56 CLASSIFICATION OF THE PUBLIC LANDS. 



and the geologic map may be constructed simultaneously — ^that is, 

 in tracing the exposures of some particular bed, such as a coal or 

 phosphate bed or an oil sand, the geologist may sketch in the roads 

 or streams or topographic features near which his instrument happens 

 to be set. 



In mapping a particular area geologically as many points are 

 located as are necessary for the degree of accuracy desired on the 

 map. In critical parts of an oil field, where all dip readings are 

 located by the plane table, observations may be taken every 400 or 

 500 feet, but in parts of the area outside of the probable extent of 

 the oil pool dip readings taken every mile may be amply sufficient if 

 the dip is regular. Places at which fossils are collected are marked 

 on the map and also places at which rock specimens and coal, phos- 

 phate, or oil samples are taken, as well as localities where sections 

 of the beds or other geologic observations are made. In the con- 

 struction of such a map any land corners that may be found are 

 located from the signals previously established, just as rock out- 

 crops and other features of" interest are located. The land net is 

 afterward drawn in from the corners which have been found, and 

 any inaccuracies and irregularities in the old surveys will thus be 

 detected. In land classification the tracts are always described with 

 reference to these surveys, so it is very essential to locate as many of 

 the corners as possible in order that the position of the land lines 

 may be accurately known. The time required to construct a map 

 of this kind varies greatly with the nature of the country and the 

 character of the geology. One man can, as a rule, establish primary 

 control over a township in one to three days, the time required being 

 dependent entirely on the number of locations made and the nature 

 of the country traversed. 



The method of locating important rock outcrops by stadia traverse 

 may be successfully employed in conjunction with location by trian- 

 gulation, or it may be used alone without triangulation. In running 

 a traverse with stadia the instrument man can not work alone but 

 must employ the services of a rodman. The plane table is set at some 

 known point, the stadia rod is held on the point whose location is de- 

 sired, and the direction from the point occupied to the other point is 

 drawn on the map by means of the alidade. The distance is read by 

 means of the stadia and platted on the map by the use of a scale, 

 in the direction already recorded ; this locates the second point. The 

 instrument is then transferred to a third point, the location of which 

 is determined by a reversal of the method used in locating the second. 

 The rod is then moved ahead and the process is repeated. Such 

 a method is most useful when a coal or phosphate bed or an oil sand 

 is to be outlined in great detail or when the work is done in deep 



