Publications of the United States Geological Survey, by years-Continued.

15 geologic folios (86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100)....

Grand total..

119, 353

8,727

Section of Geologic Maps.

The classification and geologic nomenclature adopted by the United States Geological Survey on its organization was based on the works of such eminent geologists as Dana and Le Conte, and has been the foundation of all later systems of the Survey. A scheme of colors for use on maps was provisionally adopted in 1881. It consisted of a limited number of colors, each representing a definite period of geologic time, and of a few patterns, which were used indiscriminately for all classes of rock.

The printing of geologic maps in colors was at that time an experiment, and the plan adopted by the Survey was tentative. It was soon found inadequate, and but few geologic maps were printed before 1889. Such as were published prior to that date have since been reissued upon a more comprehensive plan which was adopted in that year as a result of a conference of geologists.

This conference adopted rules of geologic nomenclature and a system of classification which accorded with the progress made in the

science of geology to that time. The method of publishing the Geologic Atlas of the United States was thoroughly revised. The engraving division of the Survey having been established, it was possible, with the aid of the chief of that division, to prepare color patterns and tints, and the geologists in charge were enabled to exercise closer supervision of the engraving and printing of geologic maps. Thus greater accuracy of engraving and better register of the intricate colors were obtained. It was not until 1892, however, that any of the final maps of the atlas could be issued.

To supervise the editing of this material and to devise and apply rules and regulations, a geologist was selected to act as editor of geologic maps, and from this beginning the section of geologic map editing has developed.

These rules and plan of publication remained in force until December, 1903, when the growth of the science demanded further changes, and new rules were formulated and put into effect in January, 1904. As the number and intricacy of the folios increased, the work of the section also increased and the force was enlarged. At present it consists of a geologist acting as editor, assisted by three draftsmen.

The section of geologic maps has charge of the publication of the geologic folios. Its duties are to examine geologic maps and other illustrations to see that they are adequate for publication, and, if not, to properly prepare them; to read the proofs and compare them with the manuscripts; and to select the colors and patterns chosen for the representation of geologic formations. Errors and inaccuracies occurring in the manuscript or on the proof sheets are eliminated, and by careful study and selection the most effective and pleasing results of colors and patterns are obtained. The folios, which are portions of a great geologic atlas, are thus made uniform in style and form, whereas they would otherwise vary as widely as the temperaments of the various authors.

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 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 thus 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.

The folio maps are generally published on the standard scale of 1:125000, or nearly 2 miles to the inch, a scale used for most of the area of the United States. In certain localities, chiefly about cities, the scale of 1:62500, or nearly 1 mile to the inch, is employed; while in a few cases the scale of 1:250000, or nearly 4 miles to the inch, is adopted. The folios are of uniform size, 19 by 22 inches, and consequently those on the 1-mile scale represent an area one-fourth of that shown on the 2-mile scale and one-sixteenth of that shown on the 4-mile scale.

In the preparation of the geologic map the black or "culture" plate of the topographic map is electrotyped, and on the electrotype are engraved the geologic boundaries, faults, letter symbols, mines, and explanation—that is, everything that is to appear in black on the geologic map.

In the classification adopted in 1904, which is essentially that adopted in 1889 with minor modifications, there are recognized three kinds of rocks-sedimentary, igneous, and metamorphic. These classes are distinguished on the maps by colors and patterns. Of the sedimentary rocks, those which were deposited under water are represented by patterns of parallel straight lines. Eleven such patterns are in use, and other variations are produced by inclining the lines at different angles-horizontal, vertical, or oblique. Glacial, alluvial, and eolian deposits are represented by patterns of dots and circles, comprising 17 designs of different density or size. The igneous rocks are represented by patterns of crossed lines forming triangles and rhombs, of which 14 designs are in use. For metamorphic rocks of unknown origin, intricate designs of short dashes or hachures are used, of which 8 patterns have been made; and for schistose rocks these dashes may be made parallel and wavy, simulating the structure. Metamorphic rocks known to have been derived from sedimentary or from igneous rocks are indicated by suitable combination patterns.

