58 CLASSffllCATlOIir op the PtJBLiC LAKDS. 



Therefore a bed that dips 5° E. under a horizontal plain is 460 feet 

 beneath the surface at a point 1 mile east of the outcrop. 



If it appears from surface exposures of the overlying rocks that 

 the dip is not constant, allowance must be made for the variations 

 in the calculation. In areas in which drilling has been carried on 

 the records of the wells are invaluable as giving accurately the depths 

 of beds beneath the surface, thus checking the deductions from- sur- 

 face evidence. 



The locations of structure contours are based on a variety of data. 

 If a well whose altitude at the surface is 4,000 feet strikes an oil sand, 

 for example, at a depth of 1,000 feet, it is evident that the 3,000-foot 

 contour on the oil sand passes through the location of this well on 

 the map. If the rocks dip toward the east and a sandstone known 

 to be 2,000 feet above the oil sand outcrops half a mile east of the 

 well at an altitude of 4,500 feet, the oil sand should be at this point 

 2,500 feet above the sea, or, in other words, the 2,500-foot contour 

 should pass through the location of this sandstone outcrop on the 

 map. If the dip of the rocks exposed at the surface is constant be- 

 tween the mouth of the well and the outcrop of the sandstone it indi- 

 cates that probably the oil sand dips regularly to the east between 

 these points, and the contours of 2,600, 2,700, 2,800, and 2,900 feet 

 may be spaced evenly between the 2,500- foot contour at the sandstone 

 outcrop and the 3,000-foot contour at the well. Should the dip, as 

 indicated by the surface exposures, be irregular between the two 

 points, the contours are spaced accordingly, being placed nearest 

 together where the dip is greatest and farthest apart where the dip 

 is least. 



After a structure-contour map is completed it is possible to esti- 

 mate from it the depth of a bed of economic importance at any 

 required point, if the surface altitude of the point is Icnown. 



HBTHODS FOLLOWED VTSBSK THE IiAJSTD OHBTOE StTBVEY APPOKDS A BASE MAP. 



More than half of the coal land still owned by the Government 

 lies in the rather sparsely settled plains region which has not beeij 

 mapped topographically but has been covered by the cadastral sur- 

 veys of the General Land Office. For such land, a topographic map 

 not being essential, a method of survey different in some ways from 

 those described above has been adopted. Plane table, telescopic 

 alidade, and stadia rod are used as in the other work, but the results 

 of the surveys made by the General Land Office are used as a base 

 for the work, and the township forms a unit, the results being suffi- 

 ciently detailed and accurate to permit classification by legal sub- 

 divisions. As previously stated, some of these surveys are not accu- 

 rate, but they are official and in general they provide a suitable base 



