GEOLOGICAL SURVEY, United States, a bureau of the Interior Department created for the purpose of preparing a map of the United States, classifying the public lands, examining the geological structure, mineral resources and the products of the country. To these duties were added those of investigating the extent to which the arid lands of the West can be re deemed by irrigation, segregating the irrigable from the nonirrigable lands, and the selection of sites for reservoirs and canals for the pur poses of irrigation. The maps made by the Geological Survey are all on a large scale, and have a degree of accuracy and a minuteness in detail incomparably greater than ordinary maps. The smallest scale is 1-250,000, or about four miles to the inch, and this scale has been em ployed for regions of the West which are thinly settled, and where the topography is mountain ous. But it has been superseded by scales of 2 miles and one mile to the inch, the latter for populous regions with slightly or moderately diversified topography, like Massachusetts and New Jersey. The maps are engraved on sheets which, with the 4-mile scale, embrace 1° of latitude and 1° of longitude. The 2-mile maps embrace tracts of half the above linear or one fourth the areal dimensions; the 1-mile maps embrace one-fourth of the above linear and one-sixteenth the areal dimensions. The to pography is represented by °contours° or °grade curves!" that is, by lines of equal altitude above the sea. The contour intervals are uni form for each sheet, but vary in different sheets according to the character of the country. In some tracts the contour intervals represent a difference of elevation of 200 feet, these being in very mountainous countries, while in flat countries and on large-scale sheets they may be as small as 20 feet. The general construction and methods of all maps are, however, the same.
There are three principal branches of the geological survey: Geology proper; (2) topography; (3) irrigation surveys. The geo logical branch investigates the stratigraphy, the geological structure and history, the lithology, mineralogy and palaeontology, the ores and mines, and in general the natural economics, re sources and physical geography of the country.
The topographic branch prepares the maps; the irrigation branch investigates the possibili ties of irrigation and selects the irrigable lands and sites available for reservoirs and canals. The work of the topographic branch is the basis of the work of the other two, and all the results of the latter are projected on the maps. The publications of the survey are: (1) the annual report of the director, which, besides the administrative report, contains memoirs on geologic subjects by members of the survey, and is distributed according to the regulations of the Interior Department; (2) monographs on the leading subjects of special investigation by the geologists; (3) bulletins on more lim ited special subjects of research; (4) an annual volume of mineral statistics. The last three are distributed gratuitously only to designated libraries and to learned corporate societies, which send their own publications in exchange. Otherwise they are sold by the director and the money deposited in the treasury. See GEOLOGY ; IRRIGATION ;' TOPOGRAPHY.
Methods and Publications.— The preced ing paragraphs state in a general way the func tions of this Survey as originally defined by Congress, together with subsequent modifica tions which included within the scope of its duties the study of the hydrographical condi tions relating to water power, and to the irriga tion of the arid lands of the western States.
As the geologic and hydrographic work de pends upon the topograhpic work, the prepara tion of a suitable topographic map received primary consideration, and the general lines of operations extended to secure it were very defi nitely outlined from the earliest period of the Survey, so that at the present time almost the total area of the United States, exclusive of Alaska, has been surveyed and mapped for this purpose.