The igneous rocks are represented by bright colors of any hue, which contrast with those used to denote surrounding formations, and indicate their intruded or extraneous character. The surficial and

metamorphic rocks are also printed in any colors, but of less brilliant tints. The water-laid sedimentary rocks are represented according to a systematic color scheme whereby the age of the beds may be recognized by the color used. These colors are more somber than those of the igneous rocks and are arranged in somewhat prismatic order, from orange at the top to brown at the base. The various systems into which the sedimentary rocks are divided according to age (the most recent at the top) and the corresponding colors are as follows:

Manuscript maps and other sheets pertaining to a folio, when completed by the author, are referred to the section of geologic maps for critical examination. The text is referred to the section of stratigraphic geology for critical reading, and is then read and prepared for the printer by the editor of texts. The maps and sections are carefully examined by the editor of geologic maps for errors, discrepancies, or omissions of any kind. Such errors may consist of either incorrect geologic interpretations or errors in drafting. Sometimes the geologic boundaries do not conform to the contours as the structure sections indicate that they should, or the sections do not accord with the map, or areas are colored incorrectly. Such errors can be detected only by one acquainted with geologic processes.

After the map is carefully examined from a geologic point of view it is scanned by a draftsman, who corrects imperfections in the drawing. This map, from which the engraving is to be made, must not be colored, as the application of color would distort the paper, and the engraving made from it would not fit the topographic base as it should. The colored geologic map, which is made for the editor's use only, is prepared on a photograph or other copy of the original map. The boundaries and faults and the mine, dip, and letter symbols are then engraved on the copper plate and proof is read in this section. In the plate-proof reading great care is taken to avoid omissions and other errors, as it is much easier to make corrections on the plate than on the stone, and the results of changes on the stone are usually not satisfactory and may necessitate the retransfer of the map. The legend is prepared and engraved separately and often requires much

attention. If the explanation appearing in the legend is not clear the map is worthless. The legend must be precise and yet concise, as the space allotted to it is limited. Besides naming and describing the formations, it must show their age and their relation to one another as far as practicable.

When engraved the map and legend are transferred to stone and the combined proof is carefully compared with the manuscript map to see that the boundaries fit the contours properly. The colors and patterns which are to represent the geologic formations are next selected. The colors for the water-laid sedimentary rocks are predetermined by the color scheme and are the same throughout the United States for rocks of the same age. Formations of the same age are distinguished from one another by their patterns. No set rules are followed in selecting these, but the distinctions are better if dense and light patterns are alternated. It is desirable also to give the larger area the lighter patterns and the smaller ones the denser patterns. This practice can not always be followed, however, because the pattern selected for a formation in one area is used for that formation wherever it occurs, although relations may be different. The direction of the lines forming the line patterns is also a means of distinction, but though the same direction is maintained for a considerable area, yet a change is made to conform with some distinct change in sedimentation, such as an unconformity, if possible. For the igneous and the surficial sedimentary rocks there is greater freedom in selecting the colors, and it is on the skillful use of this liberty that variety of color in the map and pleasing effect largely depend.

After the colors to be used are determined, sheets are prepared for the guidance of the engraver, each sheet representing a stone from which a color is to be printed. On each sheet are shown in water colors the areas which are to be printed from this particular stone, different colors being used for the various patterns to be employed. If the areas are complicated the sheets are copied on a combined or check sheet in order to avoid omissions or duplications, as any change in pattern or area requires the modification of the stone a difficult and often impossible task.

The color stones are then prepared and the proof in colors is read and compared with the sheets previously made. It is often found desirable to change some of the colors, to bring out distinctions or to harmonize harsh combinations which appear in the color proof. The precision of register, or fitting of the colors, on this map is little less than marvelous when one realizes the possibilities of error. On some maps as many as 27 colors are printed, and a slip in one of the printings would spoil the sheet. Among the sources of faults in register, change in the size of sheets of paper due to humidity is one of the most trying to overcome, as in a month-the time it takes