Figure 3.—Section and perspective view showing relations of surface features to the different kinds of rock and the structure of the beds.
A kind of cross section which is not often used but which gives a more pictorial and clearer conception of underground relations than other kinds is made by adding a sketch of the topography above the section. This sketch should be a perspective view, in which the prominent features shown hypothetically in the section below it will be reflected in the topography. Such a sketch might show, for example, not only monoclinal slopes, "hogbacks" due to steeply upturned beds, terraces, escarpments, and like features, but volcanic necks or other extruded masses in their true relations to the underground geology of the country. (See fig, 3.) In submitting the draft of such an illustration the author should, if possible, submit also a sketch or photographs of the adjacent country and indicate on the section the point of view by notes such as "Sketch A made at this point," "See photograph B." The sketch will be more useful if it is prepared on a scale consistent with the details of the section. It may be made with a pencil and should show as well as possible the relations of the features in the landscape to those in the section. Some good examples of illustrations of this type can be found in Powell's "Exploration of the Colorado River," pages 182-193. One simpler figure of the same kind is given on the cover of the geologic folios.
In preparing original drawings representing columnar sections, or sections in wells or ravines, the author should indicate all well-defined or important local features of structure, such as cross-bedding, ore bodies, or lenses. If there are no unusual features or details, the subdivisions need be identified only by names of materials, such as "thin-bedded limestone," or "slates with some coal," the coal beds being shown. The sections should, however, be so plotted and subdivided by the author that each section or group of sections will be complete in its crude form. The compilation of various parts into one unit and the construction of columnar sections by reference to tables alone is an essential part of the author's original preparation.
Figure 4.—Sections of coal beds. The Figure shows the publications size and the arrangement at the sections. Each section should be drawn three-tenths or four-tenths of an inch wide and reduced one-half. Thicknesses can be indicated by numbers, as shown on sections 1 and 10, or by bar scale.
Sections designed to show the relative thickness of beds of coal, arranged in groups for publication either as plates or figures, should be drawn in columns three or four tenths of an inch wide and reduced one-half, as shown in [figure 4]. These sections, whether correlated or not, should be drawn to some definite vertical scale and should show the thickness of the coal beds, preferably by numbers indicating feet and inches, the other material being symbolized and the symbols explained graphically, as shown in [figure 4]. The vertical scale should always be stated for the use of the draftsman. A bar scale may be used instead of figures showing the dimensions of the individual beds.
LITHOLOGIC SYMBOLS.