor Hitchcock is best known to many by his geological maps. The first efforts at mapping the geology of the United States were made independently by Edward Hitchcock and Jules Marcou in 1883--the work of Mr. Marcou extending only to the plains. Prof. H. D. Rogers, five or six years later, prepared a map for Johnston's Physical Atlas. In 1872 Prof. C. H. Hitchcock and Prof. W. P. Blake compiled a map for the ninth census of the United States, and for R. W. Raymond's report upon the mineral resources of the country. The success of his small scale map led Professor Hitchcock to undertake the preparation of a map on a scale of twenty-five miles to the inch for the whole country. For this he consulted every work that had been printed upon the geology of the United States, and obtained the privilege of using many unpublished data collected by geologists of States and Territories in which the work had never been carried to actual completion. The map prepared by the General Land Office was used as the basis for the geological coloration, and the work appeared in 1881, of a size adapted to use in the classroom. Its compiler has never seen any criticism of its accuracy. The edition prepared for the Mining Institute embodies all the information acquired for the large map, with such additional facts as had been learned since that map was published. Prof. Hitchcock's services were called into requisition in the compilation of a similar map for the United States Geological Survey, which was published in its annual report for 1886, under the editorship of W. J. McGee; in fact, the two maps were printed from the same plates, but Dr. Hitchcock's contained certain features not found in the other one--the result of different interpretations--and was more complete. In the Government edition a system of coloration devised by Major J. W. Powell, which was afterward abandoned, was employed.