DIAGRAMS AND CURVES.] Detailed drawings of this kind, though entirely conventional, can be so prepared as to give a satisfactory expression of nature. The draftsman should study well-prepared sections in Geological Survey reports and should learn the details of folding and faulting from textbooks. He should first ascertain whether or not the vertical scale in the original section has been unduly exaggerated, and if so he should confer with the author with a view to reducing the exaggeration as much as possible. He should submit to the author all questions as to doubtful points, as well as all suggestions for improvement in expression, before he makes any changes, and he should make corrections only on the author's approval. A seeming inaccuracy in an author's drawing may be a faithful representation of natural conditions. For example, a formation that seems to be omitted by inadvertence in drawing may really "pinch out" at a point represented in the section. (See a on fig. 11.) [Illustration: Figure 11.--Structure section showing method of determining the secession of folds.] Penciled lines corresponding to those shown by dots in figure 11 should be carefully added in redrawing a roughly sketched section that shows complex folding. An original indefinite sketch that shows complicated structure affords opportunities for error in preparing the new drawing, and omissions may be detected by following the formations as they would be continued above and below the section, as shown by the dotted lines in the figure. PLANS AND CROSS SECTIONS OF MINES. Plans of mines, like diagrams, should not be elaborate, and their lettering should be plain and legible, yet it should not be so conspicuous as to obscure other details. Gothic letters should generally be used, but some plans require different styles of lettering, especially for geographic or other names that should be coordinate with those on maps or other illustrations in the book. Unless there are good reasons, however, for varying the styles of lettering, plain gothic capitals, or capitals and lower-case letters, either upright or slanting, should be used. Abbreviations for the numbers of levels should generally be given thus: 3d level, 6th level, 200-foot level, etc., or the shorter terms may be spelled out, as third level, sixth level. The same general scheme of lettering should be used on all plans and cross sections that are to appear in one publication or in one series of similar papers. The reduction of such drawings to the minimum scale consistent with clearness is always advisable. DRAWINGS OF SPECIMENS OF ROCKS AND FOSSILS. METHODS USED.