tion. Bench marks also are of value in the progress of the Survey's topographic work as fixed points of reference for the extension of its level lines.

It will be noted that the act of 1896 does not require that the elevations of the bench marks above sea level be determined, but only "the elevations above a base level located in each area." Any attempt to determine elevations above sea level in the initial survey of an area might have necessitated the running of thousands of miles of precise levels in order to connect with sea level the initial points within each of the areas under topographic survey. This would have cost immense sums and would have occupied several years, during which the topographic surveys would have had to be continued without spirit leveling. The Survey, therefore, designated some point within each area under topographic survey as a central datum point for that area, and determined the elevation of that point as nearly as practicable from existing elevations adjusted by railway levels or other levels brought from the sea. In consequence, though all the elevations referred to the same central datum point agree one with the other, they may not be reduced to mean sea level because of the differences between the primary elevations on which the leveling is based. However, prior to and since 1896 preciselevel lines have been extended by the Survey and other organizations to more accurately determine inland elevations above sea level, and the elevations originally determined have been corrected from time to time, so that at present nearly all the central points have been reduced to mean sea-level datum and carry with them all the levels resting thereon. As the area referred to a single base is gradually widened, it becomes possible to correct the elevations with reference to a new base, the corrections being, if practicable, entered upon the bench mark.

The location and elevations of the bench marks are published in two ways-first, by a symbol on the atlas sheets, accompanied by the letters "B.M." and the elevation to the near

est foot; second, by lists of bench marks in the annual reports or bulletins of the Survey, with a full description of each bench mark and its exact elevation above sea level to the thousandth of a foot, as adjusted and referred to the various central datum points, these lists being corrected in publications from time to time as better connections are made with sea level. These publications are issued separately for each

state.

Geological Survey. As perhaps its basic activity, the Survey is carrying on a survey of the areal geology and geological structure of the United States. By means of this survey it seeks to ascertain the distribution, structural relations, mineralogic character, economic value, and geologic history of the formations of each area surveyed. The results of this survey it publishes in the form of maps, illustrations, and textual descriptions.

The selection of areas for geologic survey is determined by a variety of considerations, corresponding closely to those mentioned in connection with the distribution of the topographic surveys. Still another element is the desirability of giving preference to the public land areas because of the value of geologic mapping in furnishing information needed by the General Land Office in the administration of the land laws.

The degree of refinement of the geologic survey varies with the character of the area and the results desired. The scale of the map on which the results are to be published is determined by the same considerations as in the case of topographic mapping. The three scales used in the topographic mapping1:250,000, 1:125,000 and 1:62,500-are used also in the geologic mapping and according as one or the other map scale is contemplated the survey is said to be exploratory, reconnaissance, or detailed. Larger scales also are used.

The geologic survey of an area is usually begun after the topographic map has been completed, and that map is used as a base. Occasionally, however, the geologist works with the

topographer, or even at times works in advance of formal topographic mapping, but in such cases he is either supplied with some form of map, or himself constructs a rough topographic map as he works.

The field work of the geologist engaged in systematic geologic survey is ordinarily confined to examination of the surface, aided by magnetic observations in connection with magnetic ores. Samples of rocks and ores and specimens of fossils are collected. All mines or other available openings are also examined, but only in exceptional cases are independent borings made for this purpose. The field work is supplemented by office and laboratory work, which includes microscopic and chemical examinations of the rocks and ores collected and paleontologic study of the fossils.

The results of the geologic surveys are given to the public in reports accompanied by geologic maps, diagrams and other illustrations, and in "geologic folios" containing maps and descriptive text. These reports vary widely in scope and character, dealing in some cases with selected areas, in others with selected deposits, and in still others with particular geologic problems. They vary also in size and in the degree of technical knowledge assumed on the part of the reader. The shorter and less technical reports are published under the general name of Bulletins, those of intermediate character under the name of Professional Papers, and the more voluminous and technical treatises under the name of Monographs. The Bulletins and Professional Papers are distributed free for the public, while Monographs are sold.

More recently, the Survey has been issuing, as Bulletins, guidebooks to various parts of the western states, arranged according to the principal tourist routes. Although the geologic features of the country occupy a large part of the text, information is also given, in popular style, relative to the geography, including scenic features, notable historic events, Indian tribes, industrial resources and plants and animals.

A "geologic folio" is thus described by the Survey:

2

A standard geologic folio comprises a descriptive text, in which the geography, topography, and geology of the area are described; a topographic map; a geologic map, printed in colors, showing the areal distribution of the geologic formations; a geologic map showing the deposits of economic value in stronger or more brilliant colors; a geologic map on which the underground structure of the rocks is exhibited by vertical sections representing what would be seen in deep trenches cut across the area; and a columnar section, in which the rocks are represented in a vertical column in their normal relation one to another, accompanied by a condensed description of their composition, thickness, and relations.

For special areas other maps or illustrations are included. Often it is desirable to present with the description a page of illustrations reproduced from photographs or sketches. For artesian-water areas the distribution of the water-bearing strata, the areas where flowing water may be obtained, and the depths to the water horizon are shown on a special sheet. For active mining regions a map on a scale sufficiently large to show important details of the geology, mines, and ore deposits is inserted. For coal regions of economic importance special methods are used to show the structure or lay of the coal beds of so much importance to the mine worker. This is done by means of contour lines which show the elevation above sea of the coal bed throughout its occurrence in the area, and then furnish the data for determining the depth of the coal below the surface at any point. Sections of coal beds which have been opened or worked are shown on a large scale and their relation to one another is indicated.

Generally speaking, each folio relates to a single quadrangle, that term having the same meaning as when applied to a topographic map; but some folios cover two or more quadrangles.

Occasionally, the advance of geologic knowledge of a particular area, or the demand for more detailed information, necessitates a revision of maps already published.

The geologic survey now carried on discloses the distribu

2 The U. S. Geological Survey: Its Origin, Development, Organization and Operations, p. 99-100, 1904.

tion and thickness of the formations in the area covered, their chemical composition and physical characteristics, and their age—that is, the period of geologic time in which they were deposited, as well as their form—that is, their structure, and the geologic forces which caused their deposition, erosion, and deformation.

The value of facts regarding the mineral character of rock formations for the development of mineral resources need not be dwelt upon. In disposing of the public lands this information is useful not only in shaping policies but in administering the land laws, and such information concerning lands privately owned facilitates and expedites the development of mineral deposits of economic value, or indicates the non-existence of such deposits. In this connection it may also be pointed out that information as to rock formations incidentally includes information as to underground water.

The identification of the geologic positions of several strata in the geologic time-scale is also of value to the economic geologist, as it helps him to determine the probability of the occurrence, in the area, of particular economic deposits. Another value, however, lies in the light it throws on the relation of the geology of the area to that of adjacent areas and, ultimately, of the entire country.

The maps and descriptive texts resulting from geological surveys find a most important use in the hands of the Survey itself in its work of land classification and of geologic research.

It would be difficult to ascertain precisely the total amount spent by the Survey in geologic surveys of the country. The appropriations used have been made under various heads, and some have provided for other work than geological surveys. The total amount appropriated since 1889 for "geologic surveys in various portions of the United States" has been about $5,500,000; that appropriated for paleontologic researches (an item of appropriation no longer in use), and for chemical and physical researches has been over $900,000; and that ap