CLASSIFICATION OF MINERAL LANDS. 59 



for the survey. Locations and measurements are therefore accepted 

 as recorded on the Land Office plats unless the field work proves 

 them to be in error. 



If a preliminary examination of the township to be surveyed 

 shows that it contains only a few isolated outcrops of coal beds and 

 no continuous exposures, the geologist locates the outcrops by tri- 

 angulation and sketches their inferred locations. For the triangula- 

 tion two convenient land corners are selected from which to begin 

 work, and for a base line the distance given on the land plats is used, 

 it being assumed that this distance has been correctly determined 

 in the land surveys. The corners thus selected are located on a 

 plane-table sheet, and from them other triangulation stations are 

 established over the field and their elevations determined by vertical 

 angles, as already described. After enough points to control the 

 work in the township have been established the geologist makes a 

 detailed examination of the coal. He goes to each outcrop and digs 

 or bores through the bed to learn the thickness of each bench of 

 coal and of each parting and to obtain samples of the coal for study 

 or analysis. 



After the bed has been examined and the dip of the strata has been 

 measured the position of the outcrop is located on the plane-table- 

 sheet and the elevation determined. 



The examination of the coal bed is most important. It must be 

 made carefully and accurately. Each geologist who surveys coal 

 land is told that " every inch of the bed must be critically examined." 

 If the inspection shows that the coal may contain more than the 

 normal percentage of impurities a representative sample is selected 

 and taken to camp, where an ash test is made with a small portable 

 chemical outfit. If there are mines in the area a sample is procured 

 according to the prescribed method (see p. 106) and is sent to the 

 chemical laboratory for analysis. The strata in which the coal is 

 found are also carefully examined to obtain any additional infor- 

 mation they may give concerning the depth and continuity of the 

 coal beds where they are covered. 



The triangulation method of location described above suffices in 

 areas where there are only a few small outcrops of coal beds, but in 

 townships which contain more than a few outcrops or in which the 

 beds can be traced the outcrops must be accurately located by stadia 

 traverse in addition to the triangulation. If the triangulation and 

 stadia work are done properly, locations should be correct within 20 

 feet and elevations within 5 feet. 



In work of this kind the surveyor traverses the outcrop of the coal 

 bed and with the aid of an assistant and instruments he locates 

 points at short intervals along the line of outcrop and sketches its 

 position between these points. All such locations are tied to land 



