In addition to the above, areas amounting to about one hundred and six thousand square miles have been adopted from the Powell, Wheeler, Hayden, and King surveys, and published on the scale of 1:250,000. From this table it appears that during the past twelve years the Geological Survey has mapped six hundred and twenty-four thousand square miles, being more than one fifth the area of the country, excluding Alaska. Of this, more than two thirds is on the scale of 1:125,000, and nearly one sixth on the scale of 1:62,500.
Geologic Work.—The geologic work is readily classified as special investigations and areal mapping.
The first branch of the geologic work, special investigations, is illustrated by the study and report on The Tertiary History of the Grand Cañon District, by Captain C. E. Button; Lake Bonneville, by Mr. G. K. Gilbert; Geology and Mining Industry of Leadville, Colorado, by Prof. S. F. Emmons; The Palæozoic Fishes of North America, by Prof. J. S. Newberry; and the thorough investigation of the geologic phenomena of the Yellowstone National Park, by Mr. Arnold Hague. Twenty-four monographs and one hundred and sixteen bulletins have been published by the survey as the results of such investigations. They are frequently the basis of generalizations that must be obtained before the areal geologic work can be successfully prosecuted; and the areal geologist is constantly making use of the data furnished him by the specialist. Immense collections have been accumulated in the laboratories of the survey and in the National Museum, which are the basis of correlations used almost constantly in areal mapping and frequently in the solution of problems arising in connection with the study of economic questions of a high order. The interrelation of the various branches of geology are such that all must be kept up to a high standard, or all will sooner or later deteriorate and thus affect the quality of the output of results by the survey.
Under the direction given, in 1882, to complete the geological map of the United States, a comprehensive scheme of work was outlined. A large corps of geologists soon began work on various problems that arose in planning a system of mapping that would serve for all phases of geology to be met with in the three million square miles of the area of the United States. A large amount of valuable detailed local work had been done by various State surveys; several of the Government surveys had made more or less complete reconnaissances of large areas west of the Mississippi River, and a few fairly accurate geologic maps were published by them; but the State and Government surveys had been conducted each in its own way and with little regard to co-ordination with the work of the others. It was necessary to bind
