CLASSIFICATION OF MINERAL LANDS. 55 



the greatest value should show not only geology but surface relief 

 imd the positions of streams, roads, and houses, with the legal sub- 

 divisions of the public-land surveys^ in order that the geology may 

 appear in its true relation to surface features. The construction of a 

 map giving such details is, strictly speaking, the work of the topog- 

 rapher rather than of the geologist, but the latter may be called upon 

 to examine an area for which no adequate base map has been prepared 

 and he is therefore obliged to construct a map, either contoured or 

 plain, on which to record the geology. Such a map may be made by 

 the use of various instruments, perhaps the most satisfactory in both 

 accuracy and speed being the plane table and the telescopic alidade. 

 These instruments used with a system of triangulation are particu- 

 larly adapted to open country in which signals can be seen for con- 

 siderable distances. In areas where, for various reasons, it is unpos- 

 sible to sight distant signals the method of stadia traverse is employed, 

 the same instrimients being used in conjunction with the stadia rod. 



In constructing a map by a system of triangulation, the procedure 

 is briefly as follows : 



A base line is measured and the plane table is set up at one end of it 

 and properly adjusted so that one edge of the table lies in a north- 

 south line. The base line is then plotted on a selected scale, usually 

 from 1 to 4 inches to the mile, and sights are taken and lines, drawn 

 in the direction of monuments on prominent hilltops. The plane 

 table is then transferred to the other end of the base line and cor- 

 rectly oriented by back sight. Sights are then taken and lines 

 drawn in the direction of the monuments sighted from the first sta- 

 tion. The intersection of the two lines drawn in the direction of 

 each monument establishes its location in relation to the base line as 

 indicated on -the map. After the various signals are located in a 

 certain area the plane table may be set up at any point whose loca- 

 tion is not known, and after it is approximately oriented by compass 

 sights may be taken to the points already located and the position of 

 the instrument determined by lines drawn from the locations of 

 these signals on the map. These lines should exactly meet at a point. 

 If they do not so meet there is some inaccuracy either in the pre- 

 vious work or in the orientation of the plane table. It is thus pos- 

 sible to keep a close check on the work and points may be located 

 as accurately as the scale of the map allows. 



On a scale of 2 inches to the mile a distance of 10 feet would be 

 represented by the width of a fine pencil line, and it is not possible 

 to plot a distance of, say, 5 feet. A distance of 10 or 15 feet, how- 

 ever, is perceptible, and locations may be accurately made within 

 such distances. If the scale is increased to 4 inches to the mile the 

 error in the location of a point should not exceed 10 feet. After 

 the primary signals are established over the township the base map 



