54 CLASSIFICATION OP THE PUBLIC LANDS. 



somewhat variable larger figure for locations requiring a less degree 

 of accuracy and made with less precise instruments. The actual ele- 

 vation of observations along the outcrop of the valuable bed is deter- 

 mined with surveying instruments, so that the error is rarely in excess 

 of 5 feet, an amount which is practically negligible in mountainous 

 regions. In special cases, however, a higher degree of precision is 

 attained. 



The information collected by the geologist in the field relates to 

 the quality and thickness of the economic deposit, the position of its 

 outcrop relative to legal subdivisions, and its depth and accessibility 

 where covered. He maps the actual position of the outcrop of the 

 coal bed, oil sand, or phosphate bed by making careful locations at 

 many points, as described above, and where possible determines the 

 attitude of the bed by many careful dip readings with a clinometer 

 or by computation from its elevation at several points. He also maps- 

 the position of the top and bottom of the overlying and underlying 

 beds, so that the completed map shows the actual area covered by the 

 outcrop of beds of sandstone, limestone, conglomerate^ shale, or such 

 other rocks as may be present in the region. He further determines 

 the attitude of these rocks and collects typical samples and fossils 

 which serve to determine their relative age. Part of this information 

 is represented on the map by symbols whose significance is expressed 

 in the accompanying legend, and part is recorded in suitable note- 

 books for use in the preparation of reports. This information is put 

 together in a tabular form showing the normal arrangement of the 

 beds, which is technically spoken of as a columnar section. Such a 

 section shows the normal distance through the strata from the economic 

 bed to the top or bottom of any one of the constantly related overlying 

 or underlying beSs, and it is used in combination with the data on the 

 outcrop of the overlying rocks in working out the theoretical under- 

 ground position of the bed. The methods of making the computation 

 are explained in detail in the following discussion of the examination 

 of areas for which a topographic base is not available. 



The deposits of present economic importance or of promise for the 

 future are sampled at many points where the natural exposures are 

 favorable and in other places by means of pits, trenches, or drill holes. 

 All the samples from any one deposit are taken by a uniform method 

 designed for that kind of material. 



HETHODS USED WMJSN A BASE MAP MTTST BE PREPAKBD. 



In the detailed geologic study of an area it is usually necessary to 

 construct a map which shall place before the eye of the geologist all 

 the available information concerning the area in such space that it 

 can be viewed as a whole and the correct relations of details compre- 

 hended as they could not be on the ground. Such a map to be of 



