Base Maps.-A topographic map is the most desirable basis for geological work because it enables the geologist to tell about where he is simply by glancing at his working sheet; he can also sketch geologic features with reference to prominent surface points.

The map should have a system of coördinates laid out according to the true meridian, and coinciding with the system used in mapping the mine workings. This will greatly facilitate the work of comparison of surface and underground geology.

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The scale used will depend upon the nature of the geology to be mapped. If the boundaries are well marked and not complicated, the formations few in number, and the vein and fault system not complex, a relatively small scale, say 1 in. 500 ft., or even 1 in. = 1000 ft., will do, especially if the area to be covered is large. A quite large scale would be 1 in. 100 ft, and would only be necessary with a small area of very much involved geology, or in case the mine workings had been mapped to such a scale. The average scale, quite satisfactory under most conditions, for properties of moderate size may be given as 1 in. 250 ft.

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Working Sheets.-The best working sheets are white prints, made either from a negative of one of the brown print papers or directly from the tracing on a white printing paper. A working print should not be brittle, as is the case with some of the papers requiring an acid fixing bath. A tough blue print paper over-printed from a dark negative makes a good working sheet. The advantage of the white print papers which print direct from the tracing, is that the error in scale due to shrinkage is much less than where a negative is used. The negative must also be treated with a fixing solution, as must all brown process prints; whereas, with the direct white print it is only necessary to wash it with water.

Methods of Field Work and Equipment.-Rough mapping may be done by tying to claim corners or other points marked. on the map. In this work directions are taken by compass sights and distances are measured by pacing. Such work may be

accurate enough in making a reconnoissance, or for the mapping of isolated claims; but for accurate work, either transit and tape or stadia, or the plane table and stadia should be used. Another

FIG. 16.-Small plane table used in geologic mapping.

method, the best for most practical purposes where vegetation is not dense, is to use the small plane table and a simple sight alidade (Fig. 16).

The use of the transit may be desirable in wooded country or

in rainy weather, when a small note-book is easier to keep dry than a plane table sheet; but it entails the work of plotting in the office from field notes, when field conditions are bound to be hazy. Again, at least two men are required for a transit party. The field work with the transit consists in running traverses to which the main features to be mapped may be tied by offsets or stadia readings.

With the large plane table plotting is done in the field and office work lessened; but for this work also, using the stadia, two men are necessary, and the geologist must do the rodding, which interferes seriously with his work of traversing and observing the formation. It is also difficult for him to do good plotting when he only sees the map occasionally. The method has its chief application when topography and geology are to be mapped simultaneously; in this case it is well to have one rodman to cover the topographic features, leaving the geologist free to follow his own devices until he finds a point that he wants located, when he can hold the rod and signal for a reading.

The method usually adopted for geologic mapping, especially in open areas, is that of the small plane table. The apparatus consists of a board of convenient size (18 in. x 18 in.), of soft wood, on which the map may be mounted with thumb tacks. The ends of this board which are cut across the grain of the wood should be protected by strips of the same wood mitred in and fastened with strong glue. By means of a small brass plate with a threaded socket such a board may be mounted upon an ordinary camera tripod. This plane table is light and portable and big enough to hold the ordinary working sheet.

The geologist's compass, Fig. 17,1 used in this work has a circle graduated from 0 degrees to 90 degrees in the four quadrants, starting rfom the north and south points. The needle is agate mounted but should not be too sensitive. There is a device for

setting the circle to allow for the magnetic declination, so that when the needle reads zero the sides of the compass are in the true meridian. The compass edges may be used as rules and 1 Cut furnished by W. & L. E. Gurley.

should be graduated to convenient scales, preferably tenths of inches and millimeters. The clinometer needle is so placed that when the compass is held in a vertical plane the needle swings

free and points to zero of its scale when the upper side of the case is horizontal. The compass has vanes for accurate sighting, and the case is provided with level tubes by means of which the base may be made horizontal. These levels are used in setting

up the small table; care should be taken that the needle swings free when the compass is level, if it does not, the cover-glass should be removed, the case cleaned, and the little brass wire on the needle shifted until it is properly balanced. In addition to its use in leveling and orienting the table, and reading directions, the compass also serves as a clinometer in measuring the dip. This use is illustrated diagrammatically in Fig. 18.

The "strike" of a formation is the intersection of its general plane with a horizontal plane. The direction of the strike is

read with the compass and is usually given from the north point, as, for instance, N. 30 degrees E. In taking the direction, the edge of the compass may be placed against the actual rock surface, taking care that the instrument is level; or if the formation is somewhat irregular locally, it is well to stand off a little distance and place yourself in the imagined continuation of the plane to be observed, taking a sight to some point on the level of the eye. The "dip" is the angle (D, Fig. 18) which the general plane of a formation makes with the horizontal. It is read at right angles to the strike by placing the edge of the compass held in a vertical plane upon the surface, the dip of which is to be determined; or, the reading may be taken by sighting over the edge of the compass and making it correspond with the edge of the plane