For underground geologic mapping, a "work sheet" or "field sheet" is traced from the base map. A satisfactory material is a good grade of oiled tracing paper. As an alternate a semi-transparent typewriter paper of 100% rag bond may be used. Tracing cloth is entirely un-suitable for this purpose as it does not take pencil line well and a little water ruins it. Blueprints do not show pencil lines and white prints are soft when they are moist.

Before going underground to start mapping geology, the outline of the workings is traced on sheets of standard size, usually 8 1/2 x 11 inches, allowing an inch or so of overlap at the end of each sheet. As an alternative to standard-sized sheets, a strip 8 1/2 inches wide and several feet in length may be folded accordion-fashion into sections 11 inches long. The workings, survey stations, and coordinates are traced in India ink, and, for convenience in plotting strikes, it is helpful to provide a generous number of north-south lines between the coordinates, If the ink is applied on the reverse side of the sheet, erasures can be made while mapping without destroying the outline of the workings, and furthermore, any geological lines that locally coincide with the wall of the drift can be distinguished. Very little extra time is required if the map is traced in pencil and then turned over for inking. Station numbers and lettering should, of course, appear on the face side of the map.

In order to keep up to date with development and to take advantage of clean exposures near the face, the company geologist may find it desirable to do his mapping before the engineers have had time to survey and plot a new advance. In this case he will have to make his own base map as he goes and later tie it into the new transit stations, locating them on his field sheets before posting the geology on the more accurate office map.

Equipment

The work sheet is held in an aluminum sheet-holder with a hinged cover. If the mine is at all wet, it is convenient to use a sheet of blotting paper the same size as the sheet-holder; it may be held to the top cover of the sheet-holder by a rubber band, or, as practiced by the geologists at Grass Valley, the blotting paper may be cemented to the lid of the sheet-holder. At Butte and in other districts, a pencil-holder is riveted to the lower side of the sheet-holder. It consists of a sheet of leather crimped to make pockets for half a dozen or more pencils; thus hard (3H to 5H) and soft black pencils and a variety of colored pencils are handily available. Since the pencils must be kept sharp for drawing fine lines, some workers attach a strip of sandpaper to the lower side of the sheet holder. Even better, according to Farmin, is a magneto-point file which may be laced through the pencil-holder.

For plotting veins and faults, some geologists use colored pencils, which must be kept very sharp by frequent rolling on paper in order to give a fine line. Others prefer to draw all lines in black, laying on color beside the line if -necessary in order to identify it.