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The field work upon which this report is based was begun in July, 1897, and continued without interruption until December of the same year. The area studied is approximated 15 miles square and contains 234 square miles. The topographic maps, which are two in number, were prepared under the direction of Mr. R. U. Goode, Mr. S. S. Gannett doing the triangulation and Messrs. Marshall and Griswold the topography in the fall of 1896 and summer of 1897. The mapping is done on two scales; the larger area, approximately 15 miles square, is mapped on a scale of 1:62,500. This map is designed to form a part of the Geologic Atlas of the United States. The other map represents the portion of the larger area in which the majority of the mines are located. It is on a scale of 1:9,600, and covers an area of 12 square miles. The work has been greatly facilitated through the assistance rendered by the mining men of the district, among whom special thanks are to to Messrs. G. H. Robinson, W. J. Craig, W. M. Nesbit, and C. H. Blanchard. The chemical work on the ores and country rocks from the district has been done in the laboratory of the Survey by Mesrsrs. H. N. Stokes and George Steiger, adn the determination of the fossils collected is to be credited to Mr. G. H. Girty, also of the Geological Survey. In the field work the authors have cooperated constantly on every phase of the varied problems. The same is true for the office work, except that the stratigraphic and economic problems have been the especial studies of Mr. Tower, while the petrologic and remaining problems have been the special studies of Mr. Smith. In pursuance of this system of work the introduction has been written conjointly, Chapter I of Part I and all of Part II have been written by Mr. Tower, and Chapters I and III to VII of Part I by Mr. Smith.

